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HPWH set point with mixing valve

PGH_Build | Posted in Energy Efficiency and Durability on

Hello, I’m working through the planning for a PGH that I will construct in climate zone 6.  I plan for an all-electric house with a heat pump water heater (HPWH) located in an insulated basement.  I’d like to understand the energy tradeoffs in using a mixing (tempering) valve with a higher water heater set point.

A higher set point, say 140 vs 120 F, could provide several benefits:

          less use of resistance heating

          potential use of a smaller tank, e.g., 60 vs 80 gal

          reduction of Legionnaires disease risk

But comes with a few possible drawbacks:

          greater standby losses

          lower heat pump efficiency

          potential increased wear and tear on equipment, e.g., compressor, sacrificial anode

          requirement for a mixing valve to prevent scalding

There was a nice DoE paper (https://www.nrel.gov/docs/fy16osti/64904.pdf) on the field performance of heat pump water heaters that concluded the highest COPs were observed in 80 gal Stiebel Eltron units which were set to 140 F.  However, it didn’t really isolate the set point well vs. tank size and had a limited number of data points.

So, if you have a HPWH or an opinion, I’d like to know: did you include a mixing valve (may be code in some places)? What are you using as a set point temperature?  Did you see a benefit?  What decrease was observed in resistance heat use?  If water usage was not sufficient to trigger resistance heating, what was the increased energy usage due to the higher set point?

If anyone is aware of good analysis on this, please point me to it.  I’m sure there is some rule of thumb along the lines of increase setpoint until electric resistance usage occurs on <5% of water draws.  I just haven’t seen it or the analysis to support it. 

Thanks!
Greg

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Replies

  1. greenright | | #1

    I think you are overthinking it and overestimating the ashp water heater abilities..
    120 will be insufficient unless you like cold ending showers if anything over 5 mins.
    140 wjll make the heat pump run forever even with smaller loads
    I have mine set at 130 which I believe is the best compromise between capacity/ performance and efficiency.
    I know somebody who is running 140 with mixing valve and his only advantage is capacity- his efficiency numbers are far worse than mine (similar household).
    Experiment and then make up your mind.- don’t try to pre- figure it out

    1. aunsafe2015 | | #2

      Can you elaborate on saying that a setting of 120 results in cold ending showers?

      Surely an 80-gallon heat pump water heater could supply a hot shower even with the tank set to 120, no?

    2. Expert Member
      DCcontrarian | | #5

      I have a 65 gallon HPWH, set point of 120F, and four adults get showers every morning without issue. The one thing I did was replace all of the shower heads with low-flow models. With the original 2.5 GPM shower heads the fourth shower could be dodgy.

      1. aunsafe2015 | | #11

        Which low flow shower heads did you use?

        1. Expert Member
          DCcontrarian | | #12

          Right now I like the High Sierra 1.25 gpm:
          https://www.amazon.com/gp/product/B09W3X634N?th=1

          It's a great shower at 1.25 gpm.

          I used to like Speakman products but they seem to have moved away from low flow and gotten more designer. Plus they've gotten pricey.

          1. aunsafe2015 | | #13

            Thanks. Those High Sierra products have been on my radar for awhile. Maybe I'll put one in the least-used bathroom as a trial.

    3. PGH_Build | | #8

      Thanks. Appreciate the real world observation. It's interesting that your experience differs from the DoE observations that higher set points installations garnered better COPs

  2. paul_wiedefeld | | #3

    You can also adjust the setpoint as you experience it firsthand. Lower in summer, higher in winter/expected high demand periods.

    Another option to consider is a drain water heat recovery pipe - since you have a basement this would fit nicely. It preheats incoming cold water, which has the effect of increasing capacity of the water heater. 60 gallon HPWH + drain water heat recovery pipe appears less expensive than a 80 gallon HPWH according to the web.

    https://www.greenbuildingadvisor.com/green-basics/drain-water-heat-recovery

  3. Expert Member
    DCcontrarian | | #4

    To answer the question that I think is being asked, having a mixing valve has no effect on system performance. It automates the process of using the shower valve to adjust the temperature. The reason they're required is to protect people who may not be able to adjust the temperature and avoid scalding, like the very old and very young.

  4. walta100 | | #6

    A HPWH is a refrigeration system and like all refrigeration system the greater temperature difference from input to output the lower its coefficient of performance. In short, the bigger the temperature difference the less heat gets moved and more energy is required to move each unit of heat.

    If you need more capacity, you are better off with a larger storage tank of cooler water.

    So long as the system get over 120° Legionnaires disease should not be a problem.

    Walta

    1. PGH_Build | | #10

      Thanks. I'm not particularly worried about Legionnaires, but the CDC does recommend water storage at 140F as a preventative measure

      https://www.cdc.gov/legionella/wmp/control-toolkit/potable-water-systems.html
      Store hot water above 140°F (60°C) and maintain circulating hot water above 120°F (49°C).

  5. Kevin_Henry | | #7

    The Eco2 Systems (formerly Sanden) HPWH comes with a mixing valve, and it's part of their specified installation.

    The current version of the product (GS4) apparently doesn't allow you to set the output temperature, but the version I have (GS3) allows you to choose an output temperature in the 130F to 175F range. That version of the technical manual shows COP curves for four different output temperatures. They generally show modestly decreased efficiency at higher set temperatures, with the exact amount depending on the outdoor temperature.

    In the GS3 version they say: "Sanden recommends that the heat pump set-point should be at a minimum of 150°F for the combination of maximum hot water production and system efficiency." In the GS4 version they say: "The SANCO2 unit will produce hot water at temperatures between 145°F and 150°F. Therefore, it is mandatory to install the supplied 3/4" Mixing/Anti Scald valve."

    In other words, for this HPWH the system designers feel it works best at a temperature so high that you need a mixing valve to prevent scalding. Personally, I've experimented with the temperature setting and mixing valve a bit in order to use the lowest temperature (for efficiency) that's compatible with our desired capacity and fixture temperature (for warmth).

    1. PGH_Build | | #9

      Thanks. I hadn't been able to track down COP curves for HPWHs. It's encouraging that the efficiency decrease is "modest" - realizing that the Sanden is a split CO2 system and may not be 100% analogous to an integrated unit (which has the benefit of a more constant source air temperature at the cost of unwanted cooling during heating season)

  6. walta100 | | #14

    I say a heat pump is a heat pump big or small and the laws of physics apply equally.

    Pick any heat pump in the neep index of thousands of models each with lab measured COP at 4 different temps. Make a graph of the 4 data points they all go down as the difference between the indoor and outdoor temp increases. Some start higher but the curve is uniformly down.

    https://ashp.neep.org/#!/product_list/

    Walta

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