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Community and Q&A

Mini-split energy usage

stk | Posted in Mechanicals on

My first winter with a Mitsubishi ductless 15000 BTU (MSZ-GE15NA & MUZ-GE15NA). It’s been in the low 30s in Seattle the past few days. I attached the electric usage recorded by my e-gauge. In the past (warmer temps) the usage curve is pretty flat. Now it’s pretty jagged. Is it working properly (i.e.efficiently)? Thanks.

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  1. stk | | #1

    The unit has been on continuously for the past few weeks. The left side of the graphic is the typical flat curve I have seen, where I assume the unit is modulating. The flat line is affected only when the refrigerator compressor cycles on and off. Then, when the temps recently started falling into the 30s, the profile gets jagged. The electric range affects the usage amount, but the shape of the curve is what I am wondering about.

  2. Expert Member
    Dana Dorsett | | #2

    You may be seeing some spiking effects related to defrost cycling, but it does look odd to me. Looking at the daily Wunderground weather histories for 4 & 5 December don't show any clear correlations to the odd behavior.

    If this is monitoring the whole house and not just the mini-split there could be alternative explanations for the spikes (such as an old electric baseboard heater used for freeze protection in the garage that hasn't been decommissioned coming on), but if it's just the mini-split it looks like something is amiss.

    The spikes to 3kw+ have to be from something else (or a major melt down), since the GE15 running at it's maximum draws a bit less than 2kw according to the submittal page sheet:

  3. stk | | #3

    Dana, yes it is a whole house e-gauge. Main elec loads would be range, refrigerator, gas dryer motor, laser printer, lighting and (if used) a 1500W oil filled radiator portable heater. No elec resistance heat. I will check if anything was left on by mistake, or else turn off the ductless and see what the remaining usage looks like.

  4. stk | | #4

    I had my wife turn off the ductless and the powers usage went down to under 200 watts, so this usage profile is almost all of the ductless. NWS says currently 37 degrees F, mixed precip/fog.

  5. stk | | #5

    Sorry, forgot to add that the big spikes were from the stove at breakfast time!

  6. Expert Member
    Dana Dorsett | | #6

    A temp of 37F and foggy (95% + RH) results in a substantial & copious defrost cycles, which may explain the change in profile.

    When it's in the mid-40s the modulation rate is lower the incoming air at he coil is warmer, and the coil stays above freezing. There can be copious condensation at high RH in the mid-40s, but it doesn't accumulate frost until outdoor temps come down a bit. Even when it does need to defrost when it's still in the 40s out, the defrost cycles are shorter & shallower than when it's in the 30s.

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