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Community and Q&A

Heat Pump Water Heater

Faulted1 | Posted in Mechanicals on

Sizing a heat pump water heater:

Would one size a HP water heater like a gas-fired unit or electrical unit?

I have always enjoyed the fast recovery of a gas-fired unit and be disappointed by electric units. Is this outdated thinking?

If I were to replace a 50 gallon gas unit, what gallon size heat pump was be comparable?

If a larger unit is necessary, does this offset some of the energy benefits of the HP unit?



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  1. Faulted1 | | #1

    This would be for new construction Zone 4 (mixed humid)

  2. Kevin Dickson | | #2

    If you got gas, use gas, unless you are shooting for a zero energy home. Gas btu's are usually cheaper even than a HPWH, which robs heat from the house in winter. First cost is also MUCH cheaper. BTW, when you speak of sizing a HPWH, the options are very limited Your are stuck with the mfrs. decisions, and they are all about the same in terms of gallons per hour. And they are all 50 gallon tanks, except the Geyser, where you use any tank you want. Watch out, though, 120 gallon tanks are more expensive per gallon than 50 gallon tanks.

    If you ARE shooting for a ZEH, that means you have a lot of PV, and NO gas. Then you have to do a lot of analysis to choose among a conventional resistance electric water heater, solar thermal, and/or a HPWH. Here's some help:

    Since PV costs steadily keep coming down, the answer in the future is easy, and that is PV with electrical resistance.

  3. user-626934 | | #3

    John - Kevin is right - if you're using gas already, you're probably better off replacing your old unit with a high-efficiency gas water heater. Also, make sure you're practicing good conservation - see Martin Holliday's and Robert RIversong's lists over here - Heat pump water heaters have slow recovery times, though many of them have a "hybrid" setting that combines the heat pump with an electric resistance backup to help with this problem.

    Kevin - by my rough calculation, PV prices would have to come down to about $500/kW (installed) before electric resistance DHW would make sense over a HPWH for a typical family. Does that sound realistic to you?

  4. user-626934 | | #4

    Sorry, Martin & Robert...I meant to write Holladay and Riversong.

  5. Faulted1 | | #5

    Gas is available so let me change directions and ask what design consideration are necessary to use a tankless gas WH instead of a regular GWH?

  6. J99aAMQzYo | | #6

    John, in addition to a natural gas supply, you also need to check with your utility to be sure that you have both sufficient excess volume (~200K btu) and sufficient water column pressure (~8 - 12 water column) Both of those numbers may vary slightly depending on the specific brand and model you look at.

    You will also need to consider whether you may need to pay for a meter conversion for the higher volume and/or pressure or whether they will do the conversion at no charge in expectation of your higher usage. You may also not need a conversion at all depending on how new the meter is.

    They will also need to provide a step-down splitter. One side will provide the 2 lb pressure feed needed by most tankless units and the other will maintain the .5 lb feed needed by all other residential appliances (dryer, range, furnace, log sets, etc.) Again, this may or may not be an additional cost to you.

    Once you've got that info dealt with, then you can worry about how to properly size a new tankless unit.

  7. Kevin Dickson, MSME | | #7

    John S,

    If the HPWH is in the basement or garage and you're not heating it, then it's a simple calculation and your number sounds realistic. In some regions with generous PV incentives, we're almost there.

    John, in addition to Andy's gas concerns, there can be several annoying issues with living with a gas tankless. Navien makes the only unit to have solved them all (ironically by using a small tank), but their reputation will seem sketchy to a really thorough background-checker.

  8. Anonymous | | #8


    You mentioned this will be new construction correct? If you are in Zone 4 then I can only assume [I love that word][ that you have installed a ground source heat pump. If you have then a HPWH is not required since both your domestic home heating AND domestic hot water will be provided by the same unit. I still have a few relatives living in Minnesota and many are switching over to ground source heat pumps where they have sufficient space for trenching or for new construction.

    Anyway here in the desert Southwest we do the following since almost all of the homes are all electric, meaning air source heat pumps for heating and cooling and electric hot water heaters. It also means that it is extremely difficult for us to even justify a small capacity HPWH like the Geyser since during the summer months almost no electricity is used for water heating since it comes out of the tap at just about the right temperature for a shower.

    So what are most residents doing? Many are going solar. They determine their annual usage, calculate how much they want to reduce their electric bills and then size their solar systems to take care of their needs. Domestic solar how water heating system are almost never installed due to cost. A contractor installed solar hot water heating system costs between $4500-$5000.

    My recommendation - do an energy analysis and seriously consider a ground source heat pump.

  9. Daniel Ernst | | #9

    John - I'm not sure of your desire for a tankless design, but there are a couple of companies producing a hybrid type of gas water heater: their combustion systems are similar to tankless systems, but they add small tanks to eliminate some of the problems inherent to tankless designs (mainly, low flow or intermittent flow issues).

    Here is an example:

    A.O. Smith rates their Next Hybrid at 90% efficiency, almost up there with the condensing gas designs of 96%. But I don't think this figure includes the elecricity required to run the blower motors . . .

    Of course, to get the higher efficiencies with a HPWH or high efficiency gas, expect to pay $1,500 - $2,000 for a unit.

  10. Riversong | | #10


    You'll notice that almost all of us here use our real names so we can be accountable for our statements and so that we each know the others.

    Please use your name when you contribute.

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