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Community and Q&A

Vented vs. Unvented Mechanical Room for Heat-Pump Water Heater in Cold Climate

CheeseCurd | Posted in Mechanicals on

Hello – Designing an energy efficient home in northern Wisconsin with a crawl space. I want to use a Rheem HPWH, but should I leave it unvented in a conditioned mechanical room or vent it (in and out) to the crawl space? So many conflicting opinions on this on the web and the Rheem website is of no help. Thanks!

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  1. Expert Member
    Akos | | #1

    The unit can be installed in any conditioned space of sufficient size (~100sqft). In a crawlspace I would go for more area as the volume is much smaller than a typical basement. Check the install manual for the unit.

    So if your crawlspace is insulated and conditioned (which it should be, that is the insulation in on the stem walls not on the floor above the crawlspace and there is a supply duct from your furnace), you can use that without issues.

    Even if you are heating the house with a heat pump, it is still better to leave the HPWH in heat pump mode in the winter time as the overall efficiency is still higher than resistance only. A typical HPWH uses on average 2000BTU, so it will not effect the temperature in a larger space much.

    1. CheeseCurd | | #2

      Thanks for the reply! The HPWH heater will be located on the main floor. I can leave it unvented (minimum volume = 700 cubic feet) or I can vent it to the crawl space. I believe the thinking is that venting to the crawl space would minimize the cooling effect in winter on the main floor as well as provide dehumidifed air to the crawl space. The drawback is that the HPWH would be less efficient due to the colder intake temperature. Not sure which is the desired approach.

      1. Expert Member
        Akos | | #6

        Since your main floor is heated but your furnace, I would vent to there. Least likely to cause any comfort issues. Venting to the crawlspace can reduce the temperature there potentially cooling the floor above, not the best for comfort.

        HPWH have two sources of noise, vibrations from the compressor and air flow noise from the coil fan. Make sure to keep both in mind, you don't want any of the vibration to transfer to the interior walls and into the bedrooms nor do you want the fan to blow directly into the living space.

  2. user-2310254 | | #3


    HPWH generate a bit of noise. (The newest Rheems are supposed to be even louder.) If it is located near bedrooms, this noise might be an issue. On where to vent, you could include dampers that allow you to control the intake and exhaust.

    1. Expert Member
      Dana Dorsett | | #4

      >"HPWH generate a bit of noise. (The newest Rheems are supposed to be even louder.)"

      I've heard the opposite, that current generation Rheems run bout ~50 dbA which would be noticeably quieter than earlier generations, even quieter than many refrigerators. The dbA numbers are usually listed somewhere in the spec.

  3. user-2310254 | | #5

    Hi Dana,

    I should have noted that was an anecdotal comment from someone on GBA who owned an earlier and then current generation HPWH.

    FWIW, I had a second generation Rheem in my garage next to the entry door. Standing inside the entryway (dividing wall is insulated with spray foam), I could hear a low machine noise when the HP was running. It was not a big deal.

  4. CheeseCurd | | #7

    Thanks to everyone for your comments. So it seems as though leaving the HPWH unvented is the best approach. If so, the mechanical room will need to be at least 700 cubic feet. That’s pretty large, so I may just go with a smaller room with a louvered entry door being aware that it will produce some noise and cooling. I believe the newest Rheem units can be set on a timer, so perhaps it can be set to mostly regenerate during the night before hot water is needed for showers in the morning.

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