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Community and Q&A

HPWH impact on load calcs

Reid Baldwin | Posted in Mechanicals on

If one installs a heat pump water heater (HPWH) in the conditioned space, how does that impact the calculation of the design heat load and design cooling load?

For example, if the specs say that it uses 500 watts in heat pump mode and has a COP of 2.5, should the heat load include the (2.5-1)*500 = 750 watts that comes out of the house air? Or, is it not necessary to include the whole amount because the HPWH runs intermittently?

I am guessing that you can’t take a credit for it in the cooling load since it won’t necessarily be running.

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  1. Dana1 | | #1

    The effect on sensible loads are small, and you should only include the average, not the peaks.

    The effect on latent cooling loads are somewhat more significant, but again, only use the average, not the peak.

  2. charlie_sullivan | | #2

    If you wanted to calculate the average, you'd need to estimate the water usage per day. But I doubt you'll get into any trouble by ignoring it.

  3. RD3Sunworks | | #3


    Maybe you could try the free BEopt software brought to you by the US government, for the following reasons:
    --it's easy to use to get whole-house energy use, and even easier to change variables (like water heater types) to see the effect on energy usage of the whole house
    --there is a lot of waste in the US government, but at least in this case the millions of dollars spent on BEopt and its simulation engine Energy Plus has resulted in a very accurate and comprehensive program (and becoming more versatile all the time.)
    --answers on this website are often wild guesses, or feelings, or crude hand calculations. BEopt is much more quick and accurate. Several times, energy simulation has been criticized on this website, but much of that criticism is just wrong. A LOT depends on what information is put into the simulation. Just like any other program: garbage in equals garbage out. In my case, I had several years of electric bills, before a deep energy retrofit, and was able to quickly use BEopt to model my existing house, and BEopt came up with the same electricity use . After the retrofit, with many, many changes, BEopt still easily models the electricity use within a hundred kWh per year. I cannot speak for accuracy as related to energy other than electricity, but I have seen the math and complicated algorithms within Energy Plus, and they are impressive. Remember, when it comes to scientists, the government hires some of the best.

    It seems like every time I look at this website, somebody is struggling over energy calculations that can be quickly and accurately handled by BEopt. That really is unbelievable to me, but it's reality.

    Trust me, I don't work for the US government, but when we actually get something for the huge amounts of money they take, then the people that are interested in energy use should be helping themselves and using it.

  4. GBA Editor
    Martin Holladay | | #4

    Good for you for trying to have a very accurate heating load and cooling load calculation. The vast majority of such calculations are made by people who don't sharpen their pencils. That's why most heating load and cooling load calculations result in estimates that are twice as high as they should be.

    I think, however, that you can safely ignore the HPWH in your calculations. It's more or less in the level of noise. Yes, it will affect your energy bills -- but not enough to change the specifications for your heating and cooling equipment. Other factors are similarly ignored in these calculations -- internal loads like televisions and computers, for example, which are usually estimated but not quantified.

  5. Reid Baldwin | | #5

    Thanks for the responses. The feedback confirms my suspicions that the effect on a whole-house heat load that drives equipment selection would be very small. In the room-by-room load calculation that will drive duct system design, it could be significant for the room in which the HPWH is located. The water heater will share a utility room in the basement with the furnace. I am not concerned about comfort in the utility room, but do plan to occupy adjacent rooms.

    In the comments of an article about HPWHs several years ago, there was discussion of ducting the water heater exhaust behind the refrigerator. In that thread, it was presented as an idea but nobody had any experience doing it. Has anyone tried that since then? I suspect the effect would be directionally correct but the magnitude might be insignificant. The location of the rooms in my floorplan would make that easy to do.

  6. GBA Editor
    Martin Holladay | | #6


    1. Ducting the cool air from a HPWH isn't a great idea, because the energy savings are too low to justify the cost of the ductwork, fan, and controls.

    2. Because a refrigerator's compressor and the compressor on the HPWH are rarely operating simultaneously, the advantages of this approach aren't as great as some people imagine.

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