HPWH vented to *and* from attic
Question about where to duct air from for a HPWH. I’ve looked around a bit, but haven’t found an answer to this specific question, in my climate.
We are in the high desert of central Oregon (looks like zone 6b). Winter lows are typically in the 10s°F, and summer highs can reach 115°F (though not often). No humidity here to speak of. House is ~2000 sq ft, single story, metal roof. Pretty terrible insulation in the attic, but adding more soon. Ridge/gable end vents, but no soffit vents.
I want to replace our 30+ year old water heater. The house is older than water heaters, so it’s in a closet in the central part of the house that only allows one with a 20″ diameter.
I think I’d like to duct the intake air from, and then vent it to, the attic, to take advantage of all the hot air up there. I saw a study from Georgia about this, but the climate there is quite different than here. There should be no effect on the house air package, because all the air is from the attic.
Main source of heat is a wood stove, with ~8′ of triple wall in the attic that would bleed some heat. Other source of heat is mini-splits in a few rooms (including one in the stove room).
My main concern is if the attic got too cold during the dead of winter. There should be some solar gain during the day (at least until I put solar panels up?), and the wood stove should leak some heat into it. But I suppose if it does get too cold, it would just use the back up resistive elements instead, which I would consider a fair trade off.
Although since the max temperature looks to be ~147F, I may need to be concerned about that too?
The other option is to duct air from the room on one side of the closet, and expel it into the kitchen on the other side (or vice-versa).
Thoughts/concerns? Thanks in advance.
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One thought is you will lose the benefit of cooling the living space in the summer.
I'm not too concerned about not having the extra cooling during summer. It generally cools off into the 50s at night, and humidity is low, so we open all the windows and turn on the whole house fan. If we do that, most of the time we don't need to use the AC in the mini-split.
I'm mostly concerned about the attic being too cold during winter.
Get a device that records temperature or alerts on temperatures and put it in the attic.
I don’t think anyone here can tell you this is a good idea without measuring. Otherwise we might need to assume the attic gets too cold. You would also risk that the attic could get colder in than you measured this year. A temperature alert would help.
Fireplace heat can be uneven- can the HPWH take from hotter parts and or put cold back in them? Or put cold in the kitchen which benefits refrigeration, etc?
I took our outside temperature sensor and hung it in the attic. Results are... disappointing.
Yesterday highs were in the mid 60s, and lows dipped just below freezing. The attic this morning was right at 32F. Right now in the afternoon (also mid-60s outside), the attic is in the low 90s.
In the dead of winter, the attic will probably be at or under the 37F limit for the Rheem HPWHs I looked at most of the time.
I can fairly easily duct it to/from the kitchen and my office. I'm not sure it's worth having a way to switch between the attic in the summer, and the house in the winter. Although, this damper has a seal, and appears to completely block the air. Four of them would quickly & easily swap from attic to house. https://www.amazon.com/PATIKIL-Adjustment-Airflow-Backdraft-Ventilation/dp/B0BRK728W3/
To duct the input to the HPWH you'd probably need a booster fan. While they can typically support 30+ feet of duct on the output, in my experience it takes a very slight amount of resistance on the intake to cause the compressor to shut down.
Correct. The manual for the Rheem unit I was looking at gave specs for that with different sized ducts, though I'd have to look again to see if it specifically said anything about the intake vs exhaust.
Either way I'd use 8" ducting.
It seems your climate is too cold to be switched to venting to the attic for more than 2 or 3 Months of the year unless it was from an automated temperature sensor that switched it during times of day. Automated systems are A lot more effort to install and make sure they are working right. There is an article on this site that I am having trouble finding but in it Martin explains how there is very little energy savings in venting A heat pump water heater outside certain times of the year. If you were to pay HVAC company to do the ducting, you would never make your money back. It’s a lot better to spend your time and money on other aspects of your energy envelope
I looked up the average yearly temperatures for my area (should have started with that, haha), and the lows are only above freezing for about 5 months.
I'm planning to do any install myself (which I realize a lot of folks won't be able to do), but even with manual dampers, I think it may be more hassle than I want.
I think the plan now is to draw from the kitchen on the other side of the wall, then run the exhaust through the attic and back into a hallway ~18' away. Or, directly into my office opposite the kitchen.
In the winter the heat will mostly come from the (certified) wood stove, and otherwise the heat pump(s). Even with the heat pumps going twice, it should still be more efficient than straight resistive.
As I sit here, it's around 80F outside, and 107F in the attic. It would be nice to steal that heat! Ohwell.
Thanks for the thoughts!