Venting cold air from Heat Pump Water Heater to encapsulated attic?
maine_dorothy
| Posted in General Questions on
Our new Maine home , climate zone 6, will have HPWH in conditioned basement. Is there any reason we should not vent the cold air to encapsulated attic? The roof is vented using ridge vent with 2 layers of plywood separated by 2x sleepers (hope that roof terminology is right.)
Most of the ceiling follows the roof line, but there is a significant area with a dropped ceiling where the cold air would be vented.
Thank you, Dorothy
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
The output from a HPWH should go into the conditioned part of the house. If you vent to the exterior, the air you output has to be replaced, and it's going to replaced by outside air that's quite a bit colder than the air you're venting.
Is there a problem with just letting it flow into the conditioned basement?
In our last house, the basement (only used for storage) in last house got very cold from HPWH; This new house with walk out basement has much more lived-in space and I don't want it to get so chilled. Though the HPWH is in a utility room.
Also, though there is no direct heat to "attic" all insulation is under roof, not in attic floor, so there is semi-conditioned space within the envelope in "attic" areas with dropped ceiling. Make sense?
Doesn't make sense. There is no "semi-conditioned" space, it's either inside the building envelope or outside.
The heat pump water heater should be entirely inside the building envelope. It doesn't really cool that much, especially compared to the heating load in a place like Maine, but if you want to send the cold air someplace in the house where it's less noticeable, that's fine.
Most water heaters have pretty weak fans so if you're ducting any distance you'll need a booster fan.
DC Contrarian, below, mentions another thread. You should find that. I read it once. It was very long and somewhat contentious, but very helpful.
You may find this report of use, but I acknowledge it is not your specific context: https://www.nrel.gov/docs/fy16osti/64860.pdf.
This is a potentially useful resource for folks in the Pacific Northwest: https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-23526.pdf
There is a narrow band of temperature where it might make sense to use outside air -- if it's warm enough that you don't need heat, but cool enough that you don't need cooling. Only if your climate is within that band year-round does it makes sense to even consider outdoor air.
If it's hot enough outside that you need cooling, the HPWH cools -- and dehumidifies -- the inside, giving free air conditioning.
If it's cold enough outside that you need heating, it's more efficient to heat the interior of the house with the heat, and then run the HPWH with that heat, than to run the HPWH with the colder outside air.
In particular, I disagree with this statement: "Cold outdoor air reduces efficiency of water heater, but the reduction is minimal compared to overall house HVAC savings (due to not having to heat the house air from which the water heater scavenges heat)."
There's a thread in the archives where we went through this.
Re: your disagreement with my statement, I am not just speculating in those bullets, which I specifically conditioned with the context of being for my location in the PNW. I am summarizing points of the PNNL study to which I linked. I realize my post here may be confusing because OP is in Maine. I meant this to be useful specifically to anyone in the future reading this from Pacific Northwest — to which the PNNL study is pertinent.
The PNW is a unique climate. I should not have overstepped by extrapolating from these bullets by saying the OP should bring in outside air for the HPWH. I do not have enough building experience outside of the PNW to suggest that, and the archived post you mention is great. I think it’s where I found that PNNL article or at least got started in my search before I found it. Anyways, I have removed that from my post.
I read the study. A couple of things jumped out at me. First, they were pulling air from the crawlspace, not outside. They write, "Crawlspace temperatures were maintained at 44.2 ± 2.2°F throughout the heating season due to ground
coupling, several degrees warmer than the average outdoor temperature of 40.0 ± 9.0°F." It's a big leap to assume where that heat in the crawlspace is coming from, it could be coming from the house above.
But more significantly, they were heating the houses with resistance heat. I'll be the first to agree that HPWH make no sense when you're heating with resistance heat. To say "Cold outdoor air reduces efficiency of water heater, but the reduction is minimal compared to overall house HVAC savings" is really an apples and oranges comparison when you're heating with resistance heat.
These are fine points, and while I spent about a half hour re-reading the study, reconsidering my stance, conceding many points, and defending a few, I think it's not worth either of our time for me to put that out into the world :) in light of the existence of several fairly long threads on the topic (some links below).
Big picture, you're right and I had forgotten a number of important aspects of the study that are fairly context-specific, so thanks for pointing those out.
One takeaway after re-reading that I think may be useful for OP is that putting the HPWH in a closet with louvred door (or similar and to spec of manufacturer's placement guidelines) may noticeably reduce the effect of an unducted HPWH on whole-house and room temperature -- the PNNL study observed ~ 1-2 degrees F in the main spaces vs the specific ~ 5 - 10 degrees F drop in the closet. This is what I ended up doing.
Some potentially useful links:
• https://www.greenbuildingadvisor.com/article/attaching-ducts-heat-pump-water-heater
• https://www.greenbuildingadvisor.com/article/living-with-a-heat-pump-water-heater
• https://www.greenbuildingadvisor.com/article/where-does-the-heat-pump-water-heater-go
• DC mentions an old archived Q&A thread on the topic. If we're thinking of the same one, it was great, but I'm having trouble finding it. If I find it, I'll reply with it here.
This is the thread I'm talking about:
https://www.greenbuildingadvisor.com/question/where-is-the-most-efficient-place-to-get-the-heat-for-hot-water-from
The study seems to presume that you want to vent your HPWH to the outside, and asks the question whether you want to get the makeup air from inside or outside -- or more properly from outside through the building envelope or through direct vents, because it's coming from outside either way. It doesn't seem to ask the question of whether you should be venting to the outside at all.
It also gives short shrift to the benefit of cooling in the summer. In my climate we do about as much cooling as heating and that's a real benefit.
thank you for your input and links. very helpful in sorting this out.