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Replacing Electric Water Heater with Heat-Pump Water Heater

darrencope | Posted in Energy Efficiency and Durability on

Hi all,

I’m looking to replace an existing electric water heater (50 Gallon, installed in 2009) with a more efficient heat pump water heater. I have some questions and concerns, and this seems like the best place to get reliable answers, so thanks in advance! Sorry for the mix of questions–I hope this is ok. If not, I can create separate posts.

– Currently have a 50 gallon tank, and we sometimes run out of hot water (couple times a month). A larger tank would fix this obviously, but to me seems like this would be less energy efficient. However, in the “Rewiring America” guide “Electrify Everything in Your Home” they claim ‘bigger is better’ and that the most efficient thing is a bigger tank. I get that a bigger tank ‘stores’ more hot water like a battery, but.. there’s also more volume to keep warm. I suppose the tradeoff is how much is lost to surface area, which would be less per volume for a larger tank? So, is a larger tank both actually more energy efficient?
– similar to above, the Rewiring America guide also suggests keeping the tank very hot and using a mixing valve to control output temp. This makes little sense to me, as the temperature differential between temp inside and outside the tank should be the largest factor in total energy use, right? Or am I missing something?
– the Rheem units look to be the most energy efficient, at least according to the Energy Star website. However, I’ve read the noise complaints here and on Reddit. Seems like that MIGHT be fixed, but I’m hesitant to take the chance. Any recent updates on that one?
– Also, the Rheem units have app-compatibility. However, I recall reading somewhere (can’t find where right now) that some features are ONLY available in the app. If this is true, I’m hesitant to buy one, as I don’t want a water heater that is crippled in 5 years because there’s no longer app support for it. Are there functions only available in the EcoNet app, or can everything be controlled on-unit?

Thanks for any feedback!

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  1. paul_wiedefeld | | #1

    Bigger is better because you're trying to use the heat pump as much as possible, without electric resistance kicking in. Standby loss of a bigger tank will be slightly higher (maybe $1/month?) but insignificant when compared to using resistance heat because you're short on stored gallons.

    The higher temperature is used to provide greater capacity too. Heat pump water heaters are slower to recover, so the strategy is to keep the most energy stored (higher temps and/or larger tanks) so you don't run out of water.

    "This makes little sense to me, as the temperature differential between temp inside and outside the tank should be the largest factor in total energy use, right?"
    Not at all - nearly no energy is lost through an insulated tank (which is why a tankless heater saves very little energy compared to a similar tank heater). The energy use is driven by gallons used x (water temperature stored - incoming water temperature).

  2. darrencope | | #2

    Thanks Paul. So a larger tank, set to a higher temperature does make the most efficient setup then? Sounds like the best of both worlds!

    Now, to figure out if the noise of the Rheem is going to drive me crazy or if they have that fixed up!


    1. paul_wiedefeld | | #3

      Depends - lower temperature and smaller tank would be more efficient IF you don't need to run the resistance, so it depends on your water usage. If you're frequently running out of water, then bigger and hotter is better.

  3. walta100 | | #4

    Yes a larger tank will lose less heat per gallon stored but the larger tank with its larger surface area will lose more BTUs to the room than a smaller tank in the same room with the same R value.

    The larger tank only lowers the operating costs if you are likely to use the extra capacity.

    The energy guide on the Rheem 80 gallon says $149.00 and the 50 gallon says $104.00

    My guess is the tag assumes you will use more water if you buy a bigger heater. Could be right some people don’t get out of the shower before the water gets cold.


  4. brianvarick | | #5

    I have a Rheem Proterra from a couple years ago which is very loud. They sent out a new fan which I think would fix the problem but I would have to uninstall the unit to install it which I haven’t done. I have had good luck just running the unit from 12-6am and then not letting it run the rest of the time. This gives us the added benefit that it’s never actively cooling the basement while we are awake and the noise doesn’t bother us while we are sleeping. It’s also not using very much energy and we very rarely run out of our 80 gallons.

  5. bjarthur | | #6

    i have a rheem manufactured in 9/2021 and it is 60 dB at chest height a meter away and the sound is a 190 Hz tone. a friend has a GE geospring manufactured in 7/2016 and it is 50 dB and white noise. we have complained up a storm about the noise and fortunately our installer is going to replace it for free with a regular electric one.

    one way to check if you can handle the noise is to figure out a way to play a 190 Hz tone from a speaker and then measure with your phone so that it is 60 dB. then go to the living areas and see if it's bearable. i wish we had done this.

  6. nynick | | #7

    I plan to insulate my mechanical room with Safe and Sound Rockwool to cut down on the sound.

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