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Minisplit thermostat setting

timgodshall | Posted in Energy Efficiency and Durability on

I recently installed a 19-SEER rated Mitsubishi multi-zone mini split with 4 indoor 6,000 BTU units in climate zone 4A (Harrisonburg, VA. ) The customers’ previous heat system was electric radiant ceilings throughout the house. The radiant ceiling and its programmable thermostats are still in place and functional. For years, the customers have kept their bedroom set to 60 degrees all day, except for an hour in the morning and and hour in the evening when they are waking up and going to bed when they bump it up to 68 degrees.

My understanding is that heat pumps perform best at a consistent temperature setting. However, is it likely that leaving the thermostat set to 68 degrees all day would use less electricity than keeping it at 60 degrees for 22 hours and 68 for just two one-hour periods?

The idea my customer had was to keep the mini split thermostat set to 60 degrees, and then have the electric resistance heat run just for those two hours to warm up the room. My hunch is that, even in this situation, just programing the minisplit to bring the room up to 68 would still be more efficient than using resistance heat for that purpose. Is this theoretically correct?

 

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Even running at full-tilt to recover from setback the ductless would use LESS THAN HALF the power of resistance heaters at typical Harrisonburg, VA wintertime temps. On the very coldest morning it might use more than half, but even at -5F running full speed the multi-split would use only about 2/3 the power of the resistance heaters.

    If the compresor is listed in the NEEP spreadsheet it will tell you it's COP at max speed at +47F,+17F, +5F, and it's coldest rated operating temperature. It can be downloaded here:

    https://neep.org/node/3725/download/b8d1bfe31bf9ce47b653aca123c6feeb

    The fact that it's starting from 60F rather than 70F means it will have a slightly higher COP than indicated in the spreadsheet.

    The MXZ 4c36NAHZ has a COP greater than 2 at max speed even at -13F, and 2.78 @ +17F.

    The MXZ 4c36NA2 isn't a cold climate unit, but has a COP of 2.67 @ +17F:

    http://meus1.mylinkdrive.com/files/MXZ-4C36NA2_Submittal.pdf

    So even when it's in the teens it'll be using less then 1/2.67 the amount of power that resistance heaters would.

  2. timgodshall | | #2

    Thank you, Dana! That is very helpful information.

  3. user-626934 | | #3

    Tim - you (or your clients) should also measure it...here's a new-ish, very inexpensive energy monitor that would be helpful for monitoring multiple circuits.

    https://stuff.iotawatt.com

  4. timgodshall | | #4

    Thanks, John. I have a Neurio energy monitor which I suppose would work okay, too.

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