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Heat pump hot water or more PV?

joenorm | Posted in General Questions on

I’m building a new house and trying to make it net-zero.  Plumber is set to do work soon so I need to decide whether a heat pump hot water heater is the best route to go.

I have a small mechanical room/pantry where the water heater will go and I am wondering if the noise of the unit will be heard throughout the house? The room shares a wall with a bedroom and is under the stairs to the upstairs bedroom.

I have access to photovoltaic equipment at cost and can do all the work myself so Im wondering if it makes more sense to install a normal water heater and add more PV to cover it’s use?

I am not opposed to the heat pump version at all, but am not familiar with them at all so just looking for some feedback. They do seem like quite a bit of added complexity to heat water but also I know they draw very little power to do so.

thanks

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Replies

  1. user-2310254 | | #1

    Joe,

    I have a Rheem 80 gallon HPWP. The compressor only runs a couple of times a day for maybe two hours total. It is a little noisy, but that's not a problem since it is installed in the garage. If you decided to put this type of unit inside the house, you would need at least 700 square for proper operation. If your mechanical room is smaller than 700 square feet, you would have to install true louvered doors or provide make-up air from another source.

  2. Jon_R | | #2

    It depends (you could post all the relevant figures), but a rough calculation indicates that the PV solar is better.

    > trying to make it net-zero

    When it comes to green building, net energy use is of little relevance. Focus on other things.

    1. AJ__ | | #6

      This. First ask yourself why you want to make it net zero. Then figure out your energy usage current usage in months heating/cooling and months with neither, if possible and consider your current hot water usage. On the face of it net zero sounds like something that is great to strive for. When you get in to the numbers, it gets murky. Having no energy bills sounds cool but when that means spending more on equipment than you will save in its life cycle over more basic options, or spending thousands on large PV arrays that may take a couple of decades to pay off it doesn't always make good financial sense.

      Jon, Dana and others made some excellent points here when I started out my project hoping to achieve net zero.

      https://www.greenbuildingadvisor.com/question/hoped-to-get-near-net-zero-not-even-close

      Based on that, DWHR and extra PV might be a better option.

  3. Aedi | | #3

    As Steve mentions, you'll need to provide more air to the pantry somehow, and that will make it harder to isolate the noise. A heat pump water heater would have the extra benefit of keeping the pantry cool though.

    From a pure cost/benefit calculation, standard electric is almost certain to win out, especially if your hot water usage is average or less. Your climate zone and heating/cooling system can also be a factor (for example, HPWHs make no sense in combination with electric resistance space heating, but are a boon for cooling-dominated climates).

  4. user-7237131 | | #4

    I have done a fair amount of research on this issue and hear are some of my thoughts. If the heat pump sits on top of the tank it should be vented to use and exhaust outside air. Otherwise you are using air that you already paid to heat and making your house colder. ( a good thing in the summer). Not sure what your cost for installed solar will be but for me at 2.75/watt installed the heat pump water heater was the better way to go. Our system does more than heat water. It is also tied in to the electric furnace and ducts with a hydronic radiator and so is producing heat for the house at the efficiency level of a heat pump. This is three to five times more efficient than electrical resistance heating for water tank or furnace. Check out Badger Insulated Pipe for the radiator units and YouTube videos on how to install in furnace.
    Kamilche Passive House

  5. Expert Member
    Dana Dorsett | | #5

    >"I have access to photovoltaic equipment at cost and can do all the work myself so Im wondering if it makes more sense to install a normal water heater and add more PV to cover it’s use?"

    This is probably a case where more PV makes sense, but without knowing anything else about the house/climate/utility costs it's hard to refine that.

    In locations where you might get paid for demand response, a "grid-aware" or wi-fi enabled standard water heater might be worth it, even if the programs are still in the planning phase. (In most of the PJM grid region it's possible to get paid to let a third party aggregator use your water heater as a power dump as part of a "virtual powerplant" bidding into capacity and frequency & ancillary services wholesale markets.)

    So what am I even talking about? (How should I know? :-) )

    http://energystorage.org/system/files/attachments/giwh_es_wiley_april_2016.pdf

    https://www.utilitydive.com/news/utilities-in-hot-water-realizing-the-benefits-of-grid-integrated-water-hea/445241/

    https://www.rheem.com/innovations/innovation_residential/gladiator-innovation/

    There are also retrofit solutions:

    https://aquanta.io/

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