Utilizing a Heat Pump Water Heater as a cold weather backup for an Air to Water Heat Pump
Hi All. I am in the process of building a custom home in Colorado around a 5 ton Arctic Air-to-Water Heat Pump and a combo of in-slab heat and low-mass wall and ceiling panels. The sad reality is that when temps drop, the BTU capacity and COP of heat pumps also drop dramatically… just when we need them most. So, I see many installations use a several-kW resistance heater to provide additional heat when there's not enough to go around outside (OR a mod con gas, biomass, etc. backup boiler)
So… I had a "light bulb" moment that may or may not be a good idea. What if I utilize our (not purchased yet) DHW 80-gallon heat pump water heater running through an old 40-gallon boiler sidearm to pump some 140 degree water through the heat exchanger and add additional cold weather heating capacity that way?
I could purchase a stainless steel pump and a controller to pump water from my HPWH through this added "backup heat buffer tank" to my existing system's buffer tank and take advantage of the indoor temperature of the air in my mechanical room at a much-higher COP than the Arctic struggling away outside in the frigid cold. And perhaps, add another radiant heat panel in that room to account for the temperature drop from the HPWH's compressor.
Am I out of my mind here with additional complexity or something else I haven't thought through? Thanks in advance!
Comments
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The HPWH takes heat from inside the building to warm hot water. So asking it to do space heating is basically asking it to be a perpetual motion machine.
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I don't think it's a perpetual motion machine, it's just a 2-stage heatpump (assuming the mechanical room is heated by the Arctic A2W unit). To get it to do something 'useful', you would need to be running the Arctic at a lower temperature than you otherwise would so that you get higher capacity out of it.
If you're building a custom house, though, you probably just want to design it to heat with 85F water at the design temp rather than needing 140F water.
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Cascading heat pumps.
We looked at it from a different angle, using a HP to heat the home and a HPWH for DHW. You take a bit of a COP hit, but still a better option than resistance heat.
Not a lot of capacity in the HPWH, especially if you have a 120V version?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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Yes! The "2 stage heat pump" is conceptually more like I was thinking. The big unit outside giving modest SWT's and the HPWH inside bumping it up a bit, when needed.
I'm currently looking at the Stiebel Eltron Accelera 300E with its 80-gallon capacity, 1" NPT connections, and various modes of hybrid operation to potentially enlist for these concepts.
Hot Rod, I found the Idronics #30 you screenshoted there. I'm currently absorbing the section on incorporating "resilency" into modern design principles. Great stuff, there, for sure. Thanks for all you and Caleffi do to bring hydronics concepts, research, and products to the masses, sir!0 -
I think this #25 is the most relevant issue for todays designs. How to get the adequate heat at the lowest SWT. A must read for A2WHP use to stay out of trouble on design days.
My though is if you cannot adequatly heat the space/ structure with fluid temperatures at or below 120°F an A2WHP may not be you best choice. Dual fuel can help with short comings, but it gets to be an expensive, some times complex, system .
I tried a HPWH in my small 700 sq ft shop to prove the point. While providing 120 water temperature it was belching out 43° air into the same space. You will discover the same result. You can't cheat the thermodynamics at play here.
Heat pumps "pump" heat from a source to a sink. And flip depending if they are heating or cooling the space.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Let's go back to the beginning. We are suggesting a heat pump water heater — presumably inside the heated space (won't work in cold temperatures) to make hot water to heat the space…
Sorry. No joy. The heat pump water heater extracts heat from the space in which it is located to heat the water. That hot water obviously can't be used to heat the space… since the heat came from the space in the first place…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Thanks for the insights. Since I am in a new build, I do have some options on where to put the HPWH… a semi-heated garage, for one. But I understand the dilemma of fighting thermodynamics/ no free lunch, etc.
My time may well be spent better getting PV and solar water heating arrays going to offset the expense for the times when I need more heating kWh's. Or bite the bullet and bring in a natural gas feed, giving up on the dream of "net zero".
I was probably just tilting at windmills!0 -
Why not just get a heat pump that has the capacity you need?
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There are net zero homes, and now communities across the US and Canada from Maine to Florida, climate wise. Searching the Green Builder type sites will give you ideas on how it's done.
https://www.usgbc.org/education/sessions/building-net-zero-communities-12857381
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A Vermont home I remember used minisplits as the most efficient heat source. Certainly an affordable option, and you get heat and cooling. Solar thermal, solar PV, battery storage is free in some areas if you play the utilities game.
www.dsireusa.org will show the programs in your area to help with RE programs.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
It really doesn't matter where that heat pump water heater is placed. It has to be in an at least vaguely warm space (most shut off at around 50) and it will COOL the space in which it is located.
Fighting thermodynamics isn't a dilemma at all. Thermodynamics is absolute, and there is no way you can change it.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@hot_rod : "Solar thermal, solar PV, battery storage is free in some areas if you play the utilities game."
If you're lucky enough to live in a place with net metering you don't need battery storage and all-electric is going to be cheaper and simpler than solar thermal.
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If you're going to be all-electric, I would recommend a heat pump water heater for your domestic hot water.
Now, with a heat pump water heater, there's two ways that it could be configured, which I'll call one-stage and two-stage. In the one-stage configuration you use the same air-to-water heat pump as you use for space heating for domestic hot water, the heat for both is drawn from outside the house. Pretty much every vendor of air-to-water heat pumps offers a one-stage solution. In the two-stage configuration you have a standalone heat pump water heater inside the conditioned part of the house. That water heater removes heat from the house for the hot water, which has to be replaced by the heating system (during heating season, at least).
That heating system can be anything, but if it is a heat pump, you then get the question of which configuration is more efficient. I will say this is a contentious question and I'm not aware of a definitive answer. My personal belief is that in all but the mildest climates the two-stage solution is more efficient.
A discussion of the two configurations is here: https://www.greenbuildingadvisor.com/question/where-is-the-most-efficient-place-to-get-the-heat-for-hot-water-from
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It looks like I will end up needing another three-"ton" unit when I finish out my second floor, as "tons" are farily meaningless with the heat pump game how they are rated. There will be a string of days here below zero F. where my "five-ton" Arctic unit is barely pushing three tons of 95 degree F SWT.
Manual J/S/D calc was 54,128 BTU/H on a 3 deg. F. Although I have gotten more agressive with insulation than what was on my original approved drawings, it's extremely doubtful I can get it down to 36k for the Arctic.
I indeed might do mini-splits for the upstairs and zone it off, as we won't get too many visitors when it's below zero anyways. The summers and falls are the high season here for family and friends.0 -
sizing to the heat load makes for a bad air conditioner.
consider 2 systems.0 -
My feeling is that the best comfort is air-to-air sized for the cooling load and air-to-water for the rest. It's a lot simpler to get air-to-air cooling working. Something like this Mitsubishi M-Series:
https://ashp.neep.org/#!/product/249942/7/25000/95/7500/0///0
It has "HyperHeat," which allows it to keep a lot more of its capacity in cold weather. At 5F it has 87% of its capacity at 47F.
On that page, if you click on "Advanced Data — System Sizing" and put in your zip code you can get historical weather data for your location. In particular, look at the average annual low, which will be the lowest temperature they report information for. My recommendation would be to size for heating somewhere between the design temperature (99th percentile) and the average annual low.
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You can look at other units. Not sure if still being sold, but the 5ton version does close to 4tons of heat at 0F.
Most units can also be had with a built in resistance element which can get you a decent boost for those polar vortex days.
I'm with @DCContrarian get an air to water to handle floor heat in the areas you care about and have a separate ducted cold climate air to air heat pump for the rest of the heat load. This gives you cooling, fresh air distribuion (tie an ERV into it) and redundancy. Minisplits are best left for retrofit.
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Hi All. Thanks so much for all of the insight, ideas, and advise. I really appreciate this community here at H.H.
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