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What is incremental power needed to raise house temp a few degrees with Mitsubishi minisplits?

Kenneth Gartner | Posted in Energy Efficiency and Durability on

I have an all-electric house in MA heated exclusively with two MUZ-FH09NAH  ( ductless minisplit with pan heater).  This is my first heating season so I am still on a learning curve and am desirous to optimize my costs and comfort.

I was pleasantly surprised to find that the Mitsubishis generated heat even as the thermometer plummeted to minus 7.6 F a few weeks ago.  We keep them set to 64 degrees indoor but want to know what incremental power consumption would be incurred if we raised temp to 66 or 68 degrees.  In order to answer this, I tried to use the manufacturer documentation, linked at the following URL, specifically the table on page 14 and the graphs on page 16, screenshots will be attached.

https://www.manualslib.com/manual/908936/Mitsubishi-Electric-Muz-Fh09na.html

Reading the heating tables has been frustrating and counterintuitive so I hope for some GBA guidance.   I am looking to optimize TPC (total power consumption in kwh).

1. Looking at table on page 14, it looks like TPC is less than 5% higher for raising indoor temp from  65 to 70 degrees  (TPC @ 65 of .62 versus TPC @ 70 of .64).  That seems a very reasonable increment.

2. It is counterintuitive (for me) to see that TPC *increases* as outdoor intake air temp increases.  Shouldn’t the indoor heating TPC be *less* when the outdoor temp is 55 (TPC = .74) degrees than it is when the temp is 35 (TPC = .66)? I realize that the delta temp magnitude drives the heating engine, but my ears tell me that the duty cycle (ie when the outdoor unit is working hard) is more frequent at 35 than 55.  Will my Spring heating bills actually be higher than my Winter ones?

3.  My confusion is reinforced by what looks to me like an axis error for the graph of FH09NA, which shows the 65 degree interior temp has a *higher* TPC than 75 temp, while the opposite is shown for the FH09NAH graph. I think the typo is on the latter graph, no?

A brief primer on how to understand these tables would be helpful to me.

Thanks, everyone.

Replies

  1. John Semmelhack | | #1

    Kenneth, the Mitsubishi documentation isn’t going to tell you the answer to your question. The energy required to raise the temperature is a bit unique to your particular house and heat pump installation. I suggest measuring it with an energy monitor. Here’s my favorite: https://stuff.iotawatt.com/

  2. Charlie Sullivan | | #2

    Increasing the indoor temperature will:

    1. Increase the heat load--the amount of heat needed. If you've got a 7o degree indoor/outdoor temperature difference, a first-order estimate is that raising the indoor temperature will increase the heat load by 10% if you increase it by 7 degrees.

    2. Decrease the output of the heat pump, when it's running at maximum.

    3. Decrease the input power of the heat pump, when it's running at maximum.

    If the effects 2 and 3 where the same size, they cancel out and wouldn't affect the total energy consumption, and only effect 1 would matter. But the decrease in heat output is bigger than the decrease in power consumption. So those two make things a little worse as well.

    The net result of 2 and 3 is rolled in to the COP. So if you look at the data for COP, and combine that with effect 1., you'd get a reasonable estimate.

  3. Doug McEvers | | #3

    It was minus 44 F in International Falls, MN this morning and coming your way. You may have time to upgrade your electrical panel. Let us know how the minisplits do in the days ahead, I am amazed at the advancement in heat pump technology, exciting stuff.

  4. Jon R | | #4

    > Will my Spring heating bills actually be higher than my Winter ones?

    The total power consumption (TPC) they list is a maximum and should not be confused with the power consumption that you will pay for. Your spring bills will be lower - mostly because load is lower and partially because efficiency is higher.

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