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We live in western Washington state, and have an eleven year old Mitsubishi heat pump system consisting of:

We've noticed that the amber standby indicator on the ductless unit has been coming on much more frequently than it used to. I'm pretty sure that the change has been relatively sudden -- starting this fall as we've gone from cooling to heating.

Until recently, I'm pretty sure that the standby state was a relatively rare occurrence -- happening most often when the outside air temperature was close to freezing and there was significant heating demand (for example when we raised the temperature in the morning). Now the standby indicator comes on almost any time that we ask for more heat.

What I'd like to know:

  • Is the standby indicator reporting the state of the outside unit? Or, is it the inside unit that is in standby?

  • In the owner's manual, the indicator is labeled "standby/defrost." In the service manual on page 12 (section 6-4 Heat Operation), it appears that there is a Defrost mode and a Hot Adjust mode. What do these do?

  • Does the unit defrost because the heat exchanger is actually icing up (e.g., actual ice has been detected), or does it notice a loss of efficiency in the heat exchanger and assume that it ice -- in other words could the actually problem be dust or some other insulting layer on the heat exchanger?

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    Specifying the exact model of the heat pump and including a link to the manual online (or clear, readable pics of the relevant pages if it's not available online) would be most helpful. Someone here may see something in the instructions that you're overlooking that will help answer your question.
    – FreeMan
    Oct 19, 2022 at 11:58
  • This is certainly quite different from my ~2 year old units (which don't have any standby indication, and don't indicate defrost, either. They just do it, based on whatever logic they use for that decision.) I'd suggest walking outside and looking at the unit to see if defrost is doing what it should, if it's not visible through a window. I'd also be sure you have cleaned/washed your indoor filters and gently (I was told open hose, no nozzle) washed your outdoor coils.
    – Ecnerwal
    Oct 19, 2022 at 12:27
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    @FreeMan - done.
    – dlu
    Oct 20, 2022 at 1:17
  • @Ecnerwal - great suggestion, thank you. Do you know what "defrost is doing what it should" sounds or looks like?
    – dlu
    Oct 20, 2022 at 1:18
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    Is there any chance (because of the weird weather here lately) that you have a unit doing dehumidification while you are calling for heat from the other one? In that case you will end up with one of the units in standby, as it can only heat or cool at the same time.
    – KMJ
    Oct 20, 2022 at 4:51

1 Answer 1

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It appears that the light represents two different states on this unit:

  1. A 5-minute long "warming cycle" that happens when the refrigerant in the line from the outdoor unit is cold -- I think the purpose here is to delay the start up of the indoor unit until it can actually put out heat.
  2. A defrost cycle on the outdoor unit.

(My wife found this by digging through the on-line manual while the service tech was here. It was news to him, and he didn't take kindly to being asked about it...)

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