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Community and Q&A

Is make-up air req’d for a Conditoned basement heat pump water heater vented to the outside?

pNJRiLZxNH | Posted in Energy Efficiency and Durability on


I have a tight house (2.0ach50p) and want to install a heat pump water heater in the heated basement. I have no back draft appliances and the house is all electric with a fully ducted HRV system. I occasionally run the range top fan on low and a bath fan for 15 minutes or less (timer switch).

The contractor would like to vent the heat pump water heater through the rim so as not to cool the basement space and compete with the mini-split heating the area. I think this is a great idea as the plan is to move the colder air outside from the unit while still providing some drying for the basement space (the water heater is to be located next to the washer/dryer).

Do I need to be concerned with make-up air or is the CFM for the 50g heat pump water heater minimal?

thank you

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  1. GBA Editor
    Martin Holladay | | #1

    As you probably know, heat-pump water heaters don't need to be vented. What your contractor is apparently suggesting is that you install a duct to move the cold air produced by the heat pump to the exterior of your home.

    I think this is a bad idea. I assume that this ducting would occur during the winter, not during the summer. (After all, many homes can use the cool air during the summer.) If this proposed duct blows 100 cfm of air out of your house during the winter, then 100 cfm of outdoor air will leak into your house from the outdoors to replace the exhaust air. Any time that the outdoor temperature is lower than the temperature of the exhaust air in that ductwork, you lose.

  2. davidmeiland | | #2

    So... the house is heated by a heat pump, the water is heated by a heat pump in the conditioned space. The water heater is robbing heat that was generated by a 200% efficient appliance. The basement would be colder, but what's the loss here? Why duct to the outside, and actually, how would such a duct be set up? I have not seen a HPWH that is set up for an exhaust duct connection, so I assume this would be a piece of duct placed close to or against the housing of the unit. Hmmmm...

    And, Frank didn't tell us his climate and whether or not there are other places the heater could go, like an attached garage.

  3. pNJRiLZxNH | | #3

    Martin and David, thanks for the feedback. David, I'm south of you in Seattle, Marine zone 4, Puget Sound Energy ~10 cents a kwh. No attached garage. I haven't received more specifics on how the unit is configured and how it will be vented (other than a 4" hole through the rim). When I find out more, I'll pass. Wx:

  4. davidmeiland | | #4

    Frank, what about a vented attic?

  5. user-1075855 | | #5

    Frank, may I ask why you picked a heat pump water heated as apposed to an electric?

    I believe you could try to install a makeup air unit, and also work out an solution to condition the outside air while you're at it, but this is already far more complicated than it needs to ever be for a water heater!

  6. davidmeiland | | #6

    Jay, my take on HPWH is that they will pay back in a reasonable period in this climate. If he puts an electric tank in a conditioned space, he will be adding some 100% efficient heat (via standby loss) to a house that is otherwise heated at 200%, increasing his heating cost and his water heating cost (compared to a HPWH).

    Frank, could you find a way to hide a unit like this a short distance from the water tank, but outside (protected from the elements of course).

  7. user-659915 | | #7

    Standby losses are not a problem with a super-insulated high performance electric storage heater like Rheem's Marathon. I doubt you will ever see payback on a HPWH in your situation. They make sense in a majority a/c climate, elsewhere not so much.

  8. davidmeiland | | #8

    James, I have used Marathon water heaters on a couple of jobs, but I don't know that they are more efficient or better-insulated than other, cheaper units. The attraction is the plastic tank that will not rust out. If you go by energy factor, a Marathon standard 50 gallon has EF = .94, compared to a Ruud Professional series with EF = .95. I believe that insulation thickness is about the same on most of the good units.

  9. user-659915 | | #9

    Hey David, I agree that there are other less expensive products with similar performance to the Marathon. But even at the high end of these units I doubt you'd find that the cost/benefit of a HPWH would ever pencil out as a payback in the lifetime of the unit.
    It's easy to confuse unit efficiency with system effectiveness. First principles: in Seattle you don't need to be chilling your basement. Once you get into make-up air ducts, supplementary fans and other doodads, as Jay wrote, this is already far more complicated than it needs to ever be for a water heater.

  10. davidmeiland | | #10

    I think he'll get a payback if he can somehow locate the compressor outdoors. I don't think it's a big payback and in this case I would be more concerned about cutting a big hole in the basement wall, and the effect of an air leak there. Too bad the mini-split compressor doesn't have a second circuit that can be run to the water heater......

  11. pNJRiLZxNH | | #11

    Thanks All. I installed a ductless heat pump system with 3 indoor handlers last year and am happy with the system. It seems the climate here is ideal for these as it typically hovers around 40 degrees in the winter. ...but as was talked about, does that "works good" translate into a conditioned basement HPWH. Takeaways for me...don't vent it and if I have to break out the slide-rule or abacus once again, I might be on the wrong track. thanks.

  12. user-659915 | | #12

    David I agree that either an exterior compressor or a dedicated HW tap off the existing minisplit unit would be a great option. And I'm guessing the technology will be heading in that direction in the not-too-distant future.

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