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Community and Q&A

Heat pump water heater—in Maine?

Jason Mann | Posted in Mechanicals on

We are nearing the end of construction on our house. We’re on an island in Maine.

The house is 2,400 sq ft., including about 600 s.f. of walk-in basement in one half of the downstairs. (The downstairs walls are ICFs with additional blueboard insulation, 2″ in the basment and 4″ in the finished space. There are 4″ of blueboard under the slab. Upstairs walls are double studs with 11 1/2″ of dense-packed cellulose.)

Our only heat sources are two mini-split heat pumps: one upstairs, one at the opposite end of the downstairs from the basement. We have a 9kW solar array on the roof.

Here’s the conundrum. I had been planning to get a regular electric water heater, installed in the basement because it would be most centrally located to the bathrooms and kitchen. (We have no gas or oil appliances, and propane on an island is crazy expensive.)

I like the idea of a HPWH like the Rheem models, but I didn’t want our water heater and existing heat pumps to be fighting each other in the winter months, when the basement will likely be in the 50s and the heat pump will be working hardest.

However, I do like the idea of a drastically more efficient unit. There is a state rebate of $750 right now on the purchase of any heat pump water heater, which basically makes it the same price as a standard electric model.

So now I’m wondering if I should get the heat pump model, run it in heat pump mode for most of the year, but then switch it to “electric” mode—in which it uses only the electric heating elements—during colder months or if it seems to be cooling the basement too much.

The model I’m most interested in is the Rheem: https://goo.gl/Svos3Z

Am I nuts?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Jason,

    You might want to read this article: https://www.buildinggreen.com/blog/heat-pump-water-heaters-cold-climates-pros-and-cons

    Another option might be a Rheem Marathon.

  2. User avatar
    Dana Dorsett | | #2

    The summertime dehumidifying aspect of a heat pump water heater ina a Maine climate still makes it "worth t", even if the net efficiency performance of water heater + space heating declnies in winter. The annual net energy use picture is still considerably improved, even if some of the heat for the water is being sourced by the mini-splits.

    Unless your family are hot water pigs the duty cycle of the water heater's heat pump is quite low, and the total heat drawn a reasonably small fraction of the capacity of the mini split. They're not "fighting", and during the shoulder seasons the additional load of the water heater even improves the efficiency of the mini split by keeping them modulating a bit longer rather than cycling when the total load drops below the minimum modulated output of the mini split.

    During the summer the dehumidifaction allows you to operate the mini splits in their highest SEER mode most of the time rather than "DRY" or "DEHUMIDIFY" mode more of the time to manage the latent load. The ratio of latent to sensible cooling loads is much higher in high performance homes, especially in temperate New England coastal zones & islands where the temperatures are moderated by the water temperatures, which lowers the sensible load without moving the needle very far on dew points (latent load.) A basement dehumidifier would just convert the latent load to a sensible load to be removed by the mini-splits, but a heat pump water heater converts it to hot water.

    If you haven't already considered it, a drainwater heat recovery heat exchanger downstream of the most-used shower does a lot for the "apparent capacity" of any water heater, and at the slow recovery rate of heat pump water heaters in heat pump mode it mattters! At island electricity rates it'll also pay for itself in a few years in electricity savings, even at an EF of 3 if yours is a showering family. a vertical section of drain of 5' or higher to accommodate a least a 4' 4* x 48" tall heat exanger (the tallest and fattest that fits is the "right" one), yields ~50% or better energy return to the incoming water stream, sending cooler water out the drain, raising the incoming water to room temperature or so. By sending that room temperature water to both the shower and the water heater, the mix at the shower takes less draw from the water heater to achieve showering temp, and the water heater only has to raise the temperature of the incoming water 35-45 degrees rather than 70F+ in the dead of winter.

    From a showering perspective the apparent capacity of a 50 gallon heat pump water heater approaches that of an 80 gallon unit, and if that makes the difference between staying with a 50 and upsizing to an 80, the heat exchanger is mostly paid for up front. It only works for showers though, not tub baths, since the potable water needs to flow while the drain is flowing.

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/toronto-passive-some-thoughts-drainwater-heat-recovery

    https://www.greenbuildingadvisor.com/green-basics/drain-water-heat-recovery

    https://www.greenbuildingadvisor.com/articles/dept/musings/drainwater-heat-recovery-can-lower-your-hers-score

  3. Jason Mann | | #3

    This is helpful, thank you! I hadn't considered the drainwater heat recovery exchanger, but will definitely look into that as well.

    Any thoughts on the noise issues? Rheem claims to be "quietest", but not sure how to approximate that level of noise.

  4. Norman Bunn | | #4

    Rheem specs show less than 50 db, which is about the same as in home conversation.

  5. User avatar
    Stephen Sheehy | | #5

    Jason: see if you can find a water heater already installed and listen to it in operation. We opted for a conventional water heater after hearing a hpwh. But We have a small house, on one level. If we had a basement, it would have been ok.

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