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Community and Q&A

Minisplit: Leave it on or turn it off?

jackofalltrades777 | Posted in Energy Efficiency and Durability on

Very well insulated home. Nudura ICF Walls (2.5″ EPS x 6″ concrete x 2.5″ EPS) with R-40 roof, 3″ slab insulation, R7 windows, Zone 4 climate at 5,000 feet elevation. Under 800 sqft of conditioned space and I have a 1 ton ductless Mitsubishi mini-split heat pump (23 SEER).

TURNED OFF:
At 7am the interior temp is 68F and by 6PM the interior temp is 74F. Outside temps in the 92F-98F during the day with a blazing desert sun. .

Should I leave the unit off and turn it back on around 6PM to cool the house down or just leave it on all day? Which is more efficient?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Peter,
    In most cases, it's more efficient to let the minisplit run continuously rather than play with setbacks.

  2. Expert Member
    Dana Dorsett | | #2

    The part-load efficiency of mini-splits is 2-4 times higher than it is at full speed.

    When it's 92-98F outside, 75F inside a better class mini-split running full out to recover from the setback might have an COP of 3-3.5 or a bit higher.

    When it's 85F-90F outside running at minimum speed it'll have a COP of about 9-10.

    That's about three times as much heat removed per kwh as you would get removing that heat during a recovery ramp later in the day when it's hotter outside. As it gets hotter out the speed will go up and the efficiency will drop, but it's not having to move as much heat at those higher outdoor temps and higher speed either. It won't be a 2/3 savings by letting it run all day in modulating mode or even half, but it will still be well into double digit percentages.

  3. jackofalltrades777 | | #3

    Dana,

    So I should let it run all day long but set it to 74F inside during the day and late evening set it back to 70F?

    The mini sits on the east side of the home so it's shaded in the evening.

  4. Expert Member
    Dana Dorsett | | #4

    It doesn't much matter which side of the house it's on. The direct solar gain onto the chassis isn't going to be very much, and even at low speed it's moving gia-normous amounts of air.

    "Set and forget" is generally the stratege that would use less energy. The is nothing to be gained by a 4F setback. Bumping it down by more than a degree or so at a time forces it into a higher speed/lower efficiency mode for some period of time, erasing some or all of the energy savings from having been at a higher indoor temp through the day. If you're leaving for the WEEKEND, sure, let the house temps rise to 80F or more while you're away. But if the absences are only for 8-12 hours at a time, set it & forget it, let the mini-split do it's thing and modulate at it's best efficiency as the load rises and falls over the course of the day.

    With steady air temperatures the surface temperatures will be lower than in a setback situation too. If you let the house get too warm during the day the walls / floors/ ceilings are all still radiating some heat at you even when the air temp has hit the setpoint, and doesn't stabilize for a few hours. You may find that a stable 72F is as comfortable as a 70F air temp in a house that heat-soaked at 74F all day.

  5. 52Dave | | #5

    Dana,
    Not to hijack this thread, but would this also be applicable for a standard ducted heat pump situation? I realize that your standard heat pump does not modulate to the degree/at all that a mini split would. Are there advantages in maintaining a constant temp to minimize heat soak etc?

  6. Expert Member
    Dana Dorsett | | #6

    David: Standard ducted AC or heat pump systems are usually single speed, and run at pretty much a single COP (or a small range depending on the indoor/outdoor temperature difference.) If the heat pump is SO oversized that it can cool the place in a reasonable amount of time from a 5F setback it can be more efficient to use the setback from an energy use point of view, since the run times during the recovery ramp are long and continuous, rather than cycling on/off dozens of times, with some energy lost to low efficiency with every spin-up of the compressor. It takes most single speed heat pumps a good 8-10 minutes of run time or more before average efficiency during the cycle as a whole begins to approach it's steady-state efficiency.)

    In heating mode if the setback is deep enough that backup resistance heat strips are engaged during the recovery ramp the efficiency can be pretty atrocious. Even with somewhat oversized heat pumps it's usually better to "set & forget" when in heating mode.

    These guidelines are about minimum energy use, not minimum cost of operation. In locations with time of use rates or surcharges for power use during peak grid-load times it can be cheaper to adjust the above recommendations to lowest cost to optimize to the rate structures. In some locations a wi-fi thermostat can allow the homeowner to participate in the electricity demand-response markets by subscribing to a program that allows a remote party to bump the thermostat a few degrees (or even off), in return for payment or a break on the power bill.

    Of course it's always more COMFORTABLE to keep it at a constant temp, which will keep the radiant temperature from straying too far from the air temperature.

  7. 52Dave | | #7

    So, as an example, today we were in the upper 80’s, almost 90. We participate in our power companies peak demand event program. As a result, our heat pump did not turn on, due to being set back to 79, until 4:30pm. It ran until 6:30 to recover back to 75. From an energy efficiency stand point, this is the most efficient method of running a standard ducted heat pump while cooling.

  8. Dottie62 | | #8

    Is it safe to leave a ductless mitsubishi air conditioner on while away for 4 days?

    1. GBA Editor
      Martin Holladay | | #10

      Dottie,
      Of course it's safe. These appliances aren't dangerous. You may want to adjust the thermostat setting in order to save energy while you are away, but adjusting the thermostat is done to help your budget, not for safety.

  9. Jon_R | | #9

    Regarding the original question generically:

    Whether any particular heat pump set up/down/off makes sense depends on many things:

    1) how long the setback is for
    2) outside temperature vs time
    3) the COP vs load curve of the heat pump
    4) the ability to limit heat pump output during catch-up - keep it to <= 100% of rated capacity and COP remains high.
    5) time of use rates
    6) etc

    The amount of comfort reduction (or in some cases increase) depends on insulation, thermal mass, humidity, etc.

    If estimates show it's anywhere close, use measurements for the best answer.

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