What is a heat pump doing when not doing anything?

I have a ductless heat pump system with one outdoor and five indoor units. We live in a mild climate, so most of the time all indoor units are turned off. Many months ago, Sense detected a new device and was 58% sure that it was a heat pump. The signature is an “on-off” cycle with 15minutes on (at about 30W) and 30 minutes off.

To verify that that device is actually part of the heat pump, I turned the breaker off and this signal disappeared until I turned the breaker back on a day later. Conclusive: It is on the 240V circuit that only connects to the heat pump outdoor unit (which provide power to all indoor units via the bundle of power and pressure tubing).

So, with everything off, it still does consumes power, but not always. What is it actually doing??? What can possibly take 30W for 15 minutes at regular intervals? Self check? It’s a standalone system and not network connected. There are two thermostats connected to their respective units via redLINK, but that’s it. It’s more academic curiosity to understand what’s going on, a 10W average is not significant. :smiley:

Sense was never able to identify the real heat pump usage during heating or cooling. I am not sure if the pattern of “device 5” changes when we are actually heating or cooling.


Several low voltage transformers powered up your main boards, thermostat. Each transformer at item generally draws ~20% of the full load while at idle.

I would expect these to be on continuously, but we have 30 minute gaps with zero draw.

(Granted, this is just a “detected device” and I really would need some current clamps on that circuit to tell the whole story. Since I have also solar, there is no spare. As has been discussed before, we really need a viable option to monitor such circuits directly. (I do have a handheld clamp meter, maybe I could do some casual tests)

That’s a pretty low wattage, but it could be a crankcase heater: Crankcase heater - Wikipedia

Some units actually use the compressor windings themselves to provide heat when the unit is off.

Your unit may also have a drip pan defrost heater, which I would suspect would be higher wattage, and only come on below freezing.

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Not knowing how your circuit is run, but my Air Handler in the Attic has a pretty constant 15w continual wattage draw.

Interesting idea. The duty cycle did not change all summer, and we had a couple of serious heat waves. It was never really cold. (This is Southern California!). I guess if I could get my hands on the maintenance manual or a circuit diagram … :smiley:

Circuit diagram is almost always printed on the inside of an access panel in this type of equipment

Too easy. If you do not have wired remotes on all your indoor heads It is your indoor heads sampling the air (running their fans on quiet) to determine room temperature.

Or you have a sticking expansion valve in the outdoor unit that is getting constantly reset.

Lastly it could be the branch box if you have one.


No, the indoor units are off (not in fan/cool/heat mode or similar) They are in standby, just listening for their remote controls. There are no fans running. The two units connected to the thermostats have a faint LED to indicate the redlink connection. As required by the local code, all indoor units require an external physical switch. I could turn these off to see if it makes a difference. I’ll try that next…

(I have not look inside the outdoor unit for a circuit diagram yet.)

assuming your indoors are off- built- in compressor heater and/or or crank case heater.

if your indoors are on - those are the fans sampling the room air on regular basis if you are not using dedicated wall thermostats.