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Community and Q&A

Heat-pump water heater

kpklein | Posted in Green Products and Materials on

My future house will be in an rural area without natural gas lines (propane tank is an option). I’d like to go all electric (solar), and use a heat pump water heater (A.O. Smith 80 gallon). I was going to build a detached garage and have the mechanical room in the house for all the various system components, but this could be a problem for the water heater since it needs open space to function (garage). What do you think? Skip the detached garage idea?

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  1. walta100 | | #1

    Where are you located? If you are located far enough south unconditioned space is a workable plan.

    Generally home appraisers see an attached garage as more valuable than a detached. The attached garage should cost about 20% less to build.

    Understand a HP water heater will not be silent so plan its location carefully. If you are going photovoltaic a standard electric water heater and extra panels may be a better investment.


  2. kpklein | | #2

    Walter, thank you for the reply. Climate zone is 3B, inland Southern California...

    1. Expert Member
      Dana Dorsett | | #4

      In zone 3B SoCal you'll do just fine with the HPWH in an unconditioned detached garage.

      1. GBA Editor
        Martin Holladay | | #5

        The disadvantage of Dana's suggested approach is that you will need to bury the water lines in a trench, and that means (a) thermal losses and (b) long waits for hot water to arrive because of the distance.

        1. Expert Member
          Dana Dorsett | | #7

          Whether it makes any sense depends on how far away the detached garage is from the house, for sure!

          1. kpklein | | #10

            Thanks Dana, I've thought about that too. It could be connected via a breezeway, but I think it would still be too far to run the water lines...

        2. kpklein | | #8

          Thanks Martin. That's exactly what I was thinking. The only way a heat pump water heater is a practical option is if it's in an attached garage. The Sanden model was featured in a Fine Homebuilding California house, but it's just too expensive for me to consider. The choice seems to be go with a tankless (in a mechanical room inside the main house), or skip the detached garage idea and put the heat pump water heater in the attached garage. Any further thoughts?

  3. Balazs_F | | #3

    alternatively you could use a split type heat pump water heater like SanCO2 from Sanden. yes, a lot more expensive than a standard HP water heater - but the condenser can be installed remotely, indoors or outdoors in temperate climate, it is a lot more efficient (uses less electricity), less noisy and uses low GWP CO2 as refrigerant. if having some space heating is a consideration, there are combo units available (water heater for heating and domestic use)

  4. 300TTto545 | | #6

    The noise issue is a little overblown IMO.

    We are in NC (Zone 3/4 line) so pretty close to yours. We have it in a conditioned attic space over the master bedroom. I always figured I could put it on a timer and not run it while we were sleeping. Not an issue.

    In a warm climate, it obviously helps to have it in conditioned space so that you are taking advantage of the cooling effect. Mine has worked out well since we like to sleep cold and the heating is such that we never run the heat in the bedroom so that has worked out. That attic space does have the heat pump for the entire downstairs and so it tends to be a little warm this winter (our first season in the house).

    Last house, we had one in an attached garage. That garage was cold in the winter. It was a 1984 house and I was working to seal up the garage some but it got a lot of infiltration. The HPHW didn't help....

    1. kpklein | | #9

      Thanks David. The HPWH is starting to seem like a lot of trouble. May have to consider the tankless style. Not a fan of those, but maybe it's the answer? Or just go with an attached garage. The cooling would be welcome there in the hot summer, and it doesn't get cold enough in the winter to be a problem..

      1. 300TTto545 | | #12

        Interesting - I was pointing out that it really isn't any trouble.

        The only issue that I have experienced is in a high infiltration attached garage in the winter - say 30 degrees outside - it drives down the temp of the garage from say 55 to 48 which is less than ideal.

        I don't suspect that a new garage would ever the same issue especially in SoCA.

        But I say, put it in the conditioned attic.

      2. Expert Member
        Dana Dorsett | | #14

        >"The HPWH is starting to seem like a lot of trouble. May have to consider the tankless style. Not a fan of those, but maybe it's the answer?"

        Absolutely NOT "... the answer....". DON'T DO IT!

        A tankless electric water heater is in extremely heavy intermittent load that takes a lot more grid infrastructure to support than almost any other residential equipment. It saves a tiny amount on energy use, but it's abusive to the grid, and lowers the quality of the power on your local distribution grid as much or more as a 10 ton air conditioner would. It's more power than the typical Level 2 car charger, but unlike a car-charger that could become a "smart" grid-responsive load and temporarily interrupt the charging when grid circuits are at capacity, a tankless water heater is a "must run" appliance, that can't be interrupted for even 3 seconds without losing some of it's primary function.

        If/when residential rates start to include demand charges to pay for the infrastructure to support the heaviest 15 minute draw from the grid a 10 minute shower can cost more than anything you'd save over the operating cost of a plain old tank electric water heater. So far that type of rate structure is rare, but as rooftop solar get cheaper and ubiquitous utilities keep proposing residential demand charges as a means of recovering grid infrastructure costs for nearly-net-zero electricity grid-connected solar customers. So far the number of locations in the US where residential demand charges apply are small, but it those numbers are likely to grow rapidly over then next decade (particularly in California, where rooftop solar is becoming almost a requirement under Title 24.)

  5. GBA Editor
    Martin Holladay | | #11

    Here is a link to an article that will help you make a decision on which type of water heater to buy: "Domestic Hot Water: No Perfect Solution."

    1. kpklein | | #13

      Thanks Martin. The answer will be: if attached garage, heat pump water heater, A.O. Smith 80 gallon. If garage detached, electric resistance tank or tankless style water heater in home mechanical room.

  6. BillDietze | | #15

    Kurt, what are your outdoor temperatures in winter? I'm wondering if you can get away with placing the water heater in your house and ducting the heat pump water heater to the outside. You might be able to get away using outdoor air 90% of the time and the water tank's resistance heater the othe 10%. Add in a drain water heat recovery device and that might work. Depends on how many people are taking showers every day and your weather.

    1. kpklein | | #16

      I don't think the experts recommend ducting to the outside of the building...

  7. brad_rh | | #17

    If you don't want an attached garage, how about a large enough attached mechanical/storage room. The efficiency would drop in the winter, but with window or other ventilation to the outside, it would be great in the summer.

  8. AndyKosick | | #18

    Having just recently got to heard the new Rheem HPWH for the first time, it is so quiet! I live with an old GE, no comparison. I was bringing in testing equipment into the job and had to double take because I didn't realize it was on. Just put it in the mechanical room. It only needs 700 cf (about an 8 by 10 room) This model is super easy to duct, you could duct it to the attic if the room isn't big enough.

    I love a detached garage myself.

    1. fourforhome | | #19

      Does anyone else have a good report on the Rheem in a 100 ft² room (laundry + mechanicals)?

  9. mdhare | | #20

    Long time lurker, enthusiast homeowner.

    I know the parent is asking about AO Smith in a detached garage in specific, but I wanted to share my experience with a current gen Rheem unit in the basement in a cold climate (southern Wisconsin).

    Replaced a 40G gas powervent [0.63EF] with a 50G Rheem HPWH, specifically XE50T10HD50U1. A major advantage for HPWH vs my gas appliance was being able to put the tank as close to point of use as possible. I easily took out 20ft of pipe to sinks which means less hot water radiating BTUs in my insulated pipes.

    How long will your projected how water runs be from detatched garage to house? I wonder how what your water temperatures will be after running through all that pipe (that of course radiates BTUs).

    Not to fear monger, but something to consider. I have my tank set to 120F. On screen display indicates unit usually turns on when lower/upper is near an average of 112F, I commonly see 108F/114F. It heats it to about 119F and shuts off. This makes me minorly worried about things like Legionnaires but frankly the water gets cycled out frequently being a 50G tank. I will likely experiment with a tank temperature of 125 after gathering data for awhile. DOE tests claim a typical energy hit of 3 to 4% per 5F bump I think.

    This HPWH is way quieter than my power vent's exhaust fan. Frankly, that alone has brought some joy. It's a gentle hum, perhaps louder than my fridge but comparable. Definitely quieter than your typical dehumidifier.

    Considered the 80G as EF on 50G is 3.55 vs EF on 80G being 3.7 but price delta is $700. More economical for us to infrequently rely on resistance heating. Three folks in the household. Only on day 13 but averaging 1.39kwh/day with wet bulb temps around 60F. I don't think you said you're off grid electrically so consider this when choosing 80 vs 50G, etc.

    Electrical elements were fired up on initial tank fill despite being in heat pump mode. Not sure how long they stayed on for, my guess is until top thermostat registered ~100F? That day was 8kwh, not included in above average.

    Unit is in large open basement unducted, much greater than 100sqft. I may duct the intake to the first floor, but only because it would be a duct run of less than 4 feet. I haven't noticed an appreciable change in temp or humidity over the course of a day. Yes, cool exhaust while the unit is running but ambient quickly recovers thanks to thermal mass. We're insulating/air sealing so over time I suspect my heating demands to drop.

    We generally have two to three run times a day with the unit, I've tried to adapt our hot water usage around keeping the compressor running fewer, longer cycles. Not being instrumented exceptionally well I estimated my previous BTU needs based on my old gas appliance and it seems like I'm getting sticker (if not exceeding). I'm calc'ing EF 4.1. Our average daily BTU usage is only in the 20kbtu range.

    For the Rheem unit recovery is comically slow, compressor is 4200btu/h. No problem filling a normal size bathtub (5ft, 12") but recovery took 3+ hours so plan accordingly. Family is used to minor inconveniences. You might offset that with the 80G but I'm not sure if the bump in efficiency is worth the increased standby loss (larger surface area in an 80G vs 50G).

    I have natural gas but was looking to reduce point of use carbon emissions. We have a grid tied 4.5kw rooftop array so at local energy prices, factoring in my solar, energy needs are on par even taking into account absurdly cheap natural gas. Our utility prices are 15.5c/kwh or $0.55 a therm. My electric pricing includes paying a kwh premium for "green energy". Around here that mostly means trucking in wind power from Iowa. My PV system (0% loan, $2k set aside lifetime for maintenance/inverter replacement) lowers my kwh pricing to about $0.11c/kwh.

    Good luck (and enjoy!) the new home project.


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