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Musings of an Energy Nerd

Air-to-Water Heat Pumps

Should residential designers consider using an air-source chiller for space heating and cooling?

The Chiltrix is an air-to-water heat pump that can be used for space heating and cooling. The Chiltrix outdoor unit resembles the outdoor unit of a ductless minisplit system.
Image Credit: HotSpot Energy

Most air conditioners and heat pumps sold in the U.S. — including most split-system air conditioners and ductless minisplits — are air-to-air heat pumps. During the winter, these appliances extract heat from the outdoor air and deliver warm air to a house through ducts or small fan-coil units. During the summer, these appliances deliver cool air to a house and dump unwanted heat into the outdoor air.

Another type of heat pump, an air-to-water heat pump, produces hot water (or chilled water). When used for air conditioning, an air-to-water heat pump is called a chiller. Almost all air conditioners cool an air stream by blowing air past a copper coil. In a conventional split system air conditioner — called a DX (or “direct expansion”) system — the fluid in the copper coil is a refrigerant. In a chiller-based system, however, the fluid in the copper coil is water (or in some cases, a solution of water and antifreeze).

When used for space heating, an air-to-water heat pump fills the role usually held by a boiler; it requires a hydronic (water-based) distribution system. The heat pump makes hot water; the hot water can be pumped through fin-tube baseboard units, in-floor PEX tubing, or a fan-coil unit. (A system with such a fan-coil unit is called a hydro-air system.) The fan-coil unit can be a small wall-mounted unit that looks like the indoor unit of a ductless minisplit system, or it can be a large fan-coil unit connected to a conventional forced-air duct system.

Hydronic systems have pluses and minuses

The main reason that American homes usually use ducts to distribute heat is that most Americans require air conditioning. In climates where air conditioning isn’t required — for example, in northern Europe — space heating systems are usually hydronic…

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  1. Expert Member
    KOHTA UENO | | #1

    Chilled Water Comment
    Great column, Martin! In re my comment--of course, the concerns with chilled water vs. DX (refrigerant in copper pipe) are irrelevant when we're talking about the heating season.

    Most mechanical engineers love hydronic heating systems, for several reasons. For one, small-diameter water pipes can transfer heat more effectively than small-diameter ducts (because the heat capacity of water is four times the heat capacity of air). Moreover, water pipes are usually installed to a higher standard than ducts — meaning that unlike ducts, water pipes usually don’t leak. Finally, the circulators used in hydronic distribution systems usually require less energy than the fans used in forced-air distribution systems.

    Just to clarify the situation for others in the audience--that's a factor of four mass-based heat capacity. With the density difference, it becomes a huge difference. I'm also citing Siegenthaler here--his Slide 20 in this presentation does a nice job of demonstrating the difference.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Kohta Ueno
    Thanks for the link to John Siegenthaler's presentation. I'm reproducing the relevant image below.

    I appreciate the suggestion to clarify the mass/volume issue. I have edited that sentence to reflect your suggestion.


  3. albertrooks | | #3

    Sanden Specs in error.
    Hi Martin,
    Happy new year to you!
    Where did you get those Sanden specs? There are some errors there:
    -The outdoor unit will only return water to the tank at 149F. There is no range of temps.
    -Only potable water runs through the outdoor unit. No mixes.
    -The outdoor unit is rated to -15F. At this point it is still delivering 149F water. It's just got a low COP of around 1.2.
    -The system has a rated capacity of 18.5kbtu/hr

    Just FYI: My home is a trial of the COMBI heating +DHW by Washington State University and our regions electric utility: Bonneville Power Administration. There is a lot of good data coming out of that study. Essentially it asks: What else can we do with these DHW installations in low load homes since they do DHW so well, and then sit in idle 18 hours of a 24 hour cycle.

    Our company Small Planet Supply will be distributing these systems in late March so it's a topic I'm following closely.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Albert Rooks
    Thanks for the information you shared on the Sanden.

    I have corrected the article to reflect the BTU/h rating you report, and to reflect that the tubing connecting the outdoor unit of the Sanden air-to-water heat pump with the indoor equipment or tank must not contain any antifreeze. Of course, that limits the climates in which the Sanden can be used, as the danger of freezing increases (especially during power outages).

    When I researched the Sanden for my February 2015 article, I found various and contradictory reports on the minimum outdoor temperatures at which the Sanden can operate. In that article, I wrote, "The appliance works at low outside temperatures — variously reported as -4°F or -14°F."

    Right now, I'm not sure of the source for my statement that the manufacturer recommends that the unit's operation is limited to outdoor temperatures above -5°F.

    One source on cold-weather operation was “Laboratory Assessment of Sanden GAU Heat Pump Water Heater Lab” by Ben Larson of Ecotope, Inc. In that document, Larson wrote, "the manufacturer reports operation to at least -4°F."

    An Australian distributor of the Sanden tells purchasers that the appliance is "Fitted with in-built freeze protection, making it suitable for all climates (-10ºC to +43ºC)." The low end of this range (-10ºC) is equivalent to 14°F.

    In any case, I was unaware that the unit is rated for performance at -15°F; thanks for that information.

    We agree that the unit produces 149°F water.

  5. charlie_sullivan | | #5

    refrigerant release
    Excellent article. Thanks for the research and reporting.

    Another advantage of an air to water unit is that it uses less refrigerant than an equivalent split system, and it has fewer places for the refrigerant to leak. That means that on average, there should be less refrigerant released to the atmosphere, a concern because of the high global warming impact of r410a.

    In heating dominated climates that also require or benefit from some A/C, I recommend a system using a few fan coil units, supplemented by panel radiators that are used only for heating. That allows setting the fan coil fans very low so they are very quiet, and getting heat distributed as widely and evenly as you like.

    For southerners jealous of the hydronic skills in New England, and for northerners expecting an easy installation, note that plumbers used to doing hydronic heat have trouble getting their heads around the level of meticulous insulation needed for a chilled water systems. You may be better off with an installer who is learning fresh.

  6. gusfhb | | #6

    I have no direct knowledge, but in general, nothing that ever goes below 32 degrees can ever have plain water in it.
    I would contact the manufacturer for clarification.

    The most corrosive thing in a water system is the water, well the water and the oxygen in the water. Antifreeze in your car protects all the metals aluminum brass copper iron steel, from decaying. What material exactly in this unit is so fussy that it cannot tolerate antifreeze, yet so strong that it will last 10+ years without corrosion?

    I think there is a misunderstanding somewhere here. Inside the heat pump is a heat exchanger that is either copper or aluminum; a pump that is probably plastic, maybe with a stainless steel impeller, probably one or more solenoids, again plastic and stainless. I would think nothing else would touch the water.

    Lots of regular systems have propylene glycol in them after the non return valve

    While it is nice not to need an AC guy for the install, wouldn't it make more sense if you are set against anti freeze to run the freon to a heat exchanger inside the building?

    Price wise it is in the same ballpark as a high end oil install.

    Hydronic is better since it can store heat and it occurs to me with 149 degree water you could create hot water with this system, although with 149 degree water the recovery would be slow. My oil system has a max of 165 and can be run out of hot water if you try.

  7. Expert Member

    Very small typo
    (By mass, water has four times the heat capacity of air. By volume, water has a 3500-to-1 advantage over water.)

  8. GBA Editor
    Martin Holladay | | #8

    Response to Malcolm Taylor
    Small but important. Thanks for the correction, Malcolm. I appreciate it.

  9. user-971368 | | #9

    Nice overview, Martin. We
    Nice overview, Martin. We have been looking at this application for some time here in Climate Zone 7, Northern Maine where hydronics rule, and rightly so. Siggy has some great text out there on this (and other hydronic) application.

    Some parallel thoughts
    - when utilizing radiant cooling be sure to mind the cooling surface temps (ceiling, floor, wall, etc), maintain above space dew point. Consider utilizing a DOAS and/or ERV w/ cooling coil to address latent loads. Simple controls are available to maintain surface skins above dew point and dry.

    - Jaga makes a nice LWT panel rad, with a DBE (dynamic boost effect) fan, you can get it with condensate drain - nice units. There are similar offerings from other companies such as Runtal. Smith makes a LWT fin-tube

    - Siggy also makes a good point that designing systems around hydronics at appropriate temps has a way of future-proofing your system for what is to come in the future, as can be read more in-depth in his columns

    We look forward to more conversations about air to water heat pump applications and how we can best apply them. Keep up the good work. Thanks - andy

  10. GBA Editor
    Martin Holladay | | #10

    Response to Andy McPartland
    Thanks for your comments.

    I had to look up DOAS -- if any other GBA readers are also scratching their heads, it stands for "dedicated outdoor air system."

    You're right, of course, that any hydronic cooling system must either (a) maintain hydronic fluid temperatures that are higher than the dew point of the interior air, or (b) lower the indoor RH to ensure that condensation is impossible, or (c) include distribution methods that collect condensation and direct it to a drain.

    In short, designing a hydronic cooling system isn't for beginners. There are lots of ways to mess things up. Thanks again.

  11. charlie_sullivan | | #11

    Jaga vs. Chiltrix
    I bought a Jaga fan coil unit before I learned about Chiltrix. It was the only unit I could find that was capable of handling cooling AND had a low power consumption ECM fan. Chiltrix has the same features in their fan-coils for something like 1/5 the price. The Jaga looks better for sure, but I could have bought some nice handcrafted furniture that looks even better for that price difference.

    Good point about future-proofing.

  12. ethant | | #12

    Replacement for oil boilers?
    In an old building that already has a copper distribution of hot water from oil boilers, could these be swapped in and run to the existing registers?

  13. GBA Editor
    Martin Holladay | | #13

    Response to Ethan T
    The answer to your question is, "perhaps, but probably not."

    Most existing hydronic heating systems equipped with a boiler are designed for 180°F water. If you swap the boiler for an air-to-water heat pump that produces 130°F water, the distribution system won't be delivering enough heat to keep the rooms warm on the coldest days of the year.

    There are exceptions, however. Some boilers may be hooked up to distribution systems designed for 130°F water, so if you have that kind of boiler, the swap would work.

    It's also possible that some hydronic systems weren't well designed -- and ended up with distribution systems that can deliver enough heat when circulating 130°F water to work (more or less by accident). To determine whether you have that type of hydronic system, contact a mechanical engineer. An experienced hydronic installer or engineer should be able to evaluate your distribution system and perform a room-by-room heat loss calculation -- the necessary steps to answering your question.

    Manufacturers of fin-tube radiation publish tables that help engineers answer your question. For example, here is a link to a heat output table for a certain brand of fin-tube radiation showing that one foot of their product puts out 560 Btu/h with 180°F water at 1 gpm, but only 310 Btu/h with 140°F water at 1 gpm. The heat output at 180°F is only 55% of the heat output at 140°F -- and most of the air-to-water heat pumps discussed in this article can't even produce 140°F water.

  14. pls01 | | #14

    Air-to-air in California
    Great article, Martin. Glad to hear of the activity around the country. We've monitored several Althermas, which used to be approved in CA. Now only the Aermec (the Ferrari of heat pumps) and the SpacePak are approved here. Thanks to the dry climate we find they work great with radiant slab floors (very limited carpeting, no wood or vinyl). A project we did in Tucson using an Aqua Products heat pump showed that radiant floors can be used to store cooling and shift loads completely off peak while improving EER due to cooler nighttime temperatures ( We are also testing a house with two types of ceiling panels and find that even here some form of dehumidification may be needed to keep the panels below dewpoint where rooms need a lot of cooling capacity. We plan to come out of the project with some good design guidelines for ceiling panels.

    Dave Springer dba Pepper Smith

  15. charlie_sullivan | | #15

    Ways it can work

    Martin mentions that replacing 180 F water with lower temperature water could be OK if the original design was putting out more heat than it needed to. Another factor that can contribute to that possibility is any envelope upgrades that happened after the original distribution system installation, including air sealing, insulation, or window improvements. Between those two factors, you might be close but not quite able to supply the heat you need, in which case adding a few more radiators or a fan coil can get you there.

    As Martin says, figuring that all out is not simple, but it can be done--my house is one example.

  16. user-971368 | | #16

    Jaga $
    I agree (some) Jaga products are expensive - especially if ordered in piecemeal - we have been down that road - however - it was remarkable the price we got when we did a LWT project with over 10 rads. We went with the tempo, which were cheaper than the typical panel rads available (PNA, Buderus, etc) - the tempo is really just some painted metal panels erected around a nice heat emitting coil, and they can have DBE installed at a later date which makes sense if client decides they want super low water temps but require same btu/h output.

    I am not in Jaga's camp by any means. I have been looking at the runtal neo as well but i have not requested any pricing - linked below

    I think/hope we will see more offerings on this front in the future

    Thanks - andy

  17. user-971368 | | #17

    Air to Air CA
    Nice application with radiant cooling and peak shifting, and thanks for the report. I would be curious how occupant comfort was with radiant cooling, assuming it would be liken to radiant heating, it has to be more agreeable (comfort, sound, draft, etc) than forced air cooling. Looking forward to hearing about the ceiling panels.

    Thanks Again - andy

  18. pls01 | | #18

    Air-to-air in California
    We've heard no complaints from the owners of the Chico, CA home. The Tucson builder/homeowner had some discomfort during the monsoon season due to high relative humidity. Lesson learned: when using a chilled water fan coil to dehumidify make sure the system can provide lower water temperatures (~50F) so moisture will condense on the coil. No signs of condensation on the floor at either home.

  19. RayDeRaymond | | #19

    I am in New Hampshire, and am a member of a church group that is looking to replace an oil fired hydronic heating system with an air to water heat pump system and a PV panel array, with a goal to become a net zero facility of 8,368 sq. ft. It has been very difficult to find qualified contractors to engineer and install this type of system. Estimates we have been given are over $100,000. That is a hard sell to the group. Neighboring states have support for this kind of effort, but it seems lacking in NH. My question is: Are we being unrealistic? Ahead of the technology? We have already entered into a power purchase agreement for a solar array, and have been permitted to build it. We need to know our electric needs to properly size it, and that is still awaiting our heating system design.

    1. charlie_sullivan | | #21

      Sorry nobody saw this last August. Are you still looking for help with this? Is $100k for just the heating system, or is that including the PV system? How big a system you need depends on lots of details of the building, but just as a wild guess, you might need 8 to 16 tons, which will be expensive, but not all that expensive. One questions is whether you want to maintain a constant temperature or whether you do thermostat setbacks according to the use of the space. Being able to warm a large space up quickly would require a higher power system than maintaining a constant temperature, even though it would use less energy on average.

  20. charlie_sullivan | | #20

    An addition to the list of manufacturers of air-to-water: Enertech Advantage. Variable speed, units from 2.5 to 5 tons. Looks like good stuff, made by the same company that makes Hydron Module and Geocomfort water-source heat pumps.

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