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Green Building Blog

Air-to-Water Heat Pump Retrofit

A mechanical engineer shares lessons learned after replacing his propane boiler with an air-to-water heat pump to heat his home

Air-to-water heat pumps generate a lot of buzz, but is anyone installing them? When we purchased our hydronics-based home in 2015, I was intrigued by the prospect of replacing the propane boiler with an air-to-water heat pump (AWHP). But no one was doing it and the equipment options were limited: the Daikin Altherma had been pulled from the market, the Sanden was being tested at a few homes in the Pacific Northwest, and I didn’t know of any other market-ready options.

Four years on, we finally took the plunge and are now two-thirds of the way through our first winter with our AWHP. I’ve learned a lot. Given how few stories are circulating about this technology, I figured I’d share ours. Before I dive in, I’ll note that many feel the industry and contractor base isn’t there yet. So, is it? I now think the answer is “yes” or at least “pretty much.”

Project goals

During our first year in the house, we had a carbon monoxide scare that evacuated us from our home in the middle of the night. It was a leak issue easily fixed by a boiler service, but reinforced our desire to wean the house from propane completely: not only would it improve health and safety, we’d never again have to deal with propane supply contracts, winter fuel deliveries, and volatile unregulated fuel prices (currently, propane costs are low, but hovered in the $4/gallon range in recent years). Given the relatively clean generation portfolio of our electrical utility, transitioning from fossil fuel combustion to electric could also help us be better climate citizens.

We could have just installed minisplits—it certainly would have been easier, as I’ll get to—but we liked our radiant slab and wanted to reuse at least some of our hydronic infrastructure, and we didn’t want minisplits heads and linesets scattered around the interior and exterior of the house.

In sum, we wanted to eliminate fossil fuel combustion in the home, improve health and safety, use existing hydronic infrastructure, be pinned to more stable energy prices, and run our home with lower greenhouse gas impacts. There were other benefits realized along the way, but I’ll get to those later.

Fixing Distribution

Our slab-on-grade home has radiant heating tubing embedded in the slab for the main level. The second-floor zone consisted of baseboard radiators installed in series (a single loop) serving four rooms. To evaluate the system, it was critical to not only figure out the home’s heating design load, but to calculate room-by-room loads. I used REM/Rate to get an estimate of the heating design load, and then an Excel-based UA calculator for a deeper dive and to look at the room loads.

On doing this, I found that the existing system was 30% undersized in one upstairs room—the master bedroom, which was not a surprise given our comfort experience. And that’s with 180°F supply-side water from a boiler. Now, substituting in 120°F supply water—which is a reasonable upper temperature limit for an air-to-water or ground-source heat pump—the existing emitter was about 4x undersized. The other rooms upstairs would be 3x to 4x undersized with the lower temperature supply.

This part is frustrating: If the second floor of my house had been plumbed in parallel home runs (dedicated supply and return from the mechanical room to each upstairs room), upgrading heat emitters could have been relatively straightforward. But in series, it’s was pretty much impossible to get adequate heat to the second, third, and fourth rooms in the loop after starting with 120°F water and dropping degrees every time the water hits another emitter.

Upstairs layout, showing heating loads, existing baseboard, and series loop flow direction

The right way to do this was to get home runs to each room, but that meant ripping open a combination of floors, walls, and ceilings. This was a major barrier that we only justified (eventually, and not without major reservations) after committing to install an ERV and its duct work at the same time as the heating system work. And while we were busy ripping things apart, we decided to gut renovate the only full bathroom in the house. Scope creep? Absolutely. But these other factors were ultimately what helped justify moving forward with the project rather than just installing a gaggle of minisplits, dropping the idea of balanced ventilation, and calling it a day.

Onward. The new distribution system had to be properly designed. In 2016, I took a John Siegenthaler course on AWHPs and got some decent foundational knowledge on designing low water temperature distribution systems. People who work with ground-source heat pumps already know the options: radiant walls/ceilings/floors, high output baseboard, fan coil units, fan-assisted panel radiators, and just plain panel radiators.

I ultimately settled with panel radiators. They’re ubiquitous in other parts of the world, are mechanically very simple, don’t require power, and they’re relatively inexpensive. My room loads for upstairs varied from 1800 btu/h to 5600 btu/h. The final design included two large three-plate panel radiators (for the two larger bedrooms), two smaller two-plate panel radiators (for the bathroom and third bedroom), and a tiny two-plate radiator for a loft space.

It’s worth noting that someone accustomed to boiler installs would consider them grossly oversized. But for 120°F supply, a factor of about 0.28 has to be multiplied by the manufacturer listed output (at 180°F supply)—though they are perfectly capable of putting out useful heat at lower temperatures, most manufacturers don’t publish the output that low and you have to dig into technical literature to find it. Note that the large size of the radiators wasn’t a problem; hundreds of sizes are available, which allowed me to get creative in matching panel radiators to the unique space and window configurations of each room. And interestingly, selecting larger sizes didn’t come at a significant price premium.

Selecting the AWHP

At my home’s outdoor heating design temperature (-6°F) and an indoor temperature of 68°F, REM/Rate gave a design heating load of 31.1 kbtu/h. My own UA spreadsheet yields 32.3 kbtu/h.

Currently, I’m aware of five residential-sized air-to-water products available in the US: Aermec, Arctic, Chiltrix, Nordic, and SpacePak. All use R-410a refrigerant. Sanden makes an air-to-water heat pump with CO2 as its refrigerant, which was very appealing from a global warming impact perspective, but currently only markets a system for supplying domestic hot water (DHW); a modulating space heating version is being tested in the Pacific Northwest but is not available for sale.

I liked certain features of a split system, which narrowed things down to one manufacturer, Nordic. Though not familiar with installs using that equipment, I was comforted that they were a North American manufacturer (New Brunswick, Canada) with a product built around their long-established ground source heat pump line. Advantages of their system vs. a mono-bloc design were that it keeps most sensitive electronics and moving parts indoors, doesn’t require pan heat, crankcase heat, or constant circulation, and avoids the need for glycol, as only refrigerant passes through the building envelope. There are advantages to the mono-bloc design as well, which I’ll cover later.

All of Nordic’s units operate down to -6°F, though at low temperatures (5°F and lower, from my understanding) it will only heat water to 105°F. Right-sizing was important to me and I settled on the ATW-65 model; at 5°F, its capacity matches almost exactly my heating load calculation at that temperature.

Their heat pump is designed to integrate with a EcoUltra buffer tank (50 or 70 gallon options) with an electric coil to provide backup in case the heat pump is out of commission, can’t meet the set point, or the temperature drops below the operation limit. I chose the 70-gallon version and an appropriately sized coil. For what it’s worth, published heat pump performance suffers a lot at 5°F and below; the reality is we’ll mostly burn wood when it’s that cold out.

System design

Early iterations of my design (a couple years ago) were based on a mono-bloc heat pump and circulating 30% glycol through the whole system. As mentioned above, I liked the split system in part for the reason that I could avoid having to think about antifreeze and its maintenance.

Some other decisions we made:

  • Cooling: we opted against it; I didn’t want to deal with condensate lines and neither my wife nor I like A/C
  • Domestic hot water: after considering various viable ways to integrate it (the Nordic has built-in provision for DHW preheat), I decided to keep DHW independent
  • Piping: we decided on 3-pipe design for its direct-to-load pathway with less mixing in the buffer tank

My credo was simplicity, simplicity, simplicity, which also influenced decisions to leave cooling and DHW out of the picture. The result, I think, is mechanically elegant and repeatable. Many thanks to John Siegenthaler, who was a great thought partner and did the system schematic, and Nathan Mascolino of Efficiency Vermont, who is an incredibly knowledgeable advisor. (Disclosure: I work with Nathan at Efficiency Vermont.)

System schematic


Phase I of the project occurred in April-June 2018, when I selectively tore up the house interior to put in the new distribution system for the heating and ventilation systems (and redid the master bath). It was painful and took longer than we hoped. Is anyone surprised?

I used PERT (polyethylene raised temperature) tubing rather than PEX (cross-linked polyethylene). This stuff will only ever see temperatures up to 120°F and PERT can handle 180°F, so there was no need for PEX. There are advantages to both materials, but I liked that PERT is more easily recyclable and the cost difference was not significant.

After routing the distribution from the mechanical room and into the office (losing two small storage nooks), the distribution went up a level to serve the master bedroom. These two rooms both had to be padded out 1 1/2 inches to make room for the tubing to avoid a structural beam. A portion of the distribution then went up another level into a loft space and crossed into an unconditioned flat attic (covered with R-30 of air sealed insulation), before dropping down an interior wall and into the floors of three more rooms. This was all very inconvenient, but worked out well in the end and no one who sees the home would ever know.

Downstairs office wall. Ventilation (large white tubes) and heating (orange) lines coming from mechanical room (left side of photo) and heading upstairs. This room is below the master bedroom. This wall and others were measured and photographed for posterity, then insulated (for noise) and then finished.

The project paused for nearly a year at this point. We left the boiler in place to serve the downstairs through the 2018-19 winter. We burned wood to keep the upstairs warm enough. We sleep on the cool side, so this was acceptable for one winter (and one winter only).

The mechanical room was left in this state for a year. Everything was capped.

Phase II occurred April-June 2019. The boiler and indirect tank were extracted, our propane tank was disconnected and dug out of the front yard, and we put in a heat pump water heater to have continuity in DWH. The air-to-water heat pump installation then took place.

I was fortunate to work with a quality-focused and conscientious HVAC contractor Bill Chidsey of Solar Harvester throughout. We worked together to put in piers below the frost line for the outdoor unit and prep the indoors. He was diligent with his handling of ACR tubing (cut, ream, min. 15% silver, dry nitrogen purge, vacuum), which is critical in order to avoid refrigerant contamination, minimize future leaks, etc. This is where a mono-bloc system would shine—you avoid refrigerant lines altogether and the necessary precautions including great care taken to minimize risk of flexing and damaging joints in the long term.

Timewise, I was under pressure to get the job done (my wife was pregnant with twins), so it took some active management to coordinate the sequencing of heating, ventilation, and electrical work that had to take place. But it all worked out. I got the Zehnder ventilation system in and commissioned. Bill’s copper work in the mechanical room was impeccable and we put in a new manifold for the ground floor that would allow me to balance the highly variable ground floor slab loop lengths. An identical one was used for upstairs as well; these are compatible with zone valves if needed/wanted in the future, and each has an extra connection—for example, to facilitate installation of a small emitter if it’s determined in the future that the heat pump water heater over-cools the mechanical room.

Manifold for upstairs zone

Commissioning went pretty smoothly, though an error in Nordic’s instructions on the buffer tank setup cost a fair bit of time to troubleshoot (hopefully the manual has been updated by now). When we needed to contact their technical support, they were responsive.

Mechanical room and outdoors after final installation. Left side (from left to right) shows the Rheem heat pump water heater, the Nordic indoor unit, and the EcoUltra buffer tank.


The system is working well. Operation is straightforward: all the heat pump does is heat water whenever the buffer tank falls below a delta-T on the setpoint. The downstairs zone is controlled by a programmable thermostat—we chose EcoBee largely for its online portal that allows downloading of temperature and relative humidity data. A call for heat activates the single Grundfos smart circulator pump (set to constant flow) for the radiant slab loops.

The upstairs zone is also controlled by a programmable thermostat located in the mechanical room, but I’m not using it in a conventional way. I began the winter with it set to 80°F at night (when we want heat upstairs) and 50°F during the day (when we don’t need it)–basically functioning like an on-off switch on a timer. Each of the five panel radiators has a thermostatic radiator valve (TRV), and we have them dialed in to the temperatures we want: two of them are set to stay in the 50s, two keep rooms at 60°F, and the room where we all sleep is set to 68°F. This will of course change as our family grows and we start using more rooms. The TRVs are pretty slick (for more on how they work, this is a good primer). This zone is served by a single Grundfos smart circulator pump (set to constant delta-P) that adapts flow based on what the panel radiators are calling for. It’s a nice way to minimize both control complexity (i.e. things that can go wrong) and circulator electricity.

The Nordic has integral outdoor reset, which allows you to reduce buffer tank setpoint with warmer outdoor temperatures/lower building load in order to maximize COP.

Master bedroom panel radiator

One of the things I like about the design is how it minimizes extra electrical loads. As I mentioned earlier, the design of this heat pump means no constant circulation, no pan heater, no crankcase heater—items that could potentially chew up hundreds of watts 24/7. The circulator pump for the downstairs zone draws about 15W or 40W, depending on which constant speed setting I’m using (still playing with this). The upstairs circulator pump electricity use varies from about 15W-35W.


The system has a monitoring system installed: current transformers on circuits, a high accuracy turbine flowmeter on the heat pump-buffer tank loop, and some temperature sensors both integral to the heat pump and on the distribution.

The system is operating at a COP of about 3.0 at 30°F. At higher temperatures it approaches 4.0 while it dips below 2.0 when temperatures hit the single digits. My seasonal COP from November to mid-January is 2.49, which includes defrost energy.

COP vs. outdoor temperature, November 2019–January 2020. Blue dots and line show manufacturer published values.

My results closely align with the manufacturer specs and is higher than published data I’ve seen (so far) on other residential air-to-water systems. We’ve had temperatures drop below zero and it’s performing well. In fact, I’ve found that our distribution system works just fine thus far with 105°F supply water, which indicates that my heat load calculations (sized emitters assuming 120°F supply) were likely on the conservative side.

Later, I’ll process data from the entire winter and will look at total cost of this winter’s heat (new system vs. old system at current propane price), greenhouse gas impacts using 2018-19 ISO New England fuel mix data (new system vs. old system), and other items. I presented some of this—along with an overview of the project economics and several dozen photos—at the Better Buildings by Design conference in Burlington, VT in early February 2020 and will be doing a similar presentation at Building Energy Boston in late March.

For those exclusively tuned into payback, note that installing one of these is probably a big leap for an existing home, even with the generous $1000/ton rebate from (and currently still provided by) local efficiency utility, Efficiency Vermont. For new construction, I don’t think the case is hard to make. In most existing buildings, retrofitting the distribution system is a major hurdle; most of my pain (and expense) could have been eliminated had the original contractor put in home runs to the upstairs rooms.

In my opinion, anyone putting in a hydronic distribution system designed for 180°F is doing customers a huge disservice in terms of future retrofit-ability. Even if putting in a fossil fuel system today, consider:

  1. Install a home run to each emitter (or a supply and return loop with zone valves at the take offs)
  2. Size the heat emitters (radiators, baseboard, etc.) for 120°F or lower water delivery, and
  3. Mix down to deliver 120°F to the distribution (not strictly necessary, but a John Siegenthaler recommendation).

If you do nothing else, just do 1. Numbers 2 and 3 aren’t difficult with a bit of guidance.

Closing thoughts

Beyond dollars, cents, and COP, I’ll close with some thoughts on how this project has impacted our home. Simple payback be damned, this was a huge win for us. Why?

  • We no longer have comfort issues in our home; in fact, we have highly tunable individual room control that simply works. It’s an amazing upgrade. People are surprised that we can get high quality heat with nothing more than 105°F water from the heat pump.
  • We have no fossil fuels in our home—and thus a significant health and safety/risk improvement, which was very important to our family.
  • We paired this installation with a high-efficiency (balanced) fresh air ventilation system—measurements of indoor air quality before and after is a whole different topic (but has had a big impact that I’ve quantified with data).
  • Our first-floor slab heat is now balanced.
  • Removing the boiler, removing the exhaust-only ventilation system, opening up and tightening the mechanical room, and filling all the passive air inlets in the home reduced our ACH50 to 2.1 (a 10-15% improvement).
  • The heat pump water heater works symbiotically with the heating system, scavenging waste heat from the mechanical room slab, heat pump indoor unit, pumps, and buffer tank.
  • Our mechanical room has become a nice cool/dry zone where we now hang laundry (which we have a lot of, in cloth diapering our twins).

How do you put a price on all of that? Simple payback is the wrong measure.

I was fortunate to be supported by great professionals, including an HVAC contractor who did a lot of learning on his own time rather than charging the customer for it. There are things I’d do slightly differently knowing what I do now (system and buffer tank size), and I’m also excited for 3-4 new air-to-water heat pumps scheduled to hit the market later in 2020 that have some intriguing features.

In the meantime, I’ll sign off with my favorite photos from the project.

The day our propane tank was dug up and removed from the property.
Side effect: the mechanical room has become a fantastic passive clothes (and diaper) drying zone

-Brian Just is a mechanical engineer who manages a team of energy consultants at Efficiency Vermont. Photos and illustrations courtesy of the author.


  1. alexqc | | #1

    Thank you for this article. Not a lof of real user experiences on this heat pump so I'm glad to see it's a great system.

    I'll be building a new house next year north of Quebec city and this is exactly the system I'm planning to use. Since it's a new construction, I'll probably opt for radiant ceilings instead of radiators. The cost is about the same as installing a two zones minisplit here if I install all the radiant ceilings myself. And with minisplits since I'm in zone 7a, I would still need backup heating while this in built in the Nordic heat pump system.

  2. David_King | | #2

    I'm wondering how a passive house retrofit would compare in terms of payback and complexity. The same question would obviously apply to new construction. I'm also looking forward to your air quality review data.

    1. Brian Just | | #6

      Here's a quick snapshot. Typical nighttime CO2 spikes anytime bedroom door closed, in 1800 ppm range, recovering down to 600-ish during daytime (left side). Post retrofit, sails along at 800 ppm at night, gets down to nearly atmospheric levels during (unoccupied) daytime.

      1. Sandy Butterfield | | #37

        brian, this is very interesting and important for all of us who are creating very tight buildings now. we are installing an ERV. it would be fun to monitor CO2. what monitoring instrumentation did you use?

        thanks again for all this great info.

        1. Brian Just | | #38

          Hi Sandy,
          I used a TSI meter that was fairly pricey, but it was part of a larger research project (here's a short article on that work: But CO2 can be measured reasonably accurately by devices in the $100-300 range. A colleague of mine has a fleet of "Air Visual Pros" that are lent out to contractors and customers in the state of Vermont and she's had good luck with them.

          Whatever you purchase, if the device has a feature called "ABC" for Automatic Background Calibration, I recommend turning that off if possible. The way some devices to this is they take the lowest reading in the past 24 hours and call that atmospheric (~400ppm). If it's in a room that doesn't drop below 800 or 1000 ppm during the day because it's not well ventilated, I don't trust the ABC feature to maintain good accuracy.

          Good luck! In addition to being an interesting exercise, it can help you dial in flows based on actual room usage (by adjusting registers and/or flow at the ERV) if you're so inclined.

  3. Jennifer M | | #3

    Thank you for detailing your experience, Brian. I, too, am in New England (CT) and have been considering an ATWHP.

    My home is a 1950's ranch with easy(ish) access to all rooms from the basement. If this were your layout and you were planning to install only radiant panels with TRVs, would you have gone with one of the monobloc units? Did you prefer one?

    By the way, at some point in my HVAC journey I spoke to someone at Efficiency Vermont and he was incredibly helpful, even when I told him I didn't live in the state! Thanks to you and your team.

    1. Brian Just | | #5

      In a recent presentation, I suggested a decision tree for existing homes - see attached. Your home seems to fit the bill as a reasonable candidate. My personal decision to use a split system was not dependent on emitter type. If you only have radiators with TRVs, you could go either way. There are relatively few field studies at this point, so there's not a clear winner...

  4. David B | | #4

    Just my $0.02; for those predisposed to cool(er) homes in the summer, a hydronic system works equally well to heat & cool. Tekmar 406 will control the delivery of cool water temps ABOVE the dew point and you can also temper incoming ERV air too.

  5. Kieran Lavelle | | #7

    FYI Sanden CO2 HPWHs have been used in DHW+space heating applications successfully for years. They did undergo testing (successfully) and now we're seeing dozens of systems operational.

    Sanden HPWHs can deliver 175f water with no capacity loss to +5F and operation to -17F and beyond. Their output is relatively small, at 4.5KW, however, multiple units can be utilized for heat loads beyond 10kbtus/hr (leaving 6k for DHW).

  6. David Piranesi | | #8

    Great stuff, Brian. If there are any engineers or contractors in the San Francisco Bay area who are confident or adventurous enough to retrofit a residential A2WHP in the 2-4 ton range, by all means speak up. FWIW I'm in Brian Just's "new emitters" category on his decision tree. Of course the same would be true w/a minisplit or multisplit. Those might be my destiny partly because still not enough A2W talent available.

    Comment: Like Brian I don't need a/c either but if condensate drains are in place some mini splits and some A2WHPs do dehumidification, which I believe can be of benefit in marine climate zones, mine being 3C. I wonder if engineers specify controls for dehumidification mode with compressors that are not specifically rated for that mode (I could be wrong but it seems the Nordic and some other A2WHPs are not rated for it).

  7. Frank91578 | | #9

    Sounds like a nice way to waste money. System you replaced was from 2015. I know the way things are looking with fossil fuels and give incentives to replace with electric. Wish I had the money to throw away like that.

    1. Rick Evans | | #10


      Your insult-sentence fragment-insult sandwich is not very helpful.

      While the project may not be financially feasible for everyone, Brian's detailed article does demonstrate that air to water space heating is possible- even in a cold climate in today's market. As such, the article may become an important reference piece for others looking to build this type of system.

      1. User avater
        Jackson Wilkinson | | #13

        Hear, hear. Whether or not I would have gone this particular route, I'm incredibly grateful to Brian for both forging this path and giving such a helpful write-up of it.

    2. Brian Just | | #18

      The boiler I replaced was 19 years old - it was never stated that the system replaced was from 2015. I agree that had it been that new, it would have been wasteful.

  8. User avater
    Ryan Lewis - Zone 4A | | #11

    Thanks for the post!

    I’m curious: how would designs change if you did want cooling ?

    I am exploring a similar retrofit to my 1930s era Tudor.

    It seems I really need to work on lowering the loads before I can get a system like this to work.

    I also want cooling though. Any thoughts ?

    1. Brian Just | | #26

      If I wanted cooling, I would have installed condensate lines to all of the panel radiator locations and then put in fan coil units. This would have added a lot of headache given the circuitous routes followed and the fact that I would have then needed power at those locations too -- not easy in some of the rooms. But I've seen it done elsewhere (gut retrofit project with easier access to wall cavities).
      Most manufacturers and Siegenthaler's hydronics textbooks provide schematics to show how the system design would be tweaked to add in cooling.

  9. Jeremy Good | | #12

    Thanks for the detailed article. I've been watching the ATWHP space for a couple years and hoping to dispense with the gas boiler and water heater when we finish the basement. That would eliminate site fossil energy, in line with one of your motivations. While my 1930s house was almost certainly designed for 180º water, the radiators are pretty big. And the addition of Insulation and air sealing means a design day can be satisfied with 130º water... so in the ballpark for a heat pump.

    PS: You are brave to have two babies in cloth diapers simultaneously. I recognize those liners!

  10. CarsonB | | #14

    Thank you for the writeup! What was the installed cost of this system? I’m wondering if electric radiant + PV panels may make more financial sense, especially as it’s easier to DIY.

  11. User avater
    Nick Hayhoe | | #15


    First off, great article. I appreciate the analytical approach and attention to detail. I’m looking forward to updates as you gather data.

    Side note, my stress level rose when I saw the wall of tubes image, then you said you were expecting twins, then you said cloth diapers with twins! You are a brave/strong man. I have twin 3-year olds and couldn’t imagine adding in the effort of reusable diapers at that stage.

  12. Jason D | | #16

    I like the design of the Nordic with the compressor on the interior, but does that cause noise issues?

    1. Brian Just | | #27

      Nordic sells an isolation pad that goes under the indoor unit and an noise dampening jacket that gets factory installed inside the indoor unit. I wouldn't consider buying without both.
      I insulated the interior-facing walls of the mechanical room when redoing that room. But yes, we still hear it. Not loud, but it's there. We also noticed the boiler, for what it's worth.
      Ideally, I think in a slab on grade house I'd decouple the mechanical room slab. It's probably an easier sell in a basement for homes that have one.

      1. Jason D | | #28


  13. User avater
    David Goodyear | | #17


    Great article and I love Siegenthaler's work. Using constant pressure and TRVs are a great way to do a home run system. I used quite a few of john's ideas when designing our heating system with a wood boiler. I find the TRVs are fairly responsive and they make for a much cleaner mechanical room without all those zone valves, pumps, and power requirements for them. Like your system, we mix some of our return water back into the distribution supply in order to provide water temps based on our outdoor reset. The reset ratio is set pretty low. Lower than originally anticipated; at 0C we are delivering about 100F water which corresponds to about body temperature and it keeps the house quite comfortable. Once again, great article. Integrating hydronics and heat pumps is a great way to do retrofits on existing systems.

  14. User avater
    Adam McGowen | | #19

    Air source heat pumps are a no brainer for buildings with a tight envelopes. Especially those who already have solar pv installed. I always recommend integrating multiple heat sources for "staging". That way, only one unit will operate and create a minimal btu output when the heating demand for the home is low, and multiple systems working together will create the maximum btu output when the outdoor temperature drops really low and the home has a higher demand. Having a great buffer tank makes all the difference in a system as well. Check out Solar assisted Heat Pumps. They are new to the U.S. market, from England, but use only a 600w compressor to pump refrigerant through a coil inside a hot water storage tank and then outside to a black refrigerant panel which absorbs heat and sunlight to assist in heating the water before returning to the compressor. I've used them on multiple applications from radiant heating, domestic water heating, pool heating, and they are the best buffer tank I know of. They have a stainless steel coil to integrate solar thermal, or boiler backup as well. They have a backup electric element for additional btu output if needed, and a 25 year warranty.

  15. Brad Walker | | #20

    Brian - Thank you for the great article. I am looking at installing a similar system in a new build for my own house. I was a bit frustrated trying to incorporate radiant heat in my new house because I figured I would need to use a natural gas boiler until I came across the air to water heat pumps. It looks like this technology is a great alternative to mini-splits for people that want radiant heat.

  16. Jeremy Good | | #21

    Another thought and a couple comments:

    Do you have historical propane consumption data to compare with your load calcs out of curiosity? I had a Man J done with Wrightsoft and it, too, was pretty conservative (perhaps 30% over on our coldest days, which were below the design temp). That's compared to duty cycle data from our Ecobee thermostat and compared to the gas bill for reasonableness.

    "For those exclusively tuned into payback, note that installing one of these is probably a big leap for an existing home, even with the generous $1000/ton rebate from (and currently still provided by) local efficiency utility, Efficiency Vermont."

    I was quoted $10k by two contractors to replace my 1980s cast iron boiler with a lower-end mod-con boiler without DHW, buffer tank or distribution plumbing. I haven't yet sought ATWHP quotes, though I'm guessing/hoping they'll be in the ballpark. Is that folly?! I'm in the D.C. area and there aren't significant incentives, at least in Maryland.

    "In most existing buildings, retrofitting the distribution system is a major hurdle; most of my pain (and expense) could have been eliminated had the original contractor put in home runs to the upstairs rooms."

    It really depends. I have access to all radiator plumbing (two pipe) in my 1930s colonial from the basement, except for two upstairs bathrooms that share pipes. Removing the iron pipes below the basement joists and re-piping with home runs to a manifold is straightforward. (Boiler replacement is intended to coincide with finishing the basement.) The radiators were sized well, though a couple bedrooms overheat a bit and will be getting TRVs.

    1. alexqc | | #23

      To give you an idea, my quote for the 2 tons ATW was 10 750 CAD$ just for the unit itself.
      Buffer tank is 700$, pump 300$, controls 250$ and assembling all the components on a board is 500$. After that, you only need someone to install the unit and connect the refrigerant lines.

  17. Malcolm Taylor | | #22


    What a fantastic write up!

  18. Todd Hoitsma | | #24

    Great write-up and comments! I need to pull together data and do a write-up on my commercial and residential installs with Arctic HP (14 installs) and Chiltrix HP systems (3 installs). I have an Arctic HP/EcoUltra buffer tank radiant/DHW system on my house in Montana and have been very impressed with it even though this winter has not gotten much below zero F. Once I install an induction range I will be fully net zero - offsetting all electrical usage with solar PV. One thing I like with the A2W heat pumps is the service-ability. Unlike an A2A mini-split it would be very easy (!!!) to access and replace any electronics or integral components, and I much prefer having hydronic lines running to emitters (either radiant or low-temp hydronic emitters) instead of refrigerant lines. Although I have not run any COP measurements I can say that I have observed the Arctic units have output water temps of 120 to 125F at 2F outdoor temperature and that daily kWh usage on my small 20A HP at home is below what my calculations based on NG use in previous winters predicted. One other factor on the Arctic models is that there 3 output sizes (and a bigger 4th soon). This has allowed us to tackle bigger homes and higher commercial DHW loads (the commercial applications have been water heating only). Pls feel free to PM with questions.

    1. Jeremy Good | | #25

      I don't think GBA has a message feature, though I think I found your LinkedIn profile and will send you a message that way. Thanks.

      1. Todd Hoitsma | | #29

        Jeremy. You can track me down via my (outdated) Liquid Solar website. Happy to discuss

    2. Colby_Mekus | | #31

      Great article. I am designing a system now similar to ours for my house as a retrofit (switching from gas furnace to radiant. Gyp over pour). At the end you mentioned several A2W HPs coming out in 2020. Can you advise of these. So I can look out for them. Curious if daikin is one of these.

      1. Brian Just | | #33

        Hi Colby, no Daikin that I'm aware of. Check out: SpacePak (new monobloc, new split), Taco (based on EU Dimplex), Enertech (based on EU Nibe), and Stiebel Eltron.

  19. Kurt Hanushek | | #30

    The 120F supply temp has immediate payback even starting out with a gas boiler as that will operate a modulating-condensing boiler in the 95% efficiency range. Once it goes above about 140F, the condensing stops and it becomes a regular boiler at about 80% efficiency. The ability to switch to a heat pump system in the future becomes an added bonus.

  20. Mark Ganser | | #32

    Thank you for your article. The information is very useful as we start construction on our home in coastal Alaska. Utilizing John Siegenthaler’s book we have determined a combination of low temperature emitters and slab heat would be best for us, so your design looks very applicable. Hopefully you might answer a couple questions as we make final decisions.
    Could you explain why you chose not to incorporate the Nordic desuperheater function to preheat your DHW? It seems the DHW and heating system would still work independently.
    Was your choice of ventilation system made primarily on the space you had available for duct work?
    I am looking forward to seeing the performance report from your first winter.

    1. Brian Just | | #34

      Hi Mark, good questions! Here you go:
      - I like the independence and simplicity of keeping DHW completely separate. Adding another tank and more piping (see schematic from Nordic manual below) was something I wasn't interested in for the 6 months of the year I'd utilize it. But this could certainly be something you do.
      - My choice of ventilation system was based on heat exchanger performance, filtration, controllability, and space. (1) The Zehnder cores have incredible SRE and I was happy to pay a modest premium (which ended up being a minor fraction of installed cost) to have warmer air delivered on cold days. (2) It's a shame that so many HRVs and ERVs don't have the capability for MERV 13 or better filtration. I wouldn't touch a ventilation unit that doesn't allow this. (3) Rather than a single or a couple fixed speeds, the ERV I chose has great controls that can be dialed in to provide exactly what you want, where you want it. (4) It would have been impossible to get ductwork where I needed it, so the tubing was what made this retrofit possible in the first place.

      A few notes from winter #1:
      - I set up the ERV to supply 15% more than it returns. This helped the woodstove run better, on days when we used it. Last winter I did a long-term radon test (when our home was still exhaust only). Between the balanced ventilation and the fact that we're slightly pressurizing, I look forward to comparing results with the second long-term (6-month) test that I'm currently taking.
      - ERV chugs along consuming 28-30W, about double that when we boost during cooking. I like that the ERV we chose doesn't resort to recirculation as its defrost strategy. I.e. we're always getting fresh air. Its onboard resistance heating element really only kicks in below 15F (it used about 50 kWh during the whole winter).
      - We continue to love the room by room controls. We're at the portion of the year when we always stopped using the boiler, but we're leaving the nursery at 68F and still do some slab heat. It works great, and the AWHP doesn't run very often and has high COP in the shoulder season.

  21. Sandy Butterfield | | #35

    GREAT article Brian. completely relevant for my oil to AWHP retrofit project near Portland ME. i'm a wind energy engineer/researcher/CEO coming from Boulder CO that had no intention of heating with oil so our house purchase was predicated in making the conversion. i wanted a wood shop and my wife wanted a indoor endless pool and we both wanted radiant heat in these additions plus the main floor and A/C on the top floor, mostly for dehumidification. hydronics offered all these options. the house we ended up with is much bigger than we planned on but it has great southern exposure, location, real estate needed for additions so we gulped and bought it. in the end we were able to re-insulate the entire house with spray foam (we found extensive rot in the sheathing... in a 20 year old house!!!) and add new Alpen Windows (R-9 fixed and R-6.8 casements) throughout. that completely changed the heat load profile. we are using Arctic AWHPs because of their low temp performance. i want to track performance so i'm interested in how you extracted the the performance data that you published. is that data available from the Nordic controller?

    another detail, we added 23 kW of PV in December. we have been producing WAY more energy than we can use till the heat pumps going in this summer. we will have banked enough energy to keep us net positive for quite a while.

    thanks again for the great article. i hope to hear more as you (and i) go through our first winter on heat pumps!

    1. Brian Just | | #36

      Nice work on re-insulating. Nordic's indoor unit provides the water temperatures necessary for the calculation, but for your system you will need to get some temperatures so you know the water delta-T across the heat pump plus flow rate. Some meters do this in an integrated fashion, such as the Onicon BTU meter. And separately, you'd want to meter electricity usage of the circuit. Good luck with the system!

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