Anyone Own a Heat Pump Water Heater

Discussion in 'Water Heater Forum, Tanks' started by WestMIchigan, Oct 27, 2020.

  1. WestMIchigan

    WestMIchigan New Member

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    I am on the fence about buying a heat pump water heater. If anyone owns one what has been your experience? Does it cool your basement or other location too much? Do you find you run out of hot water quicker? Does it really save the money they say it will? With a COP of 4 it is hard to see how it won't save but I want to make sure it is right for my family of 6 before I take the plunge on a 80 gallon Rheem unit.
     
  2. hj

    hj Moderator & Master Plumber Staff Member

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    Family of 4 adults and the 50 gallon hybrid has been adequate so far.
     
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  4. WestMIchigan

    WestMIchigan New Member

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    Looks like you live in a fairly warm climate. Wondering how this will do in a mostly finished basement in Michigan?
     
  5. WestMIchigan

    WestMIchigan New Member

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    Let's say I emptied the water heater so I have to heat 80 gallons of water. If I did the math correctly 80 gal requires about 44600BTU. Assuming 0.018 BTU to cool 1 cu ft those 44600 BTUs will cool a 13200 cu ft basement by 187 degrees. (13200*0.18 = 237.6 BTU/°F in my basement, 44600BTU/237.6BTU/°F and you get 187.8°F) Obviously I really messed up somewhere. I would like some reasonable estimate on the cooling effect to my basement but my numbers fall apart somewhere.
     
  6. wwhitney

    wwhitney Well-Known Member

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    Most of the thermal mass inside the thermal envelope of your building is not air, it's all the solid materials.
     
  7. WestMIchigan

    WestMIchigan New Member

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    So how do I calculate this? Obviously my approach failed miserably.
     
  8. wwhitney

    wwhitney Well-Known Member

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    I'm not sure what you are trying to calculate. Here's what I think you should try to calculate.

    First, how is your basement currently conditioned, and what are the conditions? If it's 68F and 60% RH and maintained that way by passive heat exchange with the upstairs and a dedicated dehumidifier, then it's worth thinking about how much dehumidification the heat pump water heater will provide. That will provide the benefit of not running the dehumidifier as much, and reduce the sensible cooling provided.

    Then, if it's heating season, all the heat that goes into your water heater will have to come from somewhere, so it will be an extra load on your central HVAC. If your central HVAC is a COP 4 heat pump, then worst case the water heater nets out to COP 2: the heat pump uses 1 unit of energy to move 4 units of heat from outside to inside, and the water heater units another unit of energy to move those 4 units of heat from inside to the water.

    On the other hand, during cooling season, it's going to reduce your cooling load. So it's worth considering how many heating degree days per year you have versus how many cooling degree days per year you have in your climate (and forecasting how that's changing over the next 10 or 20 years).

    This discussion is based primarily on a model where heat exchange between living space and the basement dominates over heat exchange between the basement and the earth. But if your basement is not well thermally insulated from the earth, or is well insulated from the upstairs, then one would need to consider how that changes the analysis.

    Cheers, Wayne
     
  9. WorthFlorida

    WorthFlorida The wife is still training me.

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    Hybrids do not run on the heat pump alone. When demand drops the water temp to a certain temp, the electric elements are turned on and it operates just like a regular water heater along with the heat pump running. If you can program the unit for heat pump only, it probably not able to keep up with the demand since the recovery rate gets very low, therefore, you'll never get all the heat from the air space to heat the water. This energy star page may answers some of your concerns.

    https://www.energystar.gov/products...electric_storage_water_heaters/considerations
     
    Last edited: Oct 31, 2020
    phog likes this.
  10. wwhitney

    wwhitney Well-Known Member

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    OK, it's common to use a mixture of heat pump and electric resistance modes. But for analyzing the energy/heat flow for the hybrid, I think the electric resistance mode is easy to do. So we just need to analyze the heat pump only mode. Then if your water heater ends up running 2/3 of the time in heat pump mode, and 1/3 of the time under electric resistance, or whatever, you can take the weighted average of the two analyses.

    Cheers, Wayne
     
  11. WestMIchigan

    WestMIchigan New Member

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    Thanks for the replies. My geothermal system is slightly undersized and after thinking about it a little, any heat removed from the basement will just make my geo have to work that much harder and force me into strip heaters even sooner. I guess I am going to suck it up and go with the same regular tank water heaters that I have for my replacement units.
     
  12. Reach4

    Reach4 Well-Known Member

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    If you have basement space the busy shower, you want to consider a drain water heat recovery unit. Dana has made some great posts on that, and here is one: https://terrylove.com/forums/index.php?threads/purchase-advice-tankless-or-tank.77391/#post-565836

    [​IMG]
     
  13. WestMIchigan

    WestMIchigan New Member

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  14. wwhitney

    wwhitney Well-Known Member

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    So that's a net loss when the heat pump water heater forces you into strip heaters on your HVAC: 4 kWh to make 4 kWh of hot air, and another 1 kWh to move that heat to the water, so 5 kWh instead of 4 kWh to put 4 kWh of heat into the water.

    But at all other times the heat pump water heater will be at least a small win, so it may still make sense. You could even put the heat pump water heater into resistance-only mode during the coldest winter months.

    Cheers, Wayne

    P.S. If you have geothermal HVAC, do you have a "superheat" tank to preheat domestic hot water? (Not sure on the details on that, maybe only works in the summer when you are cooling the space?)
     
  15. WestMIchigan

    WestMIchigan New Member

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    I turn it off in the winter because I am undersized and need every BTU I can get for the house.
     
  16. phog

    phog Active Member

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    Rochester NY
    You sound savvy. In the winter you can switch the hybrid water heater to run off the electric element only (so that you're not stealing BTUs from the geothermal). Then switch it back to hybrid operation in the summer. It's usually just a setting on the front touch panel. For Rheems you would set the touch panel to "Electric" mode when the house heat is on in the winter, and "Energy Saver" mode in the warm months. Also recognize that much of the heat you "capture" from the air is in the form of latent heat (phase change / dehumidification) rather than sensible heat (the actual temperature of the air against your skin). So the water heater captures heat from, for example, steam from cooking activities, or from showers, that doesn't cool down the air, but rather just makes it dryer. Or even the moisture from your breath. So it's not 100% of the BTUs into the water heater come out of your geothermal. You might be fine with the hybrid mode active year round, despite your limited geothermal. But even if not, you can always just change the setting on the water heater.
     
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The latent heat of condensation of water is not to be ignored! Changing the phase of water from vapor to liquid or liquid to solid requires considerably more energy (or releases it) than the nominal definition of what's required to change it one degree away from the freezing or boiling point. So, in the process of extracting heat from the moist air, there is more energy available when it results in condensation as would happen running a hybrid condensing water heater.
     
  18. Dana

    Dana In the trades

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    The upcharge for a drainwater heat exchanger is about the same as the upcharge going from a 50 gallon HPWH to an 80 gallon. If it's a predominantly showering 6 person family taking daily showers within a 2 hour time frame you'll probably still need the 80 gallon version. If it's only 3-4 people showering using low flow showerheads in that time frame you probably won't outrun a 50 gallon unit in heat pump-only mode even without the drainwater heat exchanger. In "hybrid" mode the recovery time is fast enough that you might squeak in 6 low flow showers with a 50 gallon + heat exchanger, though it takes a hit in efficiency when the heater element is engaged.

    Don't sweat the additional load on the geothermal system that would represent, unless you are using a MASSIVE amount of hot water every day. The thermal mass of a basement slab stores a lot of heat, even if it's insulated from the even more thermally massive soil under the slab. The additional heat load to the geo from the water heater is distributed over many hours- it's not driving a peak heating load, and that average load adder is quite modest at "normal" levels of hot water use.
     
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  19. MikeQ

    MikeQ Member

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    Good point on the effective capacity increase of the water heater when using a heat recovery drain pipe! Also on the thermal mass of the basement. Another factor in favor of the basement heat pump water heater is the drier air in the basement will make the basement feel more comfortable even if the temperature is slightly cooler. Nothing is worse than a dank, cool basement!

    I love my heat recovery drainpipe! I have three showers in my ski cabin and the upstairs shower drains into a heat recovery drain inside the wall of the main floor. This allows my ELECTRIC on demand water heater to power both showers simultaneously even though my water supply is a constant 41 degrees! The colder your water supply, the more effective the heat recovery drain becomes and the more it helps. I've even run the three showers simultaneously with slightly reduced flows in the showers but the water remains hot and the flow reduction is barely noticeable. The hot water never runs out due to the water heater being on-demand.
     
  20. Dana

    Dana In the trades

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    The only situations where adding a heat recovery unit becomes a potential liability is with gas/propane fired tankless water heaters in areas with already tepid /warm incoming water in summer. The minimum modulation range of a fossil-fired isn't zero, and when the incoming water at the tankless is too high to regulate the output temp well the high-limit safety controls cut the burner.

    That isn't likely to happen at a ski condo, but a real risk at a surfing shack on the Gulf Coast.
     
  21. MikeQ

    MikeQ Member

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    Jul 17, 2012
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    Yeah, I doubt heat recovery drainpipes are very popular on the Gulf Coast, LOL! One early company in this market was started when the Canadian future founder noticed the area above his septic tank was the only area of his lawn that was green all winter long. He knew there was heat there to be recovered so he went right to the source of the sewage.
     
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