Hydronic Systems

UPDATED on April 18, 2014

Hydronic Systems Circulate Hot Water

Bird's Eye View

Hot-water systems are clean and quiet

Hydronic heating systems work by circulating hot water to wall-mounted or baseboard radiators, cast-iron radiators, or through loops of tubing embedded in the floor. Hot water can come from a variety of sources, including a gas- or oil-fired boiler, a water heater, a solar hot-water system, or a combination of these sources. Hydronic systems provide heat, but in most climates they can't supply air conditioning.

See below for:

System Types

Heat distributed via radiators or in-floor PEX tubing

Hydronic radiant floors are warmed by fluid circulating in tubing (usually made of a tough plastic called cross-linked polyethylene, or PEX). The tubing can be embedded in concrete or lightweight (gypsum) concrete, or can be installed above or below a plywood or OSB subfloor. All in-floor radiant heating systems require a thick layer of insulation directly under the heated floor.

In-slab systems. If the tubing is embedded in a slab, the concrete can be a slab on grade or a thin slab placed on top of a wood subfloor. Slab installations are very good at storing heat, although a layer of concrete installed over subflooring may require beefier framing than would otherwise be needed.

Staple-up systems. When tubing is attached below the subflooring (in a so-called “staple-up” installation), it's possible to include aluminum diffuserIn a forced-air heating/cooling system, the diffuser is a register or grille attached to ducting through which heated or air conditioned air is delivered to the living space. In a tubular skylight or an electric light fixture, the diffuser is a cover plate through which scattered light is delivered. plates to ensure that heat distribution is uniform. It's also possible to use special types of subflooring with preformed slots permitting PEX tubing to be installed between the subfloor and the finish flooring.

Because of the increased thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. , slab installations are slower to respond to thermostat changes or changes in the outdoor temperature than staple-up systems.

A baseboard radiator (or convector) consists of a finned copper tube hidden by a thin metal cover. Air warmed by the tubing and fins rises through louvers or vents while cooler air is drawn into the bottom of the radiator, setting up a convective current of air that warms the room.

Design Notes

Radiant floor systems don't get in the way

Design an efficient and effective system
Although hot water pipes generally take up less space than air ducts, hydronic systems still need to be thoughtfully laid out. Resource efficiency and energy efficiency may be your first concerns, but because many hydronic heating devices such as free standing radiators or baseboards take up living space, wall, window, and even furniture layouts need to integrate with the heating plan.

Radiant surfaces are the least obtrusive
Radiant floors are the most common invisible hydronic systems, but floors and ceilings can conceal hidden heat sources too. These installations leave the living space free of obstructions, but they can take more work to install. Communication between subcontractors is extremely important because a large portion of the system becomes inaccessible once finished floors and other surfaces are installed. There is also more chance that pipes and other components can get damaged or installed improperly during construction.

Size the system right
Rules of thumb and guess-work are not effective methods for designing heating systems. Some manufacturers, trade associations, and supply houses have software that will take the parameters of a particular home and produce sizing reports or even a diagram of radiant tube layouts. There are also helpful trade publications including a design guide published by the

Builder Tips

Radiant systems limit flooring choices

Flooring options. Not all floor coverings are appropriate for radiant-heat floor systems. Thick carpeting, for instance, lowers system efficiency, and solid plain-sawn wood flooring may warp or crack when exposed to continuous heat. Quartersawn wood and bamboo flooring are suitable, as is engineered flooring. Ceramic tile and stone are ideal.

Zoning. Hydronic baseboard heat can be zoned to provide heat to different sections of the house governed by separate thermostats. Single-circulator systems use electric solenoid valves to control separate zones. Each electric zone valve is controlled by a separate thermostat. More robust systems use a dedicated circulator for each zone.

A simple way to reduce the heat output of a baseboard radiator is to close the air damper at the top of the unit to limit convective air flow past the aluminum fins.

The Code

Prevent backflow, protect pipes from freezing

Chapter 26 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. covers the installation and specifications for hydronic piping. Table 2101.1 describes approved piping materials. Pipes must be installed so the system can be fully drained (2102.2) and potable water must be protected from backflow as described in Section 2902. Pipes must be protected from corrosive materials (2101.5) and those that run through concrete and masonry must be sleeved (2101.4). Accommodations must be made for structural settlement and natural expansion and contraction (2101.8).

Piping should be supported with non-corrosive materials at intervals found in Table 2101.9. Piping must be tested at 100 psi for at least 15 minutes prior to commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. (2101.1). Baseboard convectors should be supported and fastened independent of the hydronic piping (2102.1).

Chapter 11 of the 2006 IRC includes requirements (N1103.3) for mechanical system pipe insulation: "... system pipes capable of carrying fluids above 105°F or below 55°F shall be insulated to a minimum of R-2." The 2009 IRC updates Chaper 11 N1103.3 to read "...minimum of R-3."


Air conditioning options

Hot-water systems have no ducts, so they can heat a house but not keep it cool. (Although it is technically possible to use in-floor hydronic pipes for cooling, such systems suffer from condensation problems during humid weather.) Homeowners who want to add air conditioning to a house with hydronic heat may prefer systems that don't require installation of bulky conventional ductwork. Mini-splits and high-velocity mini-duct systems are two possibilities.


“It just doesn’t make sense to put in a $10,000 heating system to provide $100 worth of heat per year.”
—Energy consultant Marc Rosenbaum


Best for limited use

While most radiant floors are hydronic, it's also possible to heat a floor with electric resistance heat. Mats of electric wires can be installed beneath a layer of ceramic tile or stone and controlled with a dedicated thermostat.

Because of the high cost of electricity, these systems are best for providing on-demand rather than full-time heat — in a bathroom or kitchen, for example, or a guest bedroom that’s used only once in a while.

But off-peak electric rates can be an ally. When wires are set in a high-mass concrete floor and the local electric utility offers off-peak rates, the U.S. Department of Energy suggests the following routine: Heat up the floor at night (when electric rates are low). Turn the system off during the day (when rates are high), and let the residual heat in the floor carry the room.


Heat from wall and ceiling panels

Those who don't like looking at baseboard radiators may want to consider the installation of hydronic wall or ceiling panels. Most are surface-mounted, but they also can be embedded behind finish materials.

They're durable as well as out of the way. The main disadvantage is higher cost.

Some hydronic radiant panels do double duty as towel warmers. Runtal makes a line of wall-mounted hydronic panels that can operate on water temperatures as low as 100° F.


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Lakesideca Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H Under EA5 (Energy & Atmosphere), insulation prerequisite for any portion of distribution system in unconditioned space; up to 3 points for superior distribution efficiency.

NGBSNational Lakesideca Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. This program has no points associated with non-forced air heating distribution systems.


Heat, but no ducts for air conditioning

Hydronic heating systems distribute heat by pumping water or a solution of water and antifreeze through tubing made from copper or a type of plastic called cross-linked polyethylene (PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating.). Most systems rely on a boiler to heat the fluid. They typically burn fossil fuels, such as natural gas, heating oil, or propane. Dual-fuel boilers can burn either one of two fuels, cordwood or fuel oil, for example.

Other appliances are sometimes used to provide hot water, such as a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump., water heater, or solar collectors (see More About Hydronic Systems below).

Hydronic heating systems can include baseboard radiators, wall or ceiling panels, in-floor radiant tubing, fan-coil units, or a combination of two or three of these. Baseboard and free-standing radiators need relatively hot water — 160° F or higher — to operate while radiant-floor systems can heat a house with much cooler water temperatures.

Hydronic heating systems are clean and quiet
Because hydronic heat does not rely on the circulation of air, it does not move dust and other contaminants around the house. Most hydronic systems don't use fans, either, so they don't make much noise and they don't create drafts. The pressure imbalances that forced-air systems can inadvertently create are not a problem with hydronic systems.

The big drawback with a hot-water heating system is that it's limited to providing heat. Unlike a forced-air system, it doesn't have ductwork that also can be used for mechanical ventilation, air filtration, central dehumidification, or (in most climates) air conditioning. If homeowners want an air conditioning system, it will have to be added separately.

In-floor radiant heat is unobtrusive
Radiant-floor heat usually relies on loops of plastic tubing embedded in the floor. (Some in-floor radiant systems use electric resistance coils embedded in the floor.) Radiant-floor systems tend to be quite expensive to install. But they are all but invisible, operating with little or no noise and without any ducts or other obstructions to get in the way.

Hydronic systems with fin-tube baseboard radiators are much more affordable than in-floor radiant systems. Fin-tube baseboard units do most of their heat transfer via convection. Although no fan is involved, air is still the transfer medium for heat. Contemporary wall-mounted radiators are another option.


Out of sight, but more expensive

In-floor radiant heat is quiet and completely out of sight. Most people find houses with in-floor radiant systems comfortable, but installation is labor-intensive and usually expensive.

Tubing can be stapled to the bottom side of floor sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , inserted in specially grooved subflooring or cast into a slab. Heat tends to be even, but radiant floor systems don't warm a room as quickly as other types of hydronic heat, or as fast as forced-air systems.

Claims of higher efficiency
Although marketers of radiant-floor equipment sometimes claim that radiant systems require less energy to operate than hydronic baseboard or forced-air systems, there is no compelling evidence to justify the claim. A home's heat loss is determined by the characteristics of the home's shell. The heat can be produced by many means, but a 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. is Btu, regardless of the heat distribution system.

Improving performance
The best way to make an in-floor radiant heating system more efficient is to beef up the layer of insulation directly below the floor. In a cold climate, the underside of a heated slab should be insulated with a continuous layer of extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) foam between 4 inches and 6 inches thick.

More information
For more information on in-floor hydronic systems, see All About Radiant Floors.


Furniture placement can be an issue

Baseboard radiators (or convectors) consist of a copper tube and thin aluminum fins hidden by a metal cover. Air warmed by the tubing and fins rises through louvers or vents while cooler air is drawn into the bottom of the radiator. This sets up a convective current of air that warms the room.

There isn’t much mass in these radiators, so they warm up and cool off fairly quickly. They are made in various sizes and come with a range of heat outputs.

Baseboard radiators are usually installed beneath windows and along exterior walls where they produce a curtain of warm air that keeps the room comfortable. In a well insulated home with high-performance windows, however, radiators can be located on interior walls without sacrificing comfort.

Cast iron is still around
Less common, and more expensive, are baseboard radiators made from cast iron. Burnham says its one-piece “Baseray” cast-iron baseboard radiators, which are filled completely with water, supply five times the radiant energy as the fin-tube variety. Runtal makes a similar type from welded steel.

Free-standing cast-iron radiators (some converted from the days of steam heat) are used in many older homes. Some are beautifully detailed. More contemporary styles are available from Runtal and others.

Watch the furniture
One disadvantage of baseboard and free-standing radiators is that they limit furniture placement. For example, baseboard units prevent dressers from being pushed tight to a wall. Moreover, the performance of baseboard radiators can be compromised by sofas or thick carpeting.

Water for hydronic baseboard radiators can be heated with a number of fuels. Heat output varies with the temperature of the water, but a water temperature of 160° F is common. That makes these systems less suited to solar hot-water collectors than radiant-floor systems.


Lower cost 'direct' systems use a water heater

In most installations, hot water for the heating side is strictly segregated from hot water used for cooking or bathing (domestic hot water). Proponents of lower-cost radiant floor heating, however, advocate “open-direct” systems in which water used for the heating system is not separated from the domestic hot-water supply.

Most gas or electric water heaters have enough capacity to supply hot water for an in-floor radiant system. Phoenix high-efficiency gas water heaters are one type used in such systems. Open-direct systems are simpler and less expensive to install, especially for owner-builders who are willing to do some of the work themselves.

System components can be ordered from several companies on the Internet.

Health risks concern some. Open-direct systems are permitted by code, but not by all local inspectors or in all jurisdictions. Critics claim these systems increase the risk of Legionnaires' disease, a pneumonia-like illness caused by bacteria that thrive in warm, stagnant water. Since no one is known to have ever contracted Legionnaires' disease from a radiant floor heating system, however, the risk is more theoretical than actual.

Image Credits:

  1. Toby Welles/Fine Homebuilding #168
  2. Toby Welles/Fine Homebuilding
  3. Anna Robinson
  4. Sean Groom/Fine Homebuilding
Tags: , ,
Aug 27, 2009 1:27 PM ET

P&M Link
by Mike Guertin

Gary H's post included an incorrect link. I think it should have been to Plumbing and Mechanical magazine:

Aug 3, 2009 4:12 AM ET

Electric or gas hydronic in floor heating
by MIke

I am building a new home on the water about 2800 SQft with 12 ft vaulted cieling over the living and kitchen area. My lower floor area is foam block foundation. I am trying to figure out if gas is better or electric hydronic in floor for heating. I am considering the gas unit that will do both the floor heating and hot water. an un bias opion would be helpful

Aug 1, 2009 4:52 PM ET

How to get ALL the facts about radiant heat and other uesfull

The publication used by the design Engineering HVAC professional WORLD can now be yours FREE.
SO why not get the facts as published around the world rather than listen to billybob?

May 20, 2009 7:13 AM ET

Pump charts
by Michael Chandler

I've found the pump charts in the Grainger catalogue to be useful in sizing Taco pumps for these sorts of applications. With 40 feet to the roof you are looking at a Taco 009 or larger. But the floor loops would probably be served by an 006 which would likely be less expensive than the three-way valve you would need to divert the flow from the solar harvesting circuit to the radiant heating circuit, so the savings of using a single pump would likely not materialize.

Recently I've been getting some very energy-efficient French pumps from Solar H2ot in Cary, NC, and they have an engineer there, Dan Gretch, 919-656-9810, who is very helpful on phone support and pump sizing and pipe design.

May 19, 2009 3:19 PM ET

Sizing hydronic circulators
by Martin Holladay

Here are three resources for sizing hydronic circulators:

1. A good book is:
Modern Hydronic Heating for Residential and Light Commercial Buildings, by John Siegenthaler. Cengage Learning, 2003. ISBN #0766816370.

2. Here's an online guide: "Sizing Circulators for Hot Water Heating Systems."

3. "Hydronic Heating and Hot Water Recirc Systems," a handbook published by Grudfos, a manufacturer of circulators, is available online.

May 19, 2009 11:42 AM ET

Pump selection
by david kurtz

Could someone point me to any literature dealing with selection of pumps? I am specifically trying to find out thoughts on saving money doing one pump instead of two. One larger one that might do double duty circulating water through an open loop solar thermal system(40 feet up to the roof) during the day and then dumping the heat into my pre-plumbed radiant slab (basement(400sf) main floor(1000sf) and master bath(150sf). Water pressure would be enough for preheating for domestic use. Thanks for any help! Great site! Cant wait to keep reading!-dk

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