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One heat pump set to 2 deg C higher than normal, or two heat pumps?

Calum Wilde | Posted in Energy Efficiency and Durability on

After reading some recent discussions in another thread, about it being more efficient for a heat pump to run constantly instead of cycle, I decided to run an experiment. It’s warm here today in my zone 6a town, 10 deg C, so I shut off my upstairs ductless mini split and ran only on the downstairs unit. It’s been 24 hours so far. It looks like to get a decent temperature throughout the whole house I’ll need to run a 1-2 deg C higher setpoint on the downstairs unit.

My question is, would it be more efficient to run the one unit with the higher setpoint, or both units as the normal setpoing?

On the main level is a Fujitsu 15RLS3H, and on the lower level is a Fujitsu 12RLS3H.

As always, thanks!

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Calum,
    If one heat pump keeps you comfortable, you should save energy by turning off the upstairs heat pump.

    Lots of homeowners with two-story houses have two minisplits: one upstairs and one downstairs. They use one unit at a time: the downstairs unit in the winter, and the upstairs unit in the summer.

  2. Calum Wilde | | #2

    My thought was the higher setpoint on the lower unit might use more power than I save by not running both.

    I highly doubt I could get away with just one all winter, but certainly in the shoulder seasons and on the odd day like this. We just finished a week of -18 deg C, I'd probably need over 20k BTU to keep up with that.

  3. User avatar
    Dana Dorsett | | #3

    If the upstairs unit is cycling a lot and the downstairs unit isn't running at more than 2/3 speed you'll almost always get better net efficiency by bumping up the set point on the downstairs unit to carry the load.

    When it's cold enough that it doesn't keep up or the upstairs to downstairs temperature difference is too much for comfort, by all means turn on the upstairs unit during those hours/days.

    When below -10C (+14F) the efficiency doesn't change very dramatically with speed. See figure 5 of this document for an idea of how the steady-state COP varies with temperature:

    https://www.nrel.gov/docs/fy11osti/52175.pdf

    But the average efficiency still falls off quite a bit (from an already low COP) if it's cycling a lot, wasting a lot of power in each spin-up from zero to even a low speed. When it's above 0C the steady-state COP changes much more dramatically with speed, but cycling will still rob that by a substantial amount.

  4. Jerry Liebler | | #4

    I'll defer to anyone who has a logical difference with my RECOMMENDATION which is: Absolutely better to run 2 degrees warmer set point on 1 than 2 units. Regardless of the characteristics of the units.
    From what I've learned the best performing of today/s units, optimally matched to the thermal load. still has about 5c where cycling will occur.
    Edit: With the same Caveat. I'll go a bit further and recommend: Whenever the smaller unit can keep "the place" comfortable thru out you should be using it alone and adjusting as needed to maintain comfort thru out.

  5. Calum Wilde | | #5

    Dana, thank you very much for the thorough answer.

    "from an already low COP" Looking at the information in that lab test, and comparing specs, it looks like the 12RLS has good COP compared to similar sized units. Were you just referring to the low COP due to the temperature range, or compared to the competition?

    Jerry, thanks for the reply. I'll be taking the recommendation.

  6. Jerry Liebler | | #6

    Dana,
    The graph, figure 5, of COP vs ODT is at constant, rated condition or actual available, thermal output NO MODULATION!. COP drops as capacity (compressor speed) is reduced! Some brands and models show COP<1@ min output.@ODT=47f.
    EDIT: After looking at the words below the graph I realize the above is erroneous, the graph is 3 graphs in one at min,mid and max compressor speed, It shows much less attrition, by modulation, of COP than the Mitsubshi and other brands.

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