Another Perspective on Air-to-Water Heat Pumps

musingsheader image

Another Perspective on Air-to-Water Heat Pumps

Jake Marin weighs the pluses and minuses of connecting an air-source heat pump to a hydronic distribution system

Posted on May 4 2018 by Martin Holladay

Green builders, preparing for a fossil-fuel-free future, are busy building all-electric homes. Most of these homes are heated and cooled by minisplit heat pumps. Occasionally, though, a builder who’s worried about uneven heat distribution in a home with ductless minisplits will post a question on suggesting the use of an air-to-water heat pump hooked up to a hydronic distribution system.

Although these systems are quite rare, a few brave pioneers have installed them, and GBA has documented their efforts with a series of articles. (See, for example, “Air-to-Water Heat Pumps” and “Split-System Heat-Pump Water Heaters.”)

One of the many energy experts who’s been investigating air-to-water heat pumps is Jake Marin, the program manager for HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. at Efficiency Vermont. (Marin helped develop Vermont’s first incentive program for ductless minisplits.) On February 8, 2018, at the Better Buildings By Design conference in Burlington, Vermont, Marin gave a presentation on air-to-water heat pumps called “Are We There Yet?”

We’re still looking for the perfect heating and cooling system

The first question Marin addressed was, “What’s wrong with ductless minisplits?”

Marin told the Burlington audience, “Ductless minisplits aren’t a silver bullet. They have some limitations. They have the same comfort issues inherent to any point-source heater. They don’t allow system integration — they can’t be integrated with the rest of your HVAC system. Many people have aesthetic objections, calling them ‘warts on the wall.’ And anyone who wants more than one zone finds that the multi-zone systems have poorer performance than single-zone units.”

Marin also pointed out that the systems aren’t very intuitive. “People don’t know how to use their remote control,” he said. “And if the house has more than one heating system, people don’t use their ductless minisplit as much as they should, because they don’t know how to operate it.”

Should Vermonters consider air-to-water heat pumps?

According to Marin, “Vermont is a great place for air-to-water heat pumps. Vermont has aggressive renewable energy goals. We are located in ‘hydronics alley’ — the Northeast has a lot of existing hydronic systems, because we don’t have a lot of central air conditioning. About 60% of homes have boilers and hydronic distribution. Vermont contractors are diversified, and many are skilled in hydronic heating. Contractors here are interested in quality, efficiency, and high performance. When I travel around the country, I hear people say, ‘I wish contractors in my area were like contractors in Vermont.’”

Marin has created a list of the theoretical advantages of air-to-water heat pump systems. These systems:

  • Can deliver domestic hot water as well as space heat.
  • Should provide better comfort than ductless minisplits.
  • Can be zoned as easily as any hydronic system.
  • Should result in less temperature stratification of indoor air.
  • Are quiet.
  • Deliver heat without duct losses.
  • Can be used by an electric utility for demand-response purposes (since a system with a large storage tank can absorb excess electricity during off-peak hours).

In spite of all of these advantages, Marin admitted that “there is almost no market for air-to-water heat pumps in the U.S. today.”

Water temperature matters

As I noted in my 2016 article, most air-to-water heat pumps struggle to make high temperature water. In general, a heating system using an air-to-water heat pump has to be designed for a water temperature no higher than 120°F.

“A typical boiler usually delivers 180 degree water,” said Marin. “Fin-tube baseboard systems, which you see everywhere, require high temperature water. These systems are cheap to install. But with 120 degree water, you only get 30% of the heat that you would get with 180 degree water. So if your existing house has fin-tube radiation, you can’t just install an air-to-water heat pump. You won’t get enough heat to heat the space.”

Marin continued, “The lower the delivered water temperature, the higher the COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. of the heat pump. So the delivery temperature of your water is absolutely critical to efficiency. You really need to design the system to use low-temperature water.”

Marin said, “You need emitters with more surface area and better heat exchange. In a radiant floor, the piping is the emitter. Heat transfer plates can help distribute the heat more evenly. You can design a radiant floor for temperatures as low as 75 or 85 degree water for a high-performance building.”

Marin continued, “Or you can use panel radiators, which are cool and hip. The panel radiator increases the surface area. Or you can install high-output baseboard. Runtal makes it. It’s pricy. Or you can use a hydronic fan-coil unit that looks like a minisplit. You can get a wall-mounted fan-coil unit or a floor-mounted unit. You can even get a ducted fan coil unit. Spacepak makes one.”

Marin predicted, “Low temperature distribution is the future of hydronic systems.”

Mono-bloc or split system?

There are two kinds of air-to-water heat pumps: “Mono-bloc” systems and split systems.

A split system resembles a ductless minisplit: the outdoor unit is connected to the indoor unit with copper tubing carrying refrigerant. The advantage of a split system is that most of the equipment is indoors, making it easier to service. The major disadvantage is that installers need to be licensed in refrigeration.

“A mono-bloc system creates hot water outside, so it requires antifreeze,” said Marin. With a mono-bloc system, a mixture of warm water and glycol circulates between the outdoor tank and a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. located indoors.

For the installer, the advantage of a mono-bloc system is that there isn’t any need to handle refrigeration tubing, nor any worries about refrigerant leaks. All of the refrigerant tubing work is completed at the factory. Another advantage: the equipment doesn’t require much interior space.

The downside of a mono-bloc system is the need for glycol, which introduces the need for an extra heat exchanger. “The entire system is outside,” said Marin. “That means that all of the service needs to happen outdoors. And service calls always seem to happen on the coldest day of the year.”

We don’t have a good way to rate performance

Most builders know that furnace efficiency is measured by a metric called “annual fuel utilization efficiency” (AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. ). Water heater efficiency is measured by a metric called “energy factor” (EF).

With air-to-water heat pumps, however, we don’t have any convenient rating method.

Marin raised a rhetorical question: “Do they work well? It’s hard to say. We don’t have a way to rate them. We don’t even have a model to complain about. We don’t have anything like HSPF or SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. . There is no Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. test. AHRI does not list or test these units.”

He continued, “What we have is COP data from manufacturers — static COPs for various outdoor temperatures and water delivery temperatures.

“The takeaway from the COP data is that performance overall appears to be pretty good,” he said. “The COPs seem to be similar to the COPs for ductless minisplits. But we need better data for low water supply temperatures. We really need a good metric and a good cold climate specification from an organization like NEEP.”

Dual-fuel systems

Marin discussed the pluses and minuses of dual-fuel heating systems. An air-to-water heat pump could be paired with an conventional boiler burning fossil fuel.

He noted that homes with a backup heating system can deliberately downsize the air-to-water heat pump. After all, for most hours in any given year, the outdoor temperature is much warmer the temperatures experienced during a cold snap.

On the plus side, having two heating systems provides redundancy. If the power goes out, it’s easier to operate a fossil-fuel boiler on a generator than it is to try to run a heat pump with a generator.

On the minus side, it’s expensive to install and maintain two heating systems.

Equipment options

Marin told the audience in Burlington that four brands of air-to-water heat pump are available in North America:

  • , a mono-bloc system. (Chiltrix is a U.S. manufacturer with headquarters in Chesapeake, Virginia.)
  • , a mono-bloc system. (SpacePak is a U.S. manufacturer with headquarters in Westfield, Massachusetts.)
  • , a mono-bloc system. (Aermec is an Italian manufacturer that has set up a distribution center in Canada.)
  • , a split system. (Nordic is a Canadian manufacturer with headquarters in New Brunswick.)

Marin also has his eye on three products that may eventually become available in North America:

  • , a split system. (The Daikin Altherma was distributed in the U.S. for a few years before its distribution was discontinued.)
  • . (Sanden is now distributing an air-to-water heat pump for domestic hot water, not space heating; for more information, see Split-System Heat-Pump Water Heaters.)
  • .

Still looking for that elusive magic box

Marin began his presentation by telling us that “Ductless minisplits aren’t a silver bullet.” He's right, of course. But most of those who heard his presentation probably concluded that air-to-water heat pumps aren't, either.

Martin Holladay’s previous blog: “Bruce Harley’s Minisplit Tips.”


Image Credits:

  1. Runtal

May 5, 2018 10:07 PM ET

by Ethan T ; Climate Zone 5A ; ~6000HDD

We are investigating hooking the Sanden HPWH to a hydronic heating loop so it can provide both domestic hot water and heating. I'll report back.

May 6, 2018 10:48 AM ET

Two Points
by Andy Kosick

The performance rating is a problem. I recently HERS rated a house with a Chiltrix and all I had to work with was manufacturer COP. I knew that this wasn't really reflective of it's seasonal performance, something made clear by the fact that the homeowner/builder wasn't currently seeing the performance he had hoped for during the single digit temps we were having. I am hoping to work with this customer next winter to get some better monitoring data.

Also, I've been thinking a lot about air-to-water lately because of the logic I've been applying to HVAC. To invoke Einstein, HVAC should be as simple as possible, but not simpler. I have arrived here because of seeing to many high performance homes with a mechanical room that looks like it belongs in a nuclear submarine, and thinking "this can't be right, it certainly can't be affordable." My current food for thought is that It seems to me there are exactly two things you cannot avoid doing in even the most high performance (electric) home. Concentrating heat into a tank of water for domestic use and exchanging air in each individual space for ventilation. So in my simple fantasy house, everything else that is needed should be provided through these two unavoidable systems in an elegant fashion. An air-to-water HP seems critical to this general concept. Thoughts?

May 6, 2018 11:05 AM ET

Response to Andy Kosick
by Martin Holladay

I agree that we need to make sure that a home is well ventilated. Balanced ventilation systems do an excellent job. One thing I've learned over the years is that you want dedicated ventilation ductwork. Avoid the temptation to use ventilation ductwork to deliver space heat! It's a dangerous temptation.

Many GBA readers have built homes that are comfortably heated and cooled with two ductless minisplits. These homes are simple. If you like simple, it's hard to beat that approach.

Many people have been tempted to use their domestic hot water tank for space heating. You can do it if you want -- but it makes your mechanical room look like a nuclear submarine, and you're trying to avoid that. So put in a Marathon water heater or a heat-pump water heater, along with PV if you have an unshaded south-facing roof and a solar-friendly utility, and you're done.

May 7, 2018 5:26 PM ET

Retrofits vs. new construction
by Dana Dorsett

Most existing hydronic heating systems are so oversized they can still deliver design-day heat to the zones with water cooler than 140F water, but most won't cut it with 120F water. Having to upgrade the heat emitters makes it a more expensive proposition for retrofits.

But it's pretty easy to design-in cheap radiation capable of design-day heat at temps well under 120F for new construction.
Slab-on-grade with PEX embedded in the first floor slab, and low-temp air coils (doesn't have to be a high-mount wall coil with the mini-split type apperance- there are many optionjs) for micro-zoning rooms without slab floors can be reasonably inexpensive and designed to work well for both heating and cooling. Micro-zoning with air coils will be more expensive up front than fin-tube baseboard but substantially cheaper than low-temp heated floors. With a central buffer tank it can also be substantially more efficient than short-cycling a ductless mini-split on a micro-load room.

Latent cooling with chilled water below the indoor dew point can work too, but it adds another layer of design and control complexity. It can still make sense to do the bulk of the latent cooling with a ductless mini-split located in a high-volume zone/space, distributing the dry air with ventilation system recirculation, even if the hydronic coils are doing the bulk of the sensible cooling of the cooling zones.

I don't believe either Andy or Martin has ever seen the insides of a nuclear sub. ( Amm i rong? :-) ) Even a ridiculously micro-zoned hydronic heating system based on a buffer tank or water heater has complexity not even remotely like that of a submarine's drive systems. (I have a relative who has spent her career as as a team member refueling nuclear subs. She can't share pictures or talk about too many details, but I trust her that the mechanical rooms are not even remotely like the typical multi-zone home heating system.)

A single or 2-3 zone hydronic system can be pretty compact, whether it's based on a water heater or a boiler or buffer tank on a reversible chiller. Putting it on a water heater only adds one pump and a heat exchanger over what typical boiler-based system needs. Single zone 2-pump 1 heat exchanger water heater solutions that look simpler and take less space than traditional old school cast iron boiler based systems. But even micro-zoned systems be that simple with some forethought:

Micro zoning with hydronic coils from a buffer tank can be controlled with the fan coil's blower controls local to the and ONE pump driving a loop. When the room is calling for more heat the blower is on, when it's not the water flows-through delivering very little heat to the room. This is nothing like the (totally imaginary) nuclear sub paradigm. This is HTP's vision of what it can look like:

This can work with heated or chilled water from any source, either direct from a modulating reversible chiller like the Chilltrix, or from a buffer tank (with any variety of heat/coolth sources, including non-modulating hydronic heat pumps) or a water heater, modulating condensing boiler, etc. It's a lot easier to route &/or hide a single pair of pipes than it is for any ducted distribution system.

Domestic hot water distribution systems with home-runs to manifolds (for efficiency) or domestic hot water recirculation system often exceed that level of plumbing complexity.

May 8, 2018 7:27 AM ET

Response to Dana Dorsett
by Martin Holladay

Since Andy was the first one to introduce the nuclear submarine analogy, I'll let him defend it.

You're correct that designing and installing a residential hydronic system is significantly less complicated than installing mechanical equipment on a Trident sub. That said, there is something to be said for radical simplicity. For more information on this issue, see Simplicity versus Complexity.

May 8, 2018 9:50 PM ET

HERS Rating Approach for Air-to-Water Systems?
by Justin Gibbs

Andy Kosick, can you provide any advice I can pass on to my HERS rater for modeling our ATW system? For the preliminary HERS rating, he guessed at HSPF/SEER numbers for the equipment assuming we could eventually get these numbers from the manufacturer. But, those ratings don't map well to this type of setup and Nordic isn't going to attempt a testing program to measure them. I believe the rater will be using the REM/Rate package. Thanks!

May 9, 2018 2:32 PM ET

My attempt
by Kevin Camfield

I tried very hard to make an air to water radiant system work for my new home. I wanted the comfort radiant systems provide and the heat pump efficiency. I talked to Chilltrix and others with similar systems. My main problem with them is that they didn’t have any real test data that showed how the systems performed in real life and it appeared that the way the systems were designed there was likely to be a lot of short cycling. Sanden has by far the best real life test data. Washington State University did 2 studies and Sanden has been running air to water radiant system testing in Washington for quite some time with long-term data collection. I even went to a 2-hour seminar with Sanden on how to use their HPWH for radiant systems. I have a stupidly large spread sheet that I put together to try to prove to myself that I could get the comfort at the same long term cost – installation plus 10-year operation cost. I failed.

That said, I really think the Sanden system can work at comparable or better cost in the right situations. What I learned:
• As others have pointed out, return temperature to the water tank is very important. It needs to be as low as possible, which means a well-designed radiant floor system with the right flooring. Panel radiators don’t work.
• Slab on grade reduces the cost upcharge of above ground floor systems like gypcrete or warmboard and helps make the accounting math work.
• The Sanden system works better for a higher efficiency home. The sizing of the system is important to keep costs down and allow the system to run most efficiently.
• Get rid of the peak loads with an electric booster unit integrated into the design. If you look at the heat degree days for your area you then can design your system so that the booster comes on very seldom but extends the overall capacity of the system.
• Sanden has developed a design using a Taco X block pump that is really great. It gives a level of control other systems don’t have and provides the separation between DHW and floor heat.
• The system works more efficiently in a household that uses a lot of domestic hot water. The cold-water intake boosts the overall efficiency.
• Sanden uses CO2 as a refrigerant which is great for the planet and causes it to operate differently than other HWHPs.
• The equipment is very well designed and has been in use in Asia for quite some time. They also produce a high percentage of compressor parts for the US auto industry. I had no worries about durability.

If my home had a lower peak heat load and was constructed slab on grade this system would have worked for me at a price comparable to other systems. That would have even been true also if I would have used a zoned system with radiant heat on the first floor and a mini-split upstairs. Let me know if anyone wants more information and I’ll try to provide it.

May 9, 2018 4:04 PM ET

short cyling
by Jon R

> Chilltrix ... likely to be a lot of short cycling.

Given that hydronic systems can be used with buffer tanks that prevent them from ever short-cycling, I'm curious how you came to this conclusion.

> Can be used by an electric utility for demand-response purposes

I'd add that they can also be used by the homeowner for similar things, without utility involvement.

Without buffering, I'd consider air-to-water heat pumps far less interesting.

May 10, 2018 11:25 AM ET

This is a dead end
by Kevin Dickson, MSME

"Marin predicted, “Low temperature distribution is the future of hydronic systems.”"

Unfortunately, hydronic systems are not the future for heating low energy houses.

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!