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Community and Q&A

Ducted heat-pump water heater

Norman Bunn | Posted in Mechanicals on

CZ 3A – Greenville, SC – mixed humid

I have been reviewing some studies online regarding ducting a heat pump water heater and think it might make sense in my new house. Here are the results from the study above, which assumes a garage located heat pump WH in central Florida:

Cooling Season Savings
Indoor to Indoor 3.8%
Outdoor to Indoor –1.2%

Heating Season Savings
Indoor to Indoor -5.9%
Outdoor to Indoor -17.5%

Based on this it looks like using indoor air for intake and exhaust makes sense during the cooling season, though since I am in a milder climate my savings may not be that much.

My heating season, which is sure to be longer than Florida’s, shows this arrangement to be a loser, so an alternative arrangement might make sense, such as Outdoor to Outdoor or Indoor to Outdoor (something they did not test).

My house has a conditioned partial basement (1,000 sq ft), a main level (2,500 sq ft), a loft (800 sq ft), and an attached garage, My current plan has the HVAC AHUs for the basement and main level in
basement/sealed crawlspace, with a mini-split handling the loft. The HPWH is currently planned for the basement under the master bath (shortest distance for hot water) and being fed by a drainwater heat recovery unit. I still think this is the best location, but wonder if I should duct the intake and exhaust elsewhere (based on the above) with the ability to change the configuration with dampers seasonally.

Scenarios I am considering:

1) Status quo. It’s in the basement, the indoor to indoor arrangement probably won’t have that much impact on your heating load and the cooling load savings is probably not enough to worry about. Why add the complexity?

2) In the winter, ducting from the conditioned crawlspace to the outside. This, of course, will create negative pressure inside, but only when the HPWH is running. It will also dehumidify the crawlspace when cycling. I could run a pressure switch from here to the fresh air damper to open it when the HPWH is running.

3) In the winter, bring outside air in and exhaust it outside. Probably not a good idea, since the HPWH are not rated below about 38 degrees, and this may create a dependence on the resistance heating elements when it is chilly outside.

4) In the summer, ducting from the main floor, where it should be warmer than the basement, generally, and exhausting to the loft (assuming the duct run is kept to specs), so the cool air falls.

5) Any other scenarios that some to mind?

Your thoughts and inputs are appreciated.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Norman,
    Q. "I wonder if I should duct the intake and exhaust elsewhere (based on the above) with the ability to change the configuration with dampers seasonally."

    A. You are asking for advice, so I'll give it: No, you shouldn't do that. The benefits of adding ducts to heat-pump water heaters are slight, and any task that requires seasonal adjustment will be forgotten within two or three years. Omit the ducts.

  2. Norman Bunn | | #2

    Thanks, Martin. I was leaning in that direction, but am always wanting to see it there is a way to kill two birds with one stone.

    If I may follow up...

    I have the option of placing the HPWH within the conditioned crawlspace or within the conditioned basement. Any advantage of one over the other? I could also put it at the juncture of the two and pull air from one to the other. The basement is a walkout, so only 2.5 of the four walls will be underground and have access to that thermal mass. The crawlspace will have 2/3 less volume, with 3 walls underground and the fourth along the basement.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Norman,
    Q. "I have the option of placing the HPWH within the conditioned crawlspace or within the conditioned basement. Any advantage of one over the other?"

    A. Not really. However, you should note that most heat-pump water heaters are quite tall. Before you plan to install a heat-pump water heater in your crawl space, make sure that it will fit.

  4. Steve Knapp CZ 3A Georgia | | #4

    Norman,

    Many HPWH are quite noisy. You may want to pick a location where the sound won't matter or can be easily isolated.

  5. Norman Bunn | | #5

    According to Rheem, their 65 gallon runs 49 DBA which is quieter than "Quiet suburb, conversation at home. Large electrical transformers at 100 feet."

  6. User avatar
    Stephen Sheehy | | #6

    Norman: I'd find one and listen to it. I was considering a HPWH (GE) and when I heard it in person, decided it was way too loud to locate in our mechanical room. If we'd had a basement, it would probably have been OK.

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