Efficiency of Heat Pump Water Heater at 45°F
HPHW in 40-45F basement
(I live in MA)
I’m slowly getting rid of all my gas appliances, and in removing our boiler and switching to a HP, the basement is now quite cold (typically about 40-45F). Yes, it is leaky, but we’re having insulation and air sealing done by MassSave in the few weeks.
I’d like to get rid of our aging gas HW heater and replace with Rheem Heat Pump Hot Water Heater (HPWH). Would a HPHW even be efficient at 45F?
We have 2 young kids… dont have a huge HW demand now, but will likely want as much capacity as we can in about 10 years.
Other options are pretty cost prohibitive: SANCO2/Sanden exterior HP with tank, HE Condensing Combi boiler, etc. If at all possible, I’d love to get rid of all things venting out the chimney.
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Mfg min operating temperature is 45F, but 40-45F seems unusually cold for a basement. It will probably be fine after insulation/airsealing basement.
Watch out for the height of the unit!
Very helpful-thank you. I'll hold off on installing until we've done the air sealing and such... crossing my fingers that the gas HW heater doesn't bow out before then.
The min temperature in the specs is for heat pump operation. Once it gets too cold, the unit switches to resistance heat and continues to run. I most cases this will only happen for a small percentage of the year, so overall, it is still the cheapest source of hot water.
I used to have a basement as cold as yours (100 year old house with 2' stone rubble foundation). Replacing all the leaky old windows and sealing up the rim joist area (especially under exterior doors on the main floor) made a huge difference. Now the basement is within two to three degrees of the main floor even with the uninsulated slab and foundation.
I would get the HPWH and work on getting the place tightened up down the road. If you are looking for extra capacity down the road, drain water heat recovery works quite well and pretty simple install if the showers are all on one stack.
The one nice side benefit of the air sealing work is the main floor is now much more comfortable since the floor is warmer.
I've noticed that rheem has a new 120 volt heat pump water heater that plugs into a 15 amp outlet and isn't a hybrid, there is no normal heating element, just the heat pump, and it is rated for use down to 37F. I could see getting the 65 or 80 gallon version of that, setting the water temp to 140, and using a mixing valve to adjust down to 110 or 120. Rheem's hyrbid versions are rated down to 40F for heat pump use but can also act like regular electric resistance water heaters is demand is higher or temps are lower. I'd try getting the basement temp up a bit even if not placing a heat pump down there. https://www.homedepot.com/p/Rheem-ProTerra-65-Gal-Tall-0-Watt-Element-Residential-Electric-Water-Heater-w-Leak-Detection-Auto-Shutoff-10-Year-Warranty-XE65T10HMS00U0/317100806
Yes, that 120v unit is the one I'm looking at. Other than electrical panel capacity and cost to install the line (which will be minimal in the grand scheme), is there any reason to not go with a 240v unit, though?
Post is a little old but wanted to share my experience. I had solar and powerwalls installed in July of last year and opted to get a Rheem 65 gallon heat pump water heater (HPWH) to reduce energy use. I am in South Carolina for reference, but it has been getting very cold here lately. Highs of freezing and lows of 18 degrees or so the last few days. My heat pump water heater has been working flawlessly. It is in my 2 car garage which is an unconditioned space, but does share 2 walls with the interior of the house. My garage stays a good 20-25 degrees warmer than the outside temperature which is perfect for a HPWH. One thing to take into account is that even in "heat pump only" mode, the element will kick in if it sense the ambient temperature is 40 degrees or less. This is a hard cutoff it seems based on my testing, as I have a temperature sensor in the garage and can check the Rheem app to see what is currently running. My life left the garage door open for 2 hours the other day so I lost a lot of heat to the outside. So I would say if you can keep the space at 45-50 degrees or more year round you'll be totally fine. Really pleased with my purchase and it certainly has reduced my energy use a ton. Went with the 65 gallon to upsize a little bit from the standard 50 gallon. I have read some families on Reddit have gone with the 80 gallon if you have the space just bc the price differential isn't that high. I got mine delivered from Home Depot for reference also.
I'm curious why you chose to put it in unconditioned space.
People in the south seem to think it never gets cold there, even though there seem to be cold spells most years. From what I understand, there is currently more snow in parts of the south than the 3" we have in Maine!
We have about 6" on the ground in DC right now. We are south of Mason-Dixon.
The thing that gets me is that here people routinely put plumbing in outside walls, and the pipes routinely freeze. Our design temp is 22F and average annual low is 6F, but people act all surprised when it goes below freezing.
A heat pump water heater should never be placed in a conditioned space as that defeats the purpose of it. You would be stealing heat from your house to heat water, discharging cool air into your house, thereby causing your HVAC system to kick on to heat your house. That is illogical. It is actually recommended by the manufacturer to place in an unconditioned or semi-conditioned space that stays above 40 degrees year round, like a garage or a basement. Basements don't really exist in the southeast like they do up north, but a lot of houses have an attached 2 car garage. My garage doesn't go below 40 F year round, except when my wife leaves the door open for 2 hours like I said earlier lol. It is a fantastic investment if you have the right property/space for it. If you just plan on sticking it in a closet inside your house, I'm not sure that really makes much logical sense. As a side note that was brought up, all my pipes in exterior walls are insulated. This isn't Miami, Florida lol.
Do you have a link to manufacturer instructions that say it should go in unconditioned space?
"All my pipes in exterior walls are insulated."
As someone born and raised in New England, this made me laugh out loud.
Sure do.
https://www.homedepot.com/p/Rheem-Performance-Platinum-65-Gal-10-Year-Hybrid-High-Efficiency-Smart-Tank-Electric-Heat-Pump-Water-Heater-XE65T10H45U0/312741511#overlay
Click the pictures and go to the tenth one and you'll see it stated almost verbatim to what I said.
"For best performance and efficiency, install in a warm, unconditioned or semi-conditioned space above 40F year-round."
Wow. I wondered where that was coming from, it turns out the marketing department is pushing it. I had assumed it was a YouTube influencer somewhere.
I gave you my experience as someone who personally owns one, you rejected it. I gave you the science behind it, you rejected it. I gave you the manufacturer recommendations, you rejected it. It seems you just enjoy being a contrarian (username checks out).
Did it occur to you that your garage isn't magically warmed in the winter, that the heat going into the garage is coming from the house? So the water heater is in effect pulling heat out of the house, just at a much lower COP than if it were inside the conditioned space.
We've discussed this before, like in this thread:
https://www.greenbuildingadvisor.com/question/where-is-the-most-efficient-place-to-get-the-heat-for-hot-water-from
Rheem is wrong about this.
Sounds like your set up is working well.
It's true that putting it in a conditioned space will introduce cold air, requiring it to be made up by the HVAC. But it would also be true that your HVAC would benefit from this in warmer months. Also the heat pump will use less energy generating hot water from 70 degree room temperature air rather than 45 degree air so that would also have to be factored in.
All that just to say it's probably not as simple as saying it is illogical to place it in a conditioned space.
Note that the actual installation instructions (at https://images.thdstatic.com/catalog/pdfImages/6d/6d277c0f-ad7f-4753-b3e1-a55fa917dedf.pdf )
say nothing of the kind. They say:
. Water Heater Location
❑ Close to area of heated water demand.
❑ Indoors and protected from moisture, wet
conditions, freezing temperatures (below 32°F
(0°C)) and High temperatures (above 140°F
(60°C)).
❑ Area free of flammable vapors.
❑ Provisions for Air Circulation (Louvered doors on
ducting).
❑ Provisions made to protect area from water damage.
❑ Sufficient room to service heater.
❑ Six inches (6") of clearance from ceiling to
top of Hybrid Water Heater to allow for filter
maintenance.
❑ Access to condensate disposal.
❑ Vibration Isolation Kit (Non-Concrete floors).
❑ Hybrid seismic Kit (if required).
Lol, I'm well aware of COP. My commentary is accurate and backed up by the manufacturer itself, which you disregard as "marketing" for some unknown reason. Rheem would want to sell as many HPWH as possible, so why would they use something as "marketing" that would narrow and limit their scope of consumer by limiting install locations? An illogical conclusion.
The heat is also not "coming from the house" when the HPWH is in the garage. The sun heats the garage and creates a greenhouse effect, similar to why your car is always warmer than the outside temperature when you park outside. Any heat the HPWH is "taking" from the house is normal, parasitic losses leaching through the walls and insulation.
The heat is however *directly* coming from the house when you place the HPWH inside your conditioned living space. You are discharging cool air *directly* into your home, thus making the thermostat kick your HVAC system on to replace the warm air you just *directly* stole. There is no barrier to the cool air you are discharging, such as walls and insulation.
There is a lot of energy required to start and then run a home heat pump, even the most efficient ones, so you would want to limit it's use as much as possible. On the flip side, my Rheem HPWH uses less than 400 watts per hour. My Trane heat pump uses over 4,000 watts per hour, a 10 fold increase in energy use. It would take 10 hours of my HPWH running to match 1 hour of my Trane heat pump running. Even if I drain my 65 gallon HPWH to empty in the winter, it takes no more than 3-4 hours to replenish it to full. And even that is an edge case as it usually only runs for an hour or so before to reach back to 120 degrees.
It's a pretty easy logical and mathematical conclusion to come to, you just seem to be making it incredibly complicated. Occam's razor wins again.
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