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Community and Q&A

Heat-pump water heater for an all-electric garage apartment?

Robert Hallenbeck | Posted in Mechanicals on

We our building a detached garage w/ 2nd story apartment (1 bedroom, ~900 sq ft conditioned space) & debating the system to heat water:
– Primary purpose for the garage apartment is for our mothers to live in as they age. Mainly for safety reasons, we’re not having gas service to the apartment (could do relatively easily – main house has gas service)
– Cooling-dominated climate (Houston TX)
– HVAC: When we installed a ground-source heat pump for the main house several years ago, we pre-drilled 2 extra wells for the garage. I wouldn’t do a GSHP ever again, however the wells are sunk costs…since we have them, might as well use them.
– Apartment won’t be occupied all of the time (mothers may go elsewhere for Houston summers; we’re assuming 50% occupancy). If we have a tank water heater, we’d turn it off when not occupied.
– To help keep garage tolerable in summer, all garage walls will be well-insulated (2 x 6″ w/ spray foam + 3/4″ exterior rigid foam) w/ insulated garage door

I like the idea of Heat Pump Water Heaters (
– System would go in the garage
– Good fit for our climate (additional cooling in the garage is desired almost year round…mid 80s in early February)
– Models seem to be ~2x as efficient as electric resistance units
– Noise isn’t an issue (condensate drain can be included in design

1. Upfront cost are higher (how much higher?)
2. Reliability unclear (most of the posts I’ve seen are 2+ years old…has reliability improved?)
3. If system isn’t used 6 months of the year, how much energy will we really save?

If we didn’t do a heat pump water heater, what’s the next best option? Tankless electric? Tank electric tied into our GSHP with a desuperheater? Something else?

Ideas/comments appreciated

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to see an energy-saving payback for more expensive water heating equipment, the payback will be faster for a house with high usage rates for hot water. If your mother and mother-in-law don't use a lot of hot water, and are only in their apartments for half the year, the payback period for the upgrade from a conventional electric-resistance water heater to a heat-pump water heater will be very long.

    You might get a better return by investing in a PV array than by investing in a heat-pump water heater for the use you suggest.

    I don't think that it makes any sense to try to supply hot water to a detached garage from the main house -- the line will be too long, and the wait for hot water will be unacceptable.

    I'm guessing that a conventional tank-style electric-resistance water heater makes the most sense for this application. That said, a heat-pump water heater would certainly work.

    -- Martin Holladay

  2. Charlie Sullivan | | #2

    It's a perfect application for a heat pump water heater, given that cooling and perhaps dehumidification of the garage are nice bonuses. Except that you say the usage will be low. So the payback will not be as good as in a heavier use scenario. You have to decide your priorities there.

    Depending on the desuperheater setup you have now, and its production compares to your usage, you might do well by feeding the tank for the apartment, conventional or heat pump, with water pre-heated by the desuperheater. I would not run a desuperheater loop to the remote tank, but for long draws such as long showers, you might do well with a small-diameter, insulated feed line fed from a pre-heated source, if the distance isn't too far. Do you have a two-tank setup now, one heated by the desuperheater and the other topped off by electric? If so, that could make sense. And if not, you might consider upgrading to that setup anyway.

    This wasn't your question, but I'd suggest reconsidering your insulation configuration. Filling a 2x6 wall with spray foam is a big expense, and you undermine its performance with the thermal bridging through the studs. You'd do better to fill the 2x6 wall with fluffy insulation and upgrade the exterior insulation to 2" polyiso. Or you could save some money by using fluffy insulation in the walls plus 1" exterior polyiso and have close to the same performance as the design you have.

    If you do use spray foam, open-cell would be a good choice, for its lower cost and lower climate impact--most closed-cell spray foam is blown with a gas that has more than a thousand times higher climate impact than CO2.

  3. Reid Baldwin | | #3

    Why are you planning to put the HPWH in the garage as opposed to in the conditioned space of the apartment?

    Are you currently using the extra two holes for the GSHP for your house system or are they sitting idle? If they are sitting idle, including them in the house system would make that system more efficient.

  4. Robert Hallenbeck | | #4

    Martin: I forgot to mention: the garage apartment will have a 5 kW PV west-facing system on the roof (biggest we could fit); should supply ~50% of electrical demand for the main house; less once we include garage apartment load.

    Charlie: The main house doesn't have a desuperheater to pre-heat the hot water; it simply has a gas-fired on-demand hot water heater from Rinnai. Other than a ~15 second delay to get hot water in the morning, the system works fine. We were always going to have has in the main house for cooking...since gas was there anyway, decided to stick with a on-demand hot water heater. After 2 years of use (albeit during a period of record-low gas prices), the variable portion of our gas bill dedicated to gas use has been ~$5/month. The real savings would've come from doing an all-electric house to avoid the fixed monthly charges for gas service...however we weren't ready to go there for cooking.

    Charlie - regarding insulation, we have quotes for blow-in fiberglass vs spray foam (open cell) for the wall cavities. Spray foam quote is only ~10% more expensive than blown-in fiberglass. I'm leaning toward spray foam for better air-sealing. Ironically, the quote for 3/4" rigid insulation is the most-expensive: 20% more than the spray foam. Rigid exterior insulation is very rare on the Gulf Coast...I suspect the contractors estimate is high, however I also believe very few contractors have experience installing it, hence the high estimate.

    Reid: the garage apartment doesn't have a ton of extra space...more space in the garage. A little cooling in the garage would be a welcome thing, especially when I'm working there. Even today while New England is getting a foot of snow, I had to turn the a/c on...

    All - 2 questions:
    1. Is there a recent review of heat pump hot water heaters? The product guide ( is a good start, however hard to compare.

    2. Does anyone know of a calculator (preferably Excel-based) that allows one to input assumptions around hot water use, CapEx/OpEx costs & lifecycle GHG impact of the choice between water heater options? I like David White's calculator for comparing additional insulation (, however it lacks any way to factor in different costs of each option. In other words, if in comparison to a code/standard-minimum option, I were willing to pay $50/MTCO2e for environmental performance, what is the optimum water heater choice (or insulation material, insulation thickness, etc)? While various assumptions would drive model, it would address the root question of prioritization & decision-making that otherwise are seem difficult to answer.

  5. GBA Editor
    Martin Holladay | | #5

    You may be interested in two resources.

    One is a decision tree that I created; you can find it at the end of this article: Domestic Hot Water: No Perfect Solution. (I'll also attach the decision tree below.)

    The other is ACEEE's life-cycle-cost chart for residential water heaters. I'll also attach that chart below. Caveat: I don't really trust ACEEE's numbers; the assumptions built into those numbers are arguable.

    Finally: Your case is special, because you are not choosing a water heater for a family. You are choosing a water heater for an apartment that will house one or two elderly women for half the year. That means the number of gallons of hot water per day will be much lower for this apartment than for an average family. That fact changes the math.

    -- Martin Holladay


  6. Robert Hallenbeck | | #6

    Martin - thanks for resources; you've given me a good homework assignment!

    However, one unexpected problem: For a 1 bedroom apartment, a 30-40 gallon tank size is all I need. However, all of heat pump hot water heater vendors I've found start at 50 gallons. We could make that work, it seems excessive.

    Are there any smaller (say ~40 gallon) heat pump hot water heaters? It looks like Rheem came out with one in 2010, however it's now nowhere to be seen...maybe discontinued?

    - Rob

  7. GBA Editor
    Martin Holladay | | #7

    The tank capacity of heat-pump water heaters tends to be larger for a reason: the heat pump doesn't put out as much heat as two electric resistance coils, and thus a larger volume of water is needed to make up for the longer recovery times. This fact is explained in my article, Heat-Pump Water Heaters Come of Age.

    -- Martin Holladay

  8. tjaykay | | #8

    All talk about mechanicals aside, the second floor is not a good place for an apartment for aging folks.

  9. Robert Hallenbeck | | #9

    "Homework assignment" attached comparing the 3 electric options. We're likely going with a 50 gallon AO Smith Voltex Hybrid Electric Heat Pump Water Heater. Under my specific situation assumptions (25 gallons/person/day, $0.08/kWh & 200 days of use/year by 2 occupants), ACEEE's annual operating costs are ~4x what I calculate. Setting aside any site-specific conditions, are ACEEE's installed costs reasonable estimates for the different options?

    N/A - A 2nd floor is only option for additional living space without sacrificing our garden (a non-starter); the garage apartment will be elevator-ready (2 temporary closets in the meantime)

  10. Robert Hallenbeck | | #10

    File attached this time...

  11. GBA Editor
    Martin Holladay | | #11

    1. You are overthinking this. Buy a water heater already.

    2. I haven't double-checked all of the inputs or formulae on your Excel spreadsheet. But I can say with confidence that your own assumptions about energy costs, water heater costs, and hot water consumption rates are more likely to be accurate than the assumptions made by ACEEE -- because they apply to your case.

    -- Martin Holladay

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