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All About Radiant Floors

Although in-floor hydronic tubing provides comfortable space heating, these systems are expensive and rarely appropriate for a well-designed house

Posted on Apr 18 2014 by Martin Holladay

So-called radiant floors have an excellent reputation. Many customers report that this type of heating system is comfortable and quiet. Moreover, some suppliers of radiant floor materials and equipment claim that these systems can save energy.

In spite of the purported benefits of this type of heating system, few green homes include a radiant floor heating system. This article will explore why.

What shall we call these systems?

We’ve all seen ads for this type of heating system, including photos showing a barefoot mom watching her baby crawl across the floor. Using photos like this as a guide, is it possible to describe the heat transfer mechanisms in such rooms?

The mother’s bare feet are being heated by conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow.. The air near the floor is also being heated; as the warm air rises to the ceiling, it creates a convective loop. So the room is being heated by convection. And, if the floor is warm enough, the mother’s bare arms are being heated by radiation.

In other words, all three heat transfer mechanisms are at work. So why is this a “radiant floor”? The phrase “radiant floor” is misleading, and should be abandoned. It’s more accurate to say that this floor has “in-floor hydronic tubing.”

Three ways to warm up your flooring

There are three types of heated floors:

  • Most heated floors use in-floor hydronic (hot water) tubing.
  • Some heated floors have embedded electric-resistance wires that heat the flooring like a toaster. Because electric-resistance heat is expensive to operate, this type of heating system is usually restricted to small rooms like bathrooms.
  • The third type of heated floor, a hypocaust, is relatively rare. Hypocausts are thick masonry floors with embedded hot-air ducts or flues. Hypocausts were once common in Korea. They were also used in a few solar homes in the 1970s and 1980s, but are rarely installed these days.

Hydronic systems

A hydronic heating system has a boiler rather than a furnace. (Boilers heat water, while furnaces heat air.) While most hydronic systems distribute heat using fin-tube baseboard units or wall-mounted radiators, some use in-floor hydronic tubing.

It’s also possible for a hydronic heating system to use a water heater rather than a boiler to heat the water.

The main disadvantage of hydronic heating systems is that they don’t provide a convenient way to integrate air conditioning. (Most air-conditioned homes in the U.S. use ductwork to distribute cool air; since these homes need ducts for cooling, they usually use the same ducts for heating as well.)

Ironically, some homes with in-floor hydronic tubing use ductless minisplit heat pumps for cooling — which raises the question, “Why not just use the ductless minisplits for heating, too?”

Where does the tubing go?

There are at least six different ways to install an in-floor hydronic system:

  • Slab-on-grade systems use hydronic tubing that is embedded in a full-thickness concrete slab placed over rigid foam insulation.
  • Systems with thin slabs over wood framing have tubing that is attached to the top of the plywood subfloor, and is then covered with a thin slab (usually 1 1/2 inches thick) of Gyp-Crete or conventional concrete.
  • Systems with aluminum heat-transfer plates use metal plates with a groove designed to accept tubing. In these systems, the plates are stapled up against the underside of the subfloor, working from below.
  • Plateless staple-up systems use tubing that has been stapled directly to the underside of the subfloor, without any aluminum heat-transfer plates.
  • Above-floor tube-and-plate systems use heat-transfer plates which are installed on 3/4-inch sleepers nailed above the subfloor.
  • Radiant subfloor panel systems use grooved plywood or composite panels; the panel grooves are designed to accept snap-in tubing. There are several brands of these panels, including , , and .

Alex Wilson’s landmark article

The best analysis of in-floor hydronic systems from a green perspective was written over a decade ago by Alex Wilson. His classic article, was published in the January 2002 issue of Environmental Building News.

I am indebted to Wilson, as well as to Marc Rosenbaum and Andy Shapiro, the two energy consultants whom Wilson interviewed when researching the article, for laying the technical groundwork for most of the arguments I’ll be making in this article.

Is there any evidence that these systems save energy?

Proponents of in-floor hydronic tubing have suggested several mechanisms by which these systems could contribute to energy savings.

Do occupants of homes with in-floor hydronic tubing choose lower thermostat settings? Some people claim that radiant floors are so comfortable that occupants voluntarily set their thermostats to a lower-than-usual setting, thereby saving energy. Suffice it to say that there is no evidence that this is true; the best research study on this issue () found no evidence to support it.

Except in very cold weather, this type of heating system only has a “warm floor” for a few hours a day, or will circulate water with a relatively low temperature, so it stands to reason that most occupants will adjust their thermostats until the indoor air temperature feels comfortable — just like occupants of homes with different heating systems.

A house with in-floor hydronic tubing can select a lower boiler temperature than a house that uses fin-tube baseboard. That’s true, but a lower boiler temperature won’t result in significant energy savings. As Alex Wilson wrote in his 2002 article, lower boiler temperatures “might reduce heat loss into unconditioned space if boiler and piping are located in an unheated basement, but experts … suggest that the savings would be very small at best — especially because of the additional electricity consumption to operate pumps for long hours.”

Homes with hydronic systems may have lower rates of air leakage that homes with forced air systems. Strictly speaking, this point applies to hydronic heating systems in general, rather than specifically to homes with in-floor tubing. The point is nevertheless worth addressing.

A forced-air system with leaky ductwork can easily interact with a leaky building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. in a way that increases infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration.. When this happens, the problem is due to the leaky duct system and the leaky thermal envelope rather than the forced-air system itself. It’s perfectly possible to design duct systems and thermal envelopes that don’t suffer from this type of leakage.

On the other hand, maybe these homes require more energy

If a house has in-floor hydronic tubing installed in a slab on grade, the house may have more heat loss to the ground than a house with a forced-air heating system — especially if the contractor didn’t install enough rigid foam under the slab (an all-too-common problem). After all, the concrete will be warmer than usual — for several hours a day, the temperature of the concrete will be above the room air temperature — potentially increasing the rate of heat loss from the slab to the soil.

Fortunately, there is a relatively simple solution to this problem: make sure to include lots of sub-slab insulation under heated slabs — between 4 and 6 inches, depending on your climate.

Disadvantages to in-floor hydronic tubing

These systems are slow to warm up and slow to cool down. Most homeowners prefer a heating system that responds quickly to thermostat changes. If your house is chilly or overheated, there is no advantage to having to wait several hours for the heating system to respond to a thermostat adjustment.

Equipment manufacturers have cleverly designed complex controls to address the fact that in-floor hydronic systems are slow to respond to thermostat adjustments. In many cases, these controls include outdoor temperature sensors as well as sensors embedded in the concrete slabs; the information from these sensors is used to adjust the boiler water temperature in anticipation of future conditions. While these sophisticated controls go a long ways towards addressing the slow-response problem, they add complexity and expense to this type of hydronic heating system.

Why isn’t my floor warm? Homeowners who look forward to walking barefoot over warm floors are often disappointed by homes with in-floor hydronic heating. That’s because these floors are rarely as warm as most homeowners expect.

Wilson described this problem in his 2002 article. “Heat is transferred from an exposed slab to the [indoor] space at a rate of about 2 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /ft2•hr•°F,” Wilson wrote. “In a well-insulated house, this rate of heat flow means that even when it is very cold outside, the slab can only be a few degrees warmer than the rest of the room or the room will keep heating up. For a concrete slab to feel warm, however, it needs to be about 80°F. Thus, for most of the heating season, the greatest feature of radiant-floor heat — a warm floor — won’t occur.”

In many well-insulated homes, a “radiant” floor may be maintained at only 75°F — which is less than the temperature of your bare feet.

These systems are incompatible with passive solar design principles. Passive solar principles established in the late 1970s recommended the installation of a dark-colored concrete slab or dark tile flooring on the south side of a house. In theory, morning sun streaming through the south-facing windows would strike the cool concrete floor, heating up the concrete. Over the next 12 hours, the warm concrete floor would slowly lose heat to the indoor air. The intent was for the mass of the concrete to act as a thermal storage system — a flywheel — to stabilize indoor air temperatures.

This approach works best (is most efficient) when a floor slab is cool during the early morning hours. Cool slabs can store more solar heat than warm slabs. Traditional passive solar homes work best when the occupants accept a wide range of indoor temperatures — from perhaps 60°F in the early morning to 80°F in the middle of the afternoon. If homeowners can tolerate these swings in indoor temperatures, and if they live in a location with lots of winter sunshine (for example, in Colorado or New Mexico), a passive solar home will have low energy bills.

If a home has in-floor hydonic tubing, however, the heating system is likely to turn on at 3:00 a.m., when outdoor temperatures are cold. When the sun strikes the floor at 9:30 a.m., the concrete is already warm, and therefore unable to store as much solar heat as it would have if it had started out cold.

Wilson discussed the problem in his 2002 article. “If a concrete slab is ‘charged’ with heat [via hydronic tubing] during the early morning hours and the surface is warmed to the point where it cannot readily absorb solar radiation striking it, that solar heat will more directly heat the air, increasing the risk of overheating,” Wilson wrote. “This isn’t a huge problem with radiant-floor heating systems, but it may mean that homeowners will have to open windows periodically in the winter and their overall energy savings from solar energy will not be as great. [Andy] Shapiro counsels against the use of radiant slabs in areas of houses with passive solar heat. ‘It’s a waste of energy,’ he says, though just how much waste occurs is unclear.”

These systems are expensive. In-floor hydronic systems cost significantly more than forced-air systems. That’s why they are more likely to be found in custom homes than in tract homes.

If you’re building a house, and are considering spending an extra $5,000 or $8,000 to upgrade from a forced-air heating system to a hydronic heating system, a strong argument can be made that you’d be better of spending the extra money for more insulation, improved air sealing, or better windows. Once you’ve done that, your heating needs will be lower, and it will be easier than ever to satisfy your home’s heating needs with a simple HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system.

Over a decade ago, Marc Rosenbaum explained the principle this way: “It just doesn’t make sense to put in a $10,000 heating system to provide $100 worth of heat per year.”

If you take some of the money you saved by not installing in-floor radiant tubing and use that money to buy thicker subslab insulation, you’ll get most of the warm-floor benefits you seek. As Andy Shapiro noted, “A house with a good enough envelope to be called green — well-insulated and tight — will have a very high level of comfort no matter what type of heating system is used, as long as that heating system is well designed.”

A case study in Massachusetts

Many designers of low-energy homes have learned the hard way about the disadvantages of using in-floor hydronic tubing for space heating. One relevant example is a net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. house built in 2007 in Colrain, Massachusetts, by Rural Development Incorporated (RDI), a nonprofit developer of homes for low-income and moderate-income families. The 1,350-square-foot slab-on-grade home includes a 3.2-kW photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array and 57 square feet of solar thermal collectors.

The designers of the home specified a hydronic heating system with in-floor tubing. The water for the hydronic system is heated by solar thermal collectors, with back-up heat provided by a propane-fueled instantaneous water heater. “On the design heating day [the coldest day of the year], the water flowing through the tubes in the floor will only be at 85 degrees,” said Robb Aldrich, an engineer at Steven Winter Associates in Norwalk, Connecticut, who helped design the heating system. “So the radiant floor allows excellent utilization of the solar hot water. If the house had a hydro-coil forced-air system, you’d need at least 120-degree water.”

Yet Aldrich is still not convinced that the in-floor distribution system was a wise choice. “Because of the radiant slab, storing direct solar gain is out of the picture,” he noted. “Having invested so much in the envelope, they could have gone with a really simple, cheaper, lower-cost heating system. In their next project, RDI does not plan to do a radiant floor because it is just too expensive.”

Anne Perkins, RDI’s director of homeownership programs, echoed some of Aldrich’s points. “I don’t think a radiant floor is appropriate for such a well insulated house,” she said. “You have such a low heat load it doesn’t make sense to spend a lot of money on a heating system. Most of the time the water running through the pipes in that expensive radiant floor will be so cool that the floor won’t even feel warm.”

Before deciding on in-floor radiant heat, the system’s “parasitic” energy load — the electricity required for pumping — must be calculated. “We looked at pumping energy a lot,” said Aldrich. “It’s a big concern of mine. I was fairly rigorous on all the friction and pressure calcs. On the solar thermal system, we decided to use a PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.-powered DC pump. We chose the smallest possible circulators for the radiant floor. There were some compromises there, though. We ended up with a constant circulation system — basically, the pumps operate continuously for the entire heating season.”

The Colrain heating system has two circulators that together draw 173 watts. Since the circulators will operate for 4,000 to 5,000 hours per year, they consume between 19% and 23% of the annual output of the home’s PV array. “The radiant pumping energy will really be a significant load, which definitely bothers me,” Aldrich said.

These systems make sense for auto-repair shops

Many authors have quoted Alex Wilson’s conclusion that a radiant floor heating system is “a great heating system for lousy houses.” These heating systems also make sense for auto-repair shops in cold climates. If you have to work on cars that are sometimes covered in ice and snow, you will certainly appreciate a heated slab in your garage.

When it comes to designing a green home, however, it’s important to follow these simple principles:

  • Design your house to be small and compact.
  • Aim for a thermal envelope that is highly insulated and close to airtight.
  • Choose windows with appropriate glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., and don’t make your windows too big.

The main benefit of these principles is that you’ll end up with a house that is comfortable. You’ll also end up with a house that is very easy to heat and cool, without spending an arm and a leg for a complicated HVAC system.

“But I really want one!”

Occasionally, after the arguments presented in this article are explained to someone planning to build a new home, the client says, “I understand why a radiant floor heating system doesn't make much sense for the house I'm about to build. But I love the idea of radiant floor heat, and I can afford to install it. I just really, really want this type of heating system.”

If you are an engineer or a designer who has a client like this, the best response is clearly, “OK. We'll help you design the system you want.”

Martin Holladay’s previous blog: “Thermal Barriers and Ignition Barriers for Spray Foam.”

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Apr 18, 2014 8:28 AM ET

Passive Solar and Underslab Insulation
by Jeff Carroll

Excellent article. We are building a 1600 sq. ft. passive solar home with slab-on-grade and were originally considering radiant heat until we came across Alex Wilson's article.

One question I have not seen consistently answered, though, is how a passive solar floor should be insulated to optimize the thermal mass while still providing some warmth to the feet during times when the sun is not out or in areas where the sun might not reach. I have seen multiple suggestions, of course, ranging from outside the slab (including shallow frost-protected versions) up to insulation under the entire slab.

We're in a mixed climate (Missouri) and the suggestion from Building Science seem to indicate semi-permeable insulation under the entire slab with no vapor barrier, however I don't know if passive solar factors in to that suggestion.

Any advice for us? Thanks in advance.

Apr 18, 2014 8:44 AM ET

Response to Jeff Carroll
by Martin Holladay

In all but the warmest climates, it makes sense to install a horizontal layer of rigid foam directly under your slab -- whether or not your slab is a component of a passive solar design strategy.

The usual layers for a slab on grade, from the bottom up, are: 4 or more inches of crushed stone, a horizontal layer of rigid foam, 6-mil polyethylene, and concrete. You will also need vertical rigid foam at the perimeter of the slab.

There is no advantage whatsoever to choosing vapor-permeable foam at this location. On the contrary: a vapor barrier (the 6-mil poly) is an essential component of this assembly.

You can use an energy modeling program to determine the optimal R-value of your sub-slab foam. Remember: you only get one chance to install sub-slab insulation. No one has ever regretted installing too much insulation in this location, but many builders and homeowners have regretting installing too little insulation.

Apr 18, 2014 9:11 AM ET

Edited Apr 18, 2014 9:19 AM ET.

Adirondack Area; Radiant tubes are a nice addition to a basement
by aj builder, Upstate NY Zone 6a

Adirondack radiant tubes in basement floors when basements are used as living space: I can tell you first hand this option is well liked, loved. Another benefit, no baseboard radiators mucking up the walls.

Radiant floors where there is no solar gain, and bathrooms... good place for radiant. And yes I agree that a net zero or PGH homes will not need radiant floor heat covering entire floors, still might work to hit the bathrooms on a timer or not if truly looking to spend zip on energy.

I do look forward though to using mini splits someday soon on a project. I have had a customer install the first mini, but the home is not quite finished and moved into.

Apr 18, 2014 9:25 AM ET

Edited Apr 18, 2014 9:29 AM ET.

So, do "radiant" heat floors emit mostly radiant heat?
by Kent Jeffery

As usual, a nicely written synopsis on a popular topic, Martin. I have a question for you, though. As you pointed out, "radiant heat floor", in its typical installation fashion, should more appropriately be called "hydronic in-floor heating", but it begs the question: is most of the heat delivered really radiant?

While it's the conduction property that appeals to so many (like auto shop workers lying on the slab to loosen the rusty exhaust nuts), there are some who feel the real benefit is radiation, however, that same nice sense of warmth a campfire provides on a chilly night. Will a hydronic in-floor system still yield this desired trait at the lower delta-T that super-insulated homes require?

What are the relative components of conduction, convection and radiation in this system? And how much do they contribute to occupant comfort (as opposed to merely warming the room)? As AJ Builder points out, people love 'em in their basements!

Apr 18, 2014 9:46 AM ET

Edited Apr 18, 2014 10:01 AM ET.

Response to Kent Jeffery
by Martin Holladay

Q. "What are the relative components of conduction, convection and radiation in this system?"

A. There is no single answer to this question. The answer will vary depending on the amount of clothing worn by any people in the room (radiation is more of a factor for nude people than clothed people); whether they are standing or curled up on a sofa; the thermostat setting; and the temperature of the flooring.

In many well insulated homes, radiation from a hydronically heated floor to bare skin will be a very minor factor. Classic examples of radiant heating require very hot sources of heat: the sun, a blazing wood stove, or a campfire. When the heat source is at 75 degrees F or 80 degrees F, you aren't likely to extend your bare hands and aim them at the floor to warm them up, the way you would when you approach a wood stove.

Apr 18, 2014 10:22 AM ET

Aha, good info, BUT...
by Armando Cobo

One must have to live or lived in a radiant-heated house to “get it”, as is not about the price, it's all about the joy of walking barefoot and feeling warm toes. For the same reason some folks buy a Lamborghini or a new $500 golf driver and shoot 100. JP Morgan said once, referring to the price of a boat, “You have no right to own a yacht if you ask that question.’”
Having said that, I found the best way to install a hydronic radiant heating system is to install a good amount of rigid foam under the entire slab, barometric thermostat, run a water line to the air handler coil to provide quick recovery and install supply-only or balanced make-up air. It goes without saying, it all works best with a tight envelope with micro heating and cooling loads.

Apr 18, 2014 10:40 AM ET

“Radiant Heat”
by Ron Keagle

I regard the term “radiant heat” to mean a heat distribution system in which radiation plays the most prominent role in the heat delivery. It has to involve a judgment call as implied in the relative term “prominent” because all three forms of heat transfer operate in any type of heat distribution system. So, generally, I would classify a heated floor as a radiant heat delivery means. A heated floor also readily conducts, but only to the extent that there are objects or people to conduct to. Although it also conducts to air and makes convection, but the geometry for that is relatively awkward.

However, aside from the controversy of calling a heated floor a “radiant floor,” I sense that the term “radiant heat” has popularly come to mean a heated floor EXCLUSIVELY. That, I think is an error in terminology. “Radiant heat” could also come from walls or ceiling.

The cast iron “radiator” or “baseboard radiation” do both radiate, but perhaps convection is equally a part of their means of heat distribution. Baseboard radiation in particular is small in terms of a radiant emitter, but as a source of convection, it effectively heats large areas of walls, and thus converts the walls into effective radiant emitters.

I believe the popularity of heated floors comes from the experience of the sense of warmth conducting to peoples’ feet most often in a building envelop that has poor-to-average levels of insulation. This sensory experience is indeed like the experience of standing by a hot campfire on a chilly night. This sensory delight is what sells people on the so-called radiant floor.

On the other hand, if a heated floor were operating efficiently in a superinsulated house, the average person might not even realize that there was a heated floor involved.

Apr 18, 2014 11:59 AM ET

Facts please... will the floor feel warm and cozy?
by Kent Jeffery

Because I have been roundly and correctly questioned regarding my initial HVAC design (thankfully, before any significant investment made), I've been exploring in detail this question. Opinions are great, but like the sign above my desk says, "Everyone is entitled to their own opinion, but not their own facts". I needed some facts.

Let's say my house is ultra-tight, and super-insulated. And let's say that the design heat load it 10 BtuH per sq. ft. Maybe that's not so "ultra" or "super", but for this discussion, it suffices as I move forward in making my point. Now, 10 BtuH/sf is the design condition, only rarely met. Most of the time the heat flux from my hydronic floor will be much less.

What if I wanted an air temp of 72F, for there I felt “comfortable”? What, then, will be the required temperature of my slab to maintain this design point? Let's say it's REALLY cold outside, and I am near my design load condition of 10 BtuH/sf. If we use Mr. Wilson's heat transfer coefficient of 2 BtuH/sf/deg F for slab to room, which I believe is for combined radiant and convective load transfer, then the Δ-T works out to be = 10 BtuH/sf ÷ 2 BtuH/sf/deg F, or 5 deg F. Add this to my target air temp and the slab temperature targeted will be 77 deg F.

How would this feel on my bare feet? Would I be happy? Well, according to Dr. Bjarne Olesen in his 1977 paper entitled, “Thermal Comfort Requirements For Floors Occupied By People With Bare Feet”, 15% of people will be unhappy at 10 minutes if the slab temperature is less than 79 deg F. On other words, the heat flux needed to keep most people happy standing barefoot on concrete, in a 72 degree room, is 14 BtuH/sf or better (= Δ-T x 2 BtuH/sf/deg F).

According to Dr. Olesen, and incorporated into ASHRAE recommendations, the optimal temperature for comfort standing on a concrete floor is between 79F and 83F ()

FACT – an ultra-tight and super-insulated home requiring heat flux of less than 14 BtuH/sq. ft., heated via hydronic in-floor radiant heat, will leave most people dissatisfied.

Apr 18, 2014 12:04 PM ET

Edited Apr 18, 2014 12:11 PM ET.

Fact is, the answer is "no"...
by Kent Jeffery

... if they expected warm tootsies, that is.

Apr 18, 2014 12:04 PM ET

Response to Kent Jeffery
by Martin Holladay

Q. "Will the floor feel warm and cozy?"

A. The leakier the house, and the thinner the insulation, the cozier the floor will feel.

On the other hand, the tighter the house, and the thicker the insulation, the less likely that the floor will feel warm and cozy.

For most homeowners, installing enough insulation under the floor will do enough good to make the home feel comfortable. Truly uncomfortable homes are usually drafty or have a high degree of thermal stratification between the air near the floor and the air near the ceiling. Building a tight, well insulated envelope gets you where you want to be from a comfort standpoint.

Apr 18, 2014 12:16 PM ET

Edited Apr 18, 2014 12:17 PM ET.

The threshhold for warm foot comfortworks out to 14 BtuH/sq. ft.
by Kent Jeffery

Yup, Martin, I've seen the writing on the wall. I really, really thought I wanted a hydronic in-floor heating system, but am fairly certain that it will not meet my expectations, having gone through the exercise in my prior post. Thus, it has been abandoned.

Next sacred cow to be slaughtered? HVAC system that I used to feel I really,really wanted?

Geothermal ground source heat pump.

Apr 18, 2014 12:25 PM ET

Edited Apr 18, 2014 12:26 PM ET.

Response to Kent Jeffery
by Martin Holladay

So, it's time to 'fess up: how high was your contractor's bid?

Some people get a $20,000 bid for a ground-source heat pump system. Other bids are closer to $40,000. Where did your contractor's dart land?

Apr 18, 2014 12:45 PM ET

Err... um.... well....
by Kent Jeffery

I solicited two bids.

First is for 3 zone forced air geothermal with nat'l gas back-up, duct work, desuperheater for DHW, loop field and HRV. That bid is $39,000.

Second is incremental bid, adding hydronic heat throughout most of the house, but retaining forced air in the great room (hydronics w/ hardwood floor debate, anyone?) and second floor. That bid came in at $61,000.

Now, you can see why I am more than happy to have the debate about the relative merit of hydronic in-floor heating settled!

Apr 18, 2014 12:53 PM ET

Please, follow me and let's explore this further
by Kent Jeffery

I don't want to hijack this excellent post discussion for personal use, so if any readers are interested in following this line of discussion, please follow me over to my other post directly concerning the GSHP question:

It's engineered to amaze!

Apr 18, 2014 12:53 PM ET

Edited Apr 18, 2014 12:54 PM ET.

Reality or Placebo? Who cares!
by Armando Cobo

I know that walking on a tight house (1ACH50), 2x6 wall with R21 cellulose, 1" rigid foam outsulation, with 2" rigid foam under the slab in CZ4, can give you warm toes and comfortable temps, and so dozens of my clients. At some point, personal experience trumps ΔTs, π, √, or Xⁿ... Do I care if its reality or placebo? NO!

Apr 18, 2014 1:04 PM ET

by Malcolm Taylor

Perhaps oddly, every client I have had who wanted in floor heating had never experienced it. They just liked the idea.

Apr 18, 2014 1:04 PM ET

In my experience,
by Armando Cobo

When it comes to Geothermal systems, I’ve done many cost estimates and TRUE energy analysis on high-performing, well insulated, super-sealed houses over the last 20 years to know that no Geothermal system can return the investment on houses from 1K to 8K square feet where the heating and cooling loads are typically 1/3-1/4 of a code house. All these houses are in CZ2-CZ5.

Apr 18, 2014 1:28 PM ET

by Jeff Carroll

Thanks Martin. My thoughts exactly on getting the foundation right, so very glad to have your advice on this. Have a good one.

Apr 18, 2014 1:33 PM ET

by Kent Jeffery

Yeah, I know where you're coming from. I have in-floor hydronic heating in my garage/shop that I erected last year. Heated with Triangle Tube natural gas condensing boiler. AND I LOVE IT. But, it is built with 2x6 walls, with 2" XPS on the exterior, no spray foam (but I caulked the piss out of it), and has 3 large overhead doors. My energy use this past winter was just over 300 therms, or 30 million Btu's. The boiler ran for 734 hours, so delivered roughly 40,800 BtuH. The sguare footage of my garage/shop is 1800 ft^2, so heat flux works out to 22.7 BtuH/sf. Am I surprised that I love it? No, because the flux exceeds the 14 BtuH/sf threshold I calculated above. I worry, however, that when I finish the insulation, add my vapor barrier, and drywall the place, I might now hardly ever see the boiler fire up, and I might find it cold lying on the floor changing oil.

Apr 18, 2014 2:10 PM ET

Edited Apr 18, 2014 2:11 PM ET.

sub-slab insulation
by Paul Eldrenkamp

A few years ago we worked on the home of a high-level researcher for a major environmental organization. The home was (and is) heated with an oil-fired boiler. Before we had been hired, the owners had built a slab-on-grade family room addition with hydronic tubing embedded in the slab, in no small part because the contractor had touted the superior efficiency of radiant floor heat. Unfortunately, the contractor had also used foil-faced bubble wrap insulation under the slab because he had been told by his supplier that it had an R-value of 10. So the homeowners ended up (in an oil-heated house) with 200 sq. ft. of in-floor heating system directly coupled to the ground. Add some furnishings and area rugs and it's quite possible that more heat ends up going into the ground than into the room. But that floor sure stays toasty.

Apr 18, 2014 2:23 PM ET

Response to Paul Eldrenkamp
by Martin Holladay

We all know that bad insulation keeps a radiant floor warm, supplying the "warm toes" effect that homeowners desire. Clearly, the combination of foil-faced bubble wrap and in-floor hydronic heating is a marriage made in heaven.... unless, of course, the homeowners want low energy bills.

If any readers want to more information on bubble wrap, here is the link: Stay Away from Foil-Faced Bubble Wrap.

Apr 18, 2014 2:29 PM ET

Pure Radiant Heating
by Ron Keagle

Warm toes touching a warm floor is does not involve radiation. It is conduction heating like a hot bath. But, I am intrigued by radiant heat for a reason I have not heard much discussion about. I am not sure what to make of it.

Warm air heats a person purely by conduction. A warm surface not touching a person heats the person by radiation.

People talk about a certain air temperature needed to feel comfortable. However, this is relative, depending on the radiant loss to colder objects not in contact with the person. Say the walls and the air are both at 72 degrees and a person feels comfortable. If you drop the walls to 40 degrees and leave the air at 72 degrees, that person will feel much colder. This is counterintuitive to many people because they basically perceive “heat” only as warm air delivering warmth by conduction.

The body loses heat by conduction to the colder air that touches it; and it loses heat by radiation to the colder surfaces facing it. The two mechanisms are independent. You could freeze to death in 100 degree air if the radiant loss were great enough.

What this means is that a person could live in a house that felt comfortable with an unusually low air temperature if the walls, ceiling, and floors were warmed directly and became radiant emitters. I don’t know what to make of this, but I have read accounts about how the lower air temperature compensated by direct radiant heat is healthier and more comfortable than the equivalent heat perception delivered mostly by warm air conduction as opposed to radiation.

I have no idea of how the thermal dynamics of that would shake out, or whether there would be some degree of extra efficiency by heating bodies primarily with radiant emitters rather than by contact with warm air. But I see these issues as the real mysteries of radiant heating.

Apr 18, 2014 4:06 PM ET

by Malcolm Taylor

Maybe Dana will comment. He has advocated using Radiant Cove Heaters, as an alternative to baseboards for exactly the reasons you suggest.

Apr 18, 2014 4:32 PM ET

Low water temps & combustion efficiency.
by Dana Dorsett

Don't underestimate the improvement in raw combustion efficiency when using gas or propane fuels when it comes to radiant heating (or any hydronic heat emitter.) A condensing boiler delivering 160F water with 140F return has raw combustion efficiency no better than 86-87%, whereas operating with return water temps in the 90F range will be delivering 98% efficiency and a fuel savings in double digits, provided the thing is sized correctly for the loads.

Alex's notion that radiant floor temps “might reduce heat loss into unconditioned space if boiler and piping are located in an unheated basement, but experts … suggest that the savings would be very small at best — especially because of the additional electricity consumption to operate pumps for long hours.” is missing some of the fundamentals- and is only relevant to the non-condensing cases. The widely accepted rule of thumb regarding distribution & standby losses for non-condensing equipment is that there is about a 2-3% fuel savings for every 10F one can reduce the average boiler & distribution water temp. (Heat-purging boiler controls on high-mass non-condensing boilers can provide in the neighborhood 10-15% net saving this way.) But distribution loss & standby jacket losses are the LEAST of it when working with condensing equipment. ECM drive pumps and outdoor reset control (now standard on condensing boilers) can squeak quite a bit of efficiency out of it, provided you have the radiation that delivers at low water temperatures, despite long pumping cycles.

Whether it's "worth it" to spend the money on radiant floors & ceilings rather than higher performance building envelopes or heat pumps & photovoltaics ( PV ) is a personal judgement call. From a purely financial point of view the heat pumps & PV are already winning (and by a bigger margin every year, with the continuing steep decline in cost of grid tied PV), but even a 71F floor is perceptibly more comfortable to bare feet than a 69F floor.

But going to super-performance sub-U0.20 windows can raise the average radiant temperature of a room at about the same measure asU0.28 windows w/radiant floors in an otherwise high-R house, and in some cases would provide an equal or better uptick in human comfort.

I'm personally in the camp of spending the real money on the building envelope, not the heating systems and consider ductless heat pumps perfectly acceptable. But that's not to say people who have the money "shouldn't" be spending it on radiant floors in high-R houses for the hint of extra comfort margin t provides for the 10% of all winter hours when they would notice it, any more than I'd tell them not to spend it on granite countertops. But if they are looking at it on purely economic or carbon-footprint basis there are almost always better ways to spend the money.

Apr 18, 2014 6:50 PM ET

Edited Apr 18, 2014 6:51 PM ET.

I can tell all of you even
by aj builder, Upstate NY Zone 6a

I can tell all of you even low temperature radiant floors feel very nice compared to cold cellar slabs!!!!! This is fact kids. Fact.

Radiant floors are a luxury and if you can afford them they are worthy.

No armchair pondering.... Twenty five years of feet, near the heat.

Apr 18, 2014 9:05 PM ET

Let's say my house is
by Morgan Audetat

Let's say my house is ultra-tight, and super-insulated.

These are facts? These are nearly meaningless expressions.

Apr 18, 2014 9:36 PM ET

Edited Apr 18, 2014 10:13 PM ET.

1st Time
by Allan Edwards

I've never seen radiant heated floors in my market (Houston), but I built a custom house couple of years ago and the client requested this for their master bath stone floor. We purchased from a company called Warmly Yours, installed our 1st radiant heated floor (heat strips). Last year I asked the client how they liked it, they responded very positively and said they use it year round, even in the summer when temps reach 100's and the AC's are cranked down to 68 degrees. I cringed a bit.

Apr 19, 2014 6:48 AM ET

Response to A.J. Builder
by Martin Holladay

You wrote, "I can tell all of you even low temperature radiant floors feel very nice compared to cold cellar slabs."

I don't doubt that. In fact, the opening sentences of my article read, "So-called radiant floors have an excellent reputation. Many customers report that this type of heating system is comfortable and quiet."

In some cases, people compare a heated basement floor with an uninsulated cold concrete floor. Of course the heated floor is more comfortable. There is a third option, however: an insulated concrete floor (with at least 2 inches of horizontal sub-slab insulation) without any hydronic tubing. Although this type of floor won't be quite as warm as a hydronically heated floor, it will be much more comfortable than the floor A.J. is talking about: "a cold cellar slab."

For those who can afford a hydronic system with in-floor tubing, go right ahead and install such a system. Others may want to consider alternative approaches that depend on high levels of insulation to achieve comfort.

Apr 19, 2014 8:26 AM ET

Radiant Floor and Passive Solar
by Daniel Beideck

We completed building our super-insulated, air tight, passive solar house in Vermont 3 years ago that also included a radiant floor in the slab. While I don't have data to say how much passive solar heat gain is wasted due to the radiant floor, my educated guess would be that it is very small. I wish a sunny day in winter would give us a 20F change in temp. However, the actual change in temp to the slab, for which I do have direct measurements, is more like 4 or 5F under the best circumstances. I don't believe that a radiant heated floor given that small of a temp swing from passive solar heating is going to be much of an issue.

Martin makes some excellent points in this article. It's important that everyone knows what the reality of the situation is going to be when deciding on what heating system is to be used. I'm glad that I didn't have the misconception that a radiant floor was going to result in "warm toes" and no one should install a radiant floor with those expectations. However, I don't believe a radiant floor and passive solar can't work together under the right circumstances. In our case, we were able to do much of the labor involved with a radiant floor ourselves and same some money that way. That was not going to be the case with some of the alternatives.

Apr 19, 2014 11:21 AM ET

The weather
by Malcolm Taylor

At least up here in Canada the weather has provided a real challenge for systems like in-floor heat as sudden temperature swings of 20 C were not uncommon. Conversely, I find a constantly maintained temperature over 24 hours uncomfortable, preferring to keep the house quite a bit colder at night. Both situations provide a real challenge for a system with such a slow response time.

Apr 19, 2014 11:37 AM ET

Question for Daniel
by Kent Jeffery

You mention that you have measured slab temperatures? What is the "typical" temp of your slab for a given thermostat setting? Have you calculated your heat flux from the slab? I'd be very interested in some real world numbers.

Apr 19, 2014 6:36 PM ET

by aj builder, Upstate NY Zone 6a

Wow. That is the first thought you would have walking into a basement even with the tstat set low at 60 while away. The basement I go in also has ICF walls. Amazing difference folks. Amazing. And to heat just a slab in the cellar is not tons of money in materials. This house by the way on the coldest day with my T gun is all the same temperature from the slab to the ridge 34' above. It was rigid foam insulated on the inside of framing (cathedral ceiling) and then T&G.

Good build, good performance, reasonable costs.... A PGH if you will.

Apr 20, 2014 7:03 AM ET

response to Kent
by Daniel Beideck

My tstat is run from my slab temp sensor. I have found that the air temp in my tight, well insulated house is very close to the slab temp. The slab temp fluctuates less than the air temp and was advised to use that to run the tstat.

Apr 20, 2014 7:35 PM ET

Edited Apr 20, 2014 8:05 PM ET.

Radiant stuff
by Robert Bean

Why are radiant floors defined as radiant systems?

The heat transfer coefficient from a radiant floor is identified in ASHRAE, REHVA, CIBSE and other engineering manuals as a range between 1.63 Btu/hr/sf F and 1.94 Btu/hr/sf F. As stated , in Section 6, of the 2012 ASHRAE Handbook—HVAC Systems and Equipment, “A temperature controlled surface is called a radiant panel if 50% or more of the design heat transfer on the temperature-controlled surface takes place by thermal radiation.” The section goes on to demonstrate how and why radiant floors, walls and ceilings meet this definition. For literature support of the testing done to derive the heat transfer co-efficients published in the handbooks, see the ASHRAE, REHVA, CIBSE et al research archives.

Do radiant floors heat people?

At the long wave lengths typical of temperatures measured from skin, clothing and infloor heating, the floor does not strictly speaking “heat” the person. Rather the resulting increase in the mean radiant temperature reduces the differential temperature between the occupant’s clothes and exposed skin to the enclosure thus reducing heat transfer from the body…i.e. it’s not so much the heat you are absorbing as it is the heat you are retaining. It should also not be ignored that the air temperature in a space conditioned with radiant floors can never exceed the source temperature of the floor (exception to air heated at surfaces heated by other sources such as short wave solar radiation). This has an effect on air buoyancy and reduced stratification...from a convection perspective...its an effective way of having tepid air temperatures without causing a cooling draft.

How much energy is released from a clothed body via radiation, convection and evaporation?

For a split between latent (evaporation) and sensible (radiant and convection) heat transfer from a clothed occupant see Table 1 Representative Rates at Which Heat and Moisture Are Given Off by Human Beings in Different States of Activity, Section 18.4 in the 2013 ASHRAE Handbook—Fundamentals. If you can’t get your hands on the book - the radiant transfer from a clothed person represents 58% to 60% of the total sensible transfer at low air velocity and at low to moderate metabolic activities. There’s more to it than that but rather than getting into a long dissertation I suggest those interested study the materials available online.

What is the ideal floor temperature?

Thermal comfort is complex and subjective and you may as well throw in circumstantial and relative. You can't take one element of thermal comfort and make it a proxy for all the others as has been done mistakenly with air temperature in many publications and educational curriculum...and you can’t talk about any of the 10 key factors of thermal comfort in a binary language using yes/no, stop/go logic…it’s all fuzzy and gray with tones of maybe…as it relates to floors, there is exhaustive research in ranges of acceptable temperatures, and those published in ASHRAE 55 Thermal Environmental Conditions for Human Occupancy define this between 66F and 84F with the least dissatisfied for most healthy people at a nominal 75F. This is for people wearing less than 0.7 clo and having a less than 1.3 met rate and wearing “normal footwear”.

For those wearing socks or having bare feet it becomes more complicated since the flooring characteristics becomes important. For example, research shows textiles and some wood types of flooring are more comfortable at cooler temperatures (74F+/- ) than masonry floors (80F+/-).

You can study this in Fanger’s 1970 thesis and publication titled, “Thermal Comfort: Analysis and Applications in Environmental Engineering”, Olesen’s 1977 ASHRAE paper, “Thermal Comfort Requirements for Floors Occupied by People with Bare Feet”, or Chapter 9 of the 2013 ASHRAE Fundamentals Handbook.

As it relates to floor heating in terrific buildings; using 75F as the “ideal” example of a surface temperature for those wearing normal footwear (slippers for example), and a space temperature of 72F, and a nominal heat transfer coefficient (HTC) of 1.80 Btu/hr/sf F, the floor could produce a flux of approximately 5 Btu/hr/sf or well within the definition of a high performance building in a heating dominant climate.

For bare feet, as demonstrated by Olesen (1982), the measured skin temperature of low met rate occupants, the skin temperature of the feet at ambient conditions approaching 72F is approximately 77F. So relative to an 84F floor required in a traditional building with say 30 Btu/hr/sf flux, will you have the "ooh-ah" warm floor effect with a 75F floor at 5 Btu/hr/sf? Not likely - but in both cases the temperature of the floor and the perception will still be within comfort ranges provided all other factors have been accounted for…

You can play around with the variables using this simple formula:

Surface temp = (Flux / HTC) + Room Temp

Regarding “energy savings”…

There is no shortage of anecdotal evidence to “energy savings” with radiant floor heating…we’ve found everything from 10% to 100%. Within our exhaustive library of research papers none can support any thermal heating claim in a side by side residential comparison. They may exist but I’ve not read them. There are numerous modelling papers showing conservation of energy with radiant systems but these must be read with care since they are very much circumstantial. There is at least one DOE side by side residential project which demonstrated conservation in electrical energy due to the physics of circulators over blowers. At the end of the day when properly design and installed, radiant as a low temperature heating system and high temperature cooling system enables maximum efficiency from the heating and cooling plant...but this ability is not exclusive to radiant floors.

I’ve only touched on some of the discussions in this post – there are many others that need to be fleshed out– perhaps some of my ASHRAE colleagues will come forth and share their time as well.

Apr 21, 2014 12:48 AM ET

by Malcolm Taylor

What an informative and useful addition to the discussion. Thanks.

Apr 21, 2014 8:20 AM ET

Edited Apr 21, 2014 8:21 AM ET.

Response to Robert Bean
by Martin Holladay

Thanks very much for your detailed comments, and for providing technical information to help bracket comfort issues as well as readers' questions about flooring temperatures and BTU/h output questions. The information you provide is very useful.

I don't see issues of disagreement. While ASHRAE has developed its own definition of a "radiant panel," ASHRAE does not yet have the ability to prevent conduction and convection from playing a role in heat transfer in homes with in-floor hydronic tubing.

And I appreciate your explanation about the way that radiation from a person's skin to cold surfaces in a room -- especially windows, of course -- affects comfort, and that this mechanism is extremely important. While in-floor hydronic tubing can raise the temperature of surfaces in a room, thereby improving comfort, good glazing can also have the same effect. In any case, I mostly agree with your statement that “the floor does not strictly speaking ‘heat’ the person.” The exception, of course, is the famous one depicted in all the ads -- conduction from a warm floor to someone's bare feet.

I also appreciate your summary of the "warm toes" question; namely, "So relative to an 84°F floor required in a traditional building with say 30 Btu/hr/sf flux, will you have the ‘ooh-ah’ warm floor effect with a 75°F floor at 5 Btu/hr/sf? Not likely - but in both cases the temperature of the floor and the perception will still be within comfort ranges provided all other factors have been accounted for." I agree. Needless to say, many people with other types of heating systems also manage to adjust their thermostats and achieve a comfort range that works very well with bare feet.

I am also grateful for your confirmation of another point raised in the article: "There is no shortage of anecdotal evidence to ‘energy savings’ with radiant floor heating. ... Within our exhaustive library of research papers, none can support any thermal heating [energy savings] claim in a side-by-side residential comparison. They may exist but I’ve not read them."

Apr 21, 2014 11:33 AM ET

Mean Radiant Temperatures not addressed
by Mark Eatherton

Excellent conversation, and having been in this industry for over 35 years, I confess that NO ONE likes to hear people call their baby UGLY. :-)

Let's start with trying to define a definition of true "COMFORT". I am sure that my interpretation of comfort is different than any other person here, because it is a subjective term. Comfortable compared to what? Living in a log cabin with poor chinking between the logs?

My definition, which I have developed over my 38 year career is fairly simple on the outside. Simply stated, being comfortable means that you are not aware of your surroundings. And it has to do with more than just the delivery of warmth or coolth. (full disclosure, Robert Bean taught me this MANY years ago, and it has served me well).

Drilling further down into my definition, you are not hot, nor are you cold. You are not over humidified nor are you under humidified. Ideally, you do not hear your comfort being delivered. Simply stated, you are not thinking about it, and if you are, then you are not comfortable. Now, all we have to do is develop a ANSI standard around this, and come up with a metric of 1-10 whereby the consumers can actually RATE their comfort feelings and their delivery system. Any volunteers?

Back to the ugly baby. While I don't disagree with Martins musings, I must say that he has not identified the one factor of comfort that has more influence over truly comfortable people. That one factor is the Mean Radiant Temperature. Simply stated, it is the temperature of those objects surrounding your body. As RB and others pointed out, and Martin confirmed, sitting next to a large glass window wall is NOT going to be comfortable under ANY reasonable air temperature conditions. It's the nature of the beast. Heat flows from hot towards cold, and our bodies can sense this much better than we perceive. And the only reasonable way to control the MRT is by influencing the surface temperatures using radiant energy, either heat OR cooling. Yes virginia, we can do radiant cooling, even in a residential setting, but that is a whole 'nother topic for a later time.

So, who built this ugly baby (radiant floor heating systems)? I like to give credit to Richard Trethewey for showing up on This Old House with a 1,000 foot roll of PEX on his shoulders and announcing to the house wives of America that they were going to do a radiant floor heating system. I consider this the kickoff of the radiant floor revolution. And boy, what a revolution it has been. The organization which i represent (Radiant Professionals Alliance) has been at it for 20 years, and there were other organizations around promoting it before that. It has the momentum of a huge freight train, and once people experience warm floors, they want it and in most cases, cost is not a consideration. After all, how can we put a price on true human comfort? Heck, we cant even come up with a metric to measure comfort… But I digress.

We, the American comfort industry have put so much effort in to promoting warm floors, that we have lost sight of the end product we are charged with delivering, that product being COMFORT. In many cases, it is NOT necessary to install 1 linear foot of tubing per square foot of living space, but we sold the consumer on the concept of "warm floors", and by golly, if their foot hits a spot on the floor that is not as warm as the rest of the space, a light bulb goes off over their head, and they think, "What did I pay $XX,XXX.00 for, and why isn't my foot landing on a warn section of floor?" You see, WE are the ones who screwed up. What the heck did we do BEFORE we had warm floors?

Hydronically, we did large surface area upright cast iron radiators, radiant ceilings, radiant walls and hot water base board CONVECTORS. (I intentionally all cap's the convectors because a very minor portion of a base board finned tube convector is delivered in the form of radiant, most of its output is truly convective energy, and this is an important part of the overall discussion.)

What I am trying to convey here is multi fold. First and most importantly, anyone who leaves MRT out of the discussion of comfort doesn't see the whole picture, and are seeing it as "heat" only. MRT drives the bus of human comfort, both as it pertains to heating AND cooling. Simple fact of life and nature.

Secondly, you can NOT manipulate the MRT in a given space using forced air, without creating even greater conditions of human discomfort, causing our bodies to go into the protective cooling mode, which I refer to as evapotranspiration. My wife refers to it as sweating… Another fact of nature that can't be changed.

So, does this mean we should throw the ugly baby out with the wash water? Heck no. I say we re-use the baby's bath water, and we redirect the radiant baby to surfaces OTHER than the floor only concept.

We need to re-educate people (homeowners, architects, designers, contractors, blog writers and a whole industry) that there is more than one way to deliver excellent human comfort by manipulating the MRT within a given space, and it can be done without having to cover every square foot of the floor with one linear foot of tubing. I strongly believe that radiant floors DO have their application, but they should be limited to those areas where people will likely be wet and poorly clothed (necked if you will), and that place is in bathrooms and walk in closets. For the balance of the areas in a given home, I can manipulate the MRT in a given space using radiant ceilings, radiant walls, sexy panel radiators, radiant towel warmers, and even an occasional radiant bath tub. We DON"T have to use floors only.

We also need to manage the consumers expectations. If we train them to expect excellent radiant COMFORT, regardless of the emitting surface, then they don't expect their floors to be perfectly warm all the time.

I own three houses, and all three are radiantly heated. I have radiant floors, walls ceilings, even radiant glass windows. Personally, I LOVE my radiant floors, but in reality, I love my radiant ceilings and walls and windows even better. It costs less to install, and it still affects the MRT, which drives the bus of human comfort. I am going to keep saying this until the world understands it...

AND, we can deliver an excellent radiant cooling comfort condition using the same surfaces that deliver warmth. WIll it require duct work, in some cases yes, but it is significantly less duct work and associated cost, noise etc because we are handling the majority of the load (sensible) with our radiant system.

This is going to require a HUGE sea change in the way we've been doing business over the past 20 + years, and its not going to go down quietly. I believe that our industry must embrace the "Good, better, best and out and out FANTASTIC" delivery of comfort. Radiant floors are FANTASTIC, and radiant ceilings, walls and panel radiators fall into the Better and Best categories, with hot water baseboard playing the roles of GOOD.

And it doesn't have to be expensive, because we are only putting in only the amount NECESSARY to counter the heating/cooling loads of the space.

For some of the best education a person can find on the internet, I ALWAYS suggest that people go by Roberts web site at Play around with is Comfort Calculator that shows the effects on human comfort for a give scenario, and you will have a much better understanding of what MRT does to human comfort. It drives the bus of human comfort…

In a conversation with a PHIUS builder, he told me his load calcs came in at "5 btu's per square foot." I said per hour? he answered, "No, per year…" I said that he could heat it with the bodies in the house, to which he replied, "True, but I can't keep them there all the time, so I need a back up plan…" The "back up plan" is a perimeter radiant ceiling delivery system that runs around the exterior of the rooms, typically 2 foot intrusion into the space. It delivers the heating and cooling needs of the building here in Colorado (extremely dry air, low latent loads). Required ACH requirements are being done with an ERV.

By the way, anecdotal information here, my radiant ceilings actually make my floors "neutral" to the touch, and when at design condition, the floor actually does feel warm to me. And it FEELS wonderful, like standing in the sun on a cool fall day...

As it pertains to cost of operation, as others have noted, that is a real wild card, and even under the most controlled of circumstances, is hard to judge. But, the Canadians did a long term study, and took all of the variables into consideration, and they came up with a 12 to 15% reduction in energy consumption of hydronic radiant versus gas forced air. NOt the 30% typically quoted, but still 12 to 15% better than the competition with EXCELLENT comfort conditions. And it works excellently with ALL of the new state of the (changing) art "alternative and Green" technologies.

It is time to take our focus off of "Warm Floors" and put them back onto the delivery of excellent radiant comfort, regardless of the energy emitter. The articles and experiences are out there people, you just need to do some research…

Thank you Martin for the opportunity to express myself.


Mark Eatherton
Executive Director,
Radiant Professionals Alliance

Apr 21, 2014 11:50 AM ET

Edited Apr 21, 2014 11:53 AM ET.

Response to Mark Eatherton
by Martin Holladay

Your reference to ugly babies confused me. Are we talking about the babies in the radiant floor heating advertisements? Or are we talking about your favorite method of space heat delivery? In any case, I think that you're the first person to call either baby "ugly."

You wrote, "As Robert Bean and others pointed out, and Martin confirmed, sitting next to a large glass window wall is NOT going to be comfortable under ANY reasonable air temperature conditions. ... Heat flows from hot towards cold, and our bodies can sense this much better than we perceive. And the only reasonable way to control the Mean Radiant Temperature is by influencing the surface temperatures using radiant energy, either heat OR cooling."

I disagree.

Assuming that we are talking about relatively new buildings that comply with the building code -- in other words, that are adequately insulated and have been built with attention to air sealing -- this issue (mean radiant temperature) is mostly about the temperature of window glass. Your drywall is basically at the same temperature as the indoor air.

So, there are two ways to influence the mean radiant temperature of your window glass. One way -- not the only way -- is to install tubing in your floor and run hot water through it.

The other way is to buy windows with better glazing. All other factors being equal, the innermost pane of a triple-glazed window will be at a higher temperature during the winter than the innermost pane of a double-glazed window.

Apr 21, 2014 12:35 PM ET

Reigning in exaggerations
by Martin Holladay

Mark Eatherton,
It's hard to pin down a definition of comfort, although I appreciate your attempt to do so.

You describe a comfortable person as someone who is "not aware of his surroundings. ... You are not hot, nor are you cold. You are not over-humidified nor are you under-humidified. ... Simply stated, you are not thinking about it." This person may be comfortable. It's also possible that this person is dead.

In other words, some people don't mind feeling alive -- knowing when they are a little warm or a little cold is not a bad thing. When I go outdoors in perfect weather, I can assure you, I'm thinking about it.

The corollary to my analysis is that Americans are so obsessed with comfort that we have raised our expectations to unreasonable levels -- to levels that are not necessarily desirable. But I digress.

You wrote, "You can NOT manipulate the mean radiant temperature in a given space using forced air, without creating even greater conditions of human discomfort, causing our bodies to go into the protective cooling mode, which I refer to as evapotranspiration." This is an exaggeration.

Not all forced-air systems result in discomfort. I assure you that a well designed, well installed forced air system will make the occupants comfortable.

If you insist that it's impossible for a house with forced-air HVAC to be comfortable, I will simply conclude that you exaggerate and are untrustworthy. Since I doubt that is the case, I hope that you can dial back your exaggerations and stick with a technical analysis of the topics we are discussing.

Apr 21, 2014 12:37 PM ET

Edited Apr 21, 2014 12:44 PM ET.

Martin, you need to reread
by aj builder, Upstate NY Zone 6a

Martin, you need to reread Mark. Being an installer of radiant over twenty years ago, and understanding all your great blogs Martin I get Mark totally. Mark certainly understands the benefit of warmer windows.
Mark, we need you here at GBA. Post and help us all out with your radiant expertise. I for one will read every word and seek out your advice in the future.

I think I have already started to understand how using less but properly placed radiant is the way forward with a PGH.

I would like to hear more details as to systems used in highly insulated homes including what you call warm or radiant windows.

Apr 21, 2014 1:14 PM ET

The ugly baby I am referring to ...
by Mark Eatherton

Is the use of radiant floor heating in highly efficient envelopes. You didn't actually call my baby ugly, but pointed out some obvious situations whereby radiant floors do not make sense, and I agree with most of your assertions in that (radiant floors) regards

Your assertions in regards to window glazings is true for the most part. A triple pane window is significantly warmer on the inner surface than is a double, but unless it is filled with a gas, is still going to experience mid glazing temperatures less than the 85 degree F skin surface temperatures, and will still result in less human comfort when in close proximity. Having been exposed to triple panes at design condition, I can tell you from personal experience that its not as comfy as the marketers would like you to believe it can be. I didn't have my IR camera with me, but my body told me so, and I believe that sensor first and foremost.

Then there's the whole issue about how long the gas remains between the panes with a gas permeable seal, and how do we test and verify that and blah blah blah.

The third way of influencing glazing temperatures, which you and your readers may not be familiar with is electric radiant windows. Google 'electric radiant windows' for more information on that topic. They've been in Europe for over 20 years. Although a person COULD literally heat their dwelling with radiant windows, it is NOT recommended. I keep mine (DPIGU's) in a "neutral" state, roughly around 70 to 75 degrees F depending.. I can hear rumblings in the back ground of people thinking "Why the heck would you want to put heat into an outside opening?"

Guess what, we've been placing heating elements in front of cold windows since the invention of hot water heat. And the reason we have been doing that is because the window represents the greatest (well in most cases) heat loss factor of a given envelope. The thought being, if we place a warm curtain of air between the glass and our bodies, our bodies can't "feel" the cold heat robbing condition of the cold glazing. True so long as the heating system is on, but as soon as it turns off, your body can "see" the cold surface again, and discomfort sets in.

But this conversation isn't about heated windows. Its about using radiant energy to control the comfort conditions in a given application without breaking the bank, making it compatible with solar thermal, GSHP, ASHP, woody biomass, what ever makes fluid warm. And yes, in a well built dwelling, the MRT will be higher than a non well built dwelling, but it still is the primary factor in determining excellent human comfort (or discomfort in some cases), and it can ONLY be influenced through the use/manipulation of MRT. We can argue over that until the cows come home, but it stands the test of time and mother Nature.

Thanks for continuing to educate people on this and other energy related matters.


Apr 21, 2014 1:46 PM ET

Response to Mark Eatherton
by Martin Holladay

Yes, I know about electric radiant windows -- marketed by Engineered Glass Products as "Hot Glass."

As one of the company's spokesmen, Peter Gerhardinger, has written, "Experience has shown that typical power usage [of the company's electric windows] will range from 15 to 30 watts per sq. ft., with a peak not to exceed 50 watts per square foot. One consideration in determining power usage is the desired response time. The higher the power rating, the faster the glass will warm up."

Needless to say, heat flow from a window to the outdoors is a function of delta-T. The hotter the glass, the faster the heat loss. I strongly advise readers of GBA to avoid this product.

Apr 21, 2014 8:41 PM ET

by Mark Eatherton

I know that the format of your blog/chat is probably not your design, but boy, what a pain. I want to thank AJ Builder for the compliment, and I also want to respond to your earlier response to my comment, so kludgy and all, here goes.

As AJ said, re-read my statement. I didn't say that forced air systems created discomfort. I said they they can not raise the MRT without creating conditions of discomfort. And this is a true statement. In order for forced air to significantly affect the MRT within a given space, you would have to run the forced air system at a temperature that would cause most (live) beings to break out into a sweat. I have lived with forced air, hot water baseboard and radiant heat, and I can tell you (along with millions of others who have experienced the comfort associated with radiant heating) that radiant is hands down more comfortable than the other alternatives. Forced air does get the job done, but as has been proven many times over, it is not as comfortable as a good hydronic radiant comfort delivery system. If you are comfy with forced air, that's great. But I guarantee you I am much more comfortable with hydronic radiant than can be achieved with a conventional forced air delivery system.

If it weren't there would be a WHOLE lot of people (hydronic heating/cooling contractors) looking for jobs in the other fields.

I am not sure why it is that you refuse to accept this radiant comfort fact, and really, it doesn't matter. As for being dead, not yet. Still alive and kicking, and keeping people comfortable and saving energy, as I have been for the last 30+ years.

As it pertains to radiant windows, as someone who has worked and lived and studied these gems for many years now, I can tell you that the person you quoted (Peter G.) is really not considered a reliable industry spokes person/ information source for the radiant window industry. To the best of my knowledge, they are no longer making radiant windows. (problems with their fritted/sputtered buss bars delaminating and losing continuity to the low e metallic coating.) And I would be more than glad to openly debate you on this subject, but I don't want to clutter up this thread. I can provide you with some studies and statistics that might convince you otherwise as it pertains to the efficiency and overall net effect of radiant windows. Just like anything else in life, they are not for everyone or every job, but they do have their applications. Having lived with 14 of them for the last 5 years in two of my homes, I can tell you that they are extremely comfortable. Imagine pumping sunshine from the sun (PV panels) through wires into areas where the sun never shines (North facing windows). The numbers Peter is quoting are numbers that were extrapolated from the reach in freezer door industry standards, and are MUCH higher than necessary to maintain the glass in a thermally opaque, or neutral condition. In any case, if you are open to it, I am more than willing to discuss, and debate with you on any subject relating to the radiant world.

Thanks for continuing the conversation and allowing me to contribute to your forum. I am not here to blow smoke up peoples skirts. I am here to help educate.


Apr 21, 2014 10:09 PM ET

Edited Apr 21, 2014 10:10 PM ET.

The vortex...
by Robert Bean

I can feel I’m getting sucked into the vortex…my last post…have work to do [:@).

Re: MRT, window and drywall temperatures and comfort

Part II: Mean radiant temperature (MRT) integrated with dry bulb (tdb) obtains in thermal comfort analysis the “operative temperature” (top) or what people actually sense.

Consider top as the fulcrum on a teeter totter where on one side you have tdb and the other MRT. To maintain an acceptable top in the presence of a dropping tdb the MRT must go up; or you can say in the rising of a tdb the MRT must go down. This is in part, the theoretical basis for the lower thermostat setting argument which is true in theory practiced by some but not all (what else is new?).

When does the temperature of the drywall equal the air temperature?

ASHRAE Handbooks do not address this specifically from a comfort perspective but the ASHRAE 55 Standard does provide a litmus test for when the MRT can be assumed to be approximately equal to the air temperature. Ergo if using ASHRAE Standard 55, the window has to be incorporated into the analysis. Before defining when the tdb can be assumed to equal the MRT for a designed operative temperature, readers unfamiliar with inside surface temperatures of glass should have a first look at Table 2, Indoor Surface Heat Transfer Coefficients, 2009 ASHRAE Handbook of Fundamentals, Chapter 15, Fenestration; or for more detailed reviews use the LBNL Window Software.

If you can’t get access to the tools; as per Table 2 examples, under no solar load, the inside surface temperature of triple pane, ½” argon filled between panes, 0.1 E on surfaces 2 and 5 with an outside temp = 0F and an inside temperature = 70F, the inside glass temperature is ≈ 63F; for double pane, 0.1 E on surface 2, the inside surface temperature is ≈ 56F.

At approximately 72F ambient temperature, the mean skin temperature is approximately 88F, so the differential between the body and glass becomes the motive force for drawing energy away resulting in the sensing and perception of cooling. This is only part of the story since the area of the glass, exposed skin area, characteristics of the clothing, geometric orientation of the person to the glass etc. etc. etc. are all non trivial elements to be considered.

… back to….when can you assume the tdb = MRT?

The basic ASHRAE Standard 55 test is tdb = MRT if Uw < 15.8/(tin-tout)

This says if the U-value of the zone enclosure (weighted average of all surfaces including windows) is less than 15.8/dt then you can assume the air temperature is approximately equal to the mean radiant temperature. Caveat…this is a very simplified test and is under review by our SSPC 55 Committee. In my own practice, it is only used as a starting point not as a definitive conclusion.

Nevertheless, readers should reflect on the window performance and its area, the wall performance and its area plus the outside design conditions because only the right combination can deliver a yea or nay assumption for tdb = MRT.

In detailed studies of MRT, the occupant’s geometric relationship to the window is considered - as it should be for accuracy since the closer the body to the critical surface the greater its affect on the sensation and perception of comfort. This principle holds true for all radiant surfaces. See UC Berkley, Center for the Built Environment and papers on thermal comfort and windows.

For those with spare time and like to play with Excel, it’s really a good exercise to do a simplified calculation of the space MRT, using an area weighted average of the surfaces in a hypothetical space playing around with framing factors, wall and window performance and window/wall areas and watch what happens. It’s not difficult math and if it matters at all to some, a seasoned designer should be able to perform the calculation anyways to demonstrate compliance with comfort Standards.
As another exercise, take the same hypothetical space and move it around the country and watch what happens in Fairbanks or Seattle or Montreal or New York…not the same beast.

You can model this topic in 2D using the ASHRAE Thermal Comfort Tool and soon you will be able to model it in a 3D space using the ASHRAE 1383 Radiant Modeler.

How does this fit in to the topic at hand?

Radiant heating and cooling is a tool just like any other tool…in the hands of a skilled designer on the right projects with the right clients it can be an elegant way of conditioning people and spaces – and yes as Mark and others can verify it can even be low cost and simple…in the wrong hands...well…don’t we all know the rest of that story …[:@).

Best of luck with the rest of the conversation...

Apr 22, 2014 7:18 AM ET

Response to Mark Eatherton (Comment #43)
by Martin Holladay

You wrote, "Forced air does get the job done, but as has been proven many times over, it is not as comfortable as a good hydronic radiant comfort delivery system."

I suspect that our disagreement is unresolvable, but I will put forward my arguments to GBA readers and let the readers reach their own conclusions.

Many advocates of in-floor hydronic tubing compare the comfort provided by these systems with run-of-the-mill forced-air systems. As GBA has reported for years, most forced-air systems in the U.S. have oversized furnaces, undersized ductwork, leaky duct seams, and lengths of ductwork installed outdoors (in unconditioned attics). These badly designed, badly installed forced-air systems often have comfort problems.

However, a forced-air system with a properly sized furnace, properly sized ductwork, sealed duct seams, and with all ductwork installed in the conditioned space is an entirely different animal. GBA advocates that readers should install the latter type of forced-air system, not the former.

Forced air, done right, is comfortable. Mark, if you tell me that you are uncomfortable in a home with a well designed, well installed forced air system, I'm going to assume that we are talking about a princess-and-the-pea problem. A certain percentage of the population is never comfortable.

This issue is similar to the debate in Maine between Passivhaus advocates and Pretty Good House advocates. Is a properly designed, properly installed forced-air system "good enough"? Or is it too uncomfortable? I'll leave that for GBA readers to determine.

Apr 22, 2014 7:36 AM ET

Edited Apr 22, 2014 7:39 AM ET.

Response to Robert Bean (Comment #44)
by Martin Holladay

Once again, I am grateful for the technical information you have shared to help our readers bracket these problems.

Once we have finished looking at our spreadsheets, however, we have to return to our living rooms and determine the best way to stay comfortable at a reasonable price.

You noted that when the outdoor temperature is 0°F, the inner pane of triple-glazed window will be 7°F warmer than the inner pane of a double-glazed window (63°F instead of 56°F). What does this mean in real terms? For the vast majority of homeowners, the difference in comfort is significant. If you live in a cold climate, you are far more likely to be comfortable sitting next to a triple-glazed window on a cold night than sitting next to a double-glazed window -- no matter what type of heating system you have.

For most of us, this choice of a triple-glazed window tips the room from uncomfortable to comfortable. As I wrote before, the "mean radiant temperature" issue really boils down to windows. (If your skin is made uncomfortable because of radiation from your skin to the Sheetrock on your walls, something is seriously wrong with your walls. For any code-legal building, you won't have radiational cooling problems with drywall. As I said, this is a window issue.)

For a small minority of Americans and Canadians, it's possible that triple-glazed windows and a 72°F thermostat setting aren't enough. They want in-floor hydronic tubing. I say, "Go right ahead and install it if you can afford it."

However, I suspect that the uncomfortable Americans and Canadians have problems with their thermal envelopes, not their heating systems. The reason they are uncomfortable is that their homes have air leaks, insufficient insulation, or cheap windows.

Very few Americans and Canadians have experienced living in a house with a good thermal envelope, so they don't believe it's possible to be comfortable in a home unless the entire floor is heated with circulating hot water. There are simpler solutions, however.

Apr 22, 2014 9:28 AM ET

Martin you keep sliding and missing and repeating
by aj builder, Upstate NY Zone 6a

First Martin, it has been mentioned by Mark that less radiant can be installed in homes that are highly insulated and well sealed. Michael Chandler and I both advocate using less radiant in a modern highly insulated home. Stop comparing "heating the whole floor" to anyone suggesting doing so in a home that is highly insulated. NO one is advocating for that Martin.

And yes if one spends to buy a very good hot air system with variable speed expensive furnace and the best duct work then those systems are very nice. Average homes do not have these systems. We contractors that build custom homes do have customers who afford high end homes and buy much more expensive HVAC systems than most have so that they can have luxury comfort. We have around here many homes that have both radiant hydronics, GEO, hot air and more all in one home. I just passed along a job for a home where the HVAC remodel price must have been near $200,000.

Radiant has it's place. No one has to have any radiant. But even lower cost homes can enjoy a few square feet of heated tile in a master bath for a cost of $5.00 a month... if they choose to buy that EXTRA COMFORT.

I build for a endless variety of people. We talk over the plans and design, and they decide what they finally want and that is what I build.

I could see someone enjoying a heated window in a specific view window by their favorite reading lounge chair....

Martin, I do love your knowledge, don't take this post wrong... but there is room in this world for Marks and Roberts and even ajs.

Apr 22, 2014 9:38 AM ET

Response to A.J. Builder
by Martin Holladay

If your customers are happy with these systems, that's great. I have no problem with anyone installing this type of heating system if that's what they want.

For other GBA readers who are debating the pluses and minuses of different heat distribution systems, however, it's possible that the information provided in my article is useful.

Apr 22, 2014 8:56 PM ET

Martin, if your blog read
by aj builder, Upstate NY Zone 6a

Martin, if your blog read like my last post 47 it would be up to date and useful to all whether they desire radiant or not. Just harping that it is not worthy is wrong. Doing a whole home in radiant with 6" spacing is what we used to do and that is wrong. Mark and I and Chandler and you Martin say to avoid whole floor radiant with a lowly insulated home. So we all agree. My point is your blog is old news and not properly up to date.

I can't explain my points any clearer... next blog.


Apr 23, 2014 4:52 AM ET

Response to AJ
by Martin Holladay

I'm glad we all agree.

For many readers, the points made in this blog probably are, as you say, old news; but for some readers who are just beginning to think about these issues, the information may still be useful.

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