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Community and Q&A

Heat pump water heater in an unconditioned attic… any issues?

cykilbourn | Posted in Green Products and Materials on

I’m working with a builder building some townhomes in Orlando. One option is for them to install heat pump water heaters in the unconditioned attic (R38 blown cellulose floor). There are a couple concerns I have:

1. Will it be too cold in the winter? The 97.5% heating design temp in Orlando is 38F. I expect this, plus the small temp buffer from being in an attic, should make freezing not a problem. Does anyone have experience with this?

2. Due to height restrictions, the water heaters would have to be installed in the middle of the attics. This means there will be a large moment in the truss members, so they will have to increase the size of the members. Has anyone seen either a short (~3′) heat pump water heater or one that can operate on its side?

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  1. Expert Member
    Dana Dorsett | | #1

    You'd have to use a retrofit heat pump on a lowboy electric tank to keep the height down.

    Marc Rosenbaum retrofitted one of these on his last house:

    I'm not sure whether freeze protection is really called for in Orlando. Even though it gets well below freezing there, it rarely stays below freezing long enough for water to freeze in an attic. As long as you have R4 pipe insulation on all exposed plumbing it shouldn't be a problem (should it?).

  2. user-2890856 | | #2

    Hope you are prepared to tolerate not enough hot water .

  3. GBA Editor
    Martin Holladay | | #3

    1. The efficiency of the water heater goes down when it is in a cold space, and rises when it is in a hot space. The ideal place for a heat-pump water heater in Florida is inside the home's thermal envelope, because the appliance provides free cooling and dehumidification.

    2. If you end up putting the water heater in an attic, make sure that the attic space meets the minimum volume requirements established by the water heater manufacturer.

  4. Dana1 | | #4

    In climate zone 2 central Florida average attic air temperature over a year is well above that of the conditioned space. It's not at all clear if putting it inside conditioned space would improve water heating efficiency. (Probably not. My gut tells me it's the other way around, but I don't have data.)

    But putting it inside of conditioned space reduces the cooling load on the house a bit, so the net power use overall MIGHT be improved by bringing it inside. TBD- I'd like to see somebody actually measure it. It may come down to how the place is heated in the winter. If it's heated with a heat pump with no resistance heating it may be a net win, but if it's resistance electric heating coils in an air handler or electric baseboards it may tip the balance the other direction, since the 1/2-2/3 of the heat going into the water drawn from the room air is not being leveraged by heat pumps. The efficiency of the air conditioning systems are comparable to or better than the efficiency of a heat pump water heater, but the efficiency of resistance heaters is less than 1/3 that of air source heat pumps.

    It's an odd fact that annual peak power grid loads in Florida occur during the heating season, due to the prevalence of inefficient resistance electric heating, whereas in the cold northeastern state the annual peak grid load is from air conditioning.

  5. cykilbourn | | #5

    Thanks guys!

    Dana, I was wondering the same thing. Initially I figured it would actually lower electricity use to have it in the attic because of the 90+ F summer temps. Either way, in terms of efficiency, the only way to get the HPWH in is to put the attic - otherwise we're stuck with a .95EF electric tank. So I'm thinking it's a win as long as no one sees any dealbreakers.

    Richard, I'm not sure how to reconcile your comment. Why would there not be enough hot water? The first hour rating on this thing is 70g, which is actually better than the electric tank option. And I wouldn't expect cold attic temperatures to change that too much, these things are pretty well insulated... Do you disagree?

  6. user-2890856 | | #6

    It may occasionally fall short when there is less heat for it to utilize . First hour is a bit different than every hour after that . A water heater only offers about 2/3 of it's content as usable . You did not say what size or make and model , that would be helpful to know what you may be considering .

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