OpenEnergyMonitor Community

Reducing heat pump standby power

Hi there…

While pondering my summer electricity bills I came upon an article by John Cantor about crankcase heaters… This explains that heat pumps use electrical power to warm the compressor crankcase while not in use. The purpose of this is to prevent refrigerant mixing with the compressor oil when idle, which could otherwise cause bearing failure on start-up.

Now, we have a monster set up here with two big Daikin high temp heat pumps (so two compressors on each one). I quickly decided to invest in an EmonPi to investigate.

A bit of online research revealed that the crankcase heaters use 33W each, and maintain the temp at 70-75C, so I was expecting maybe 150W tops. Well, that would be bad enough but actually, our system guzzles over 400W doing absolutely nothing…! OMG…! And this is meant to be good for the planet???

Now, the manual also says that the system should be left on for 8 hours to warm the compressor through before use, so it’s really not possible to hook the heating/hot water demand signals into some big power relays and clunk everything on and off when needed.

I am an experienced electronics engineer and hacker, so I’m ready to get in there and do something drastic. My current thought is to add an Arduino with a temp sensor plus a relay and bypass the built-in controls for the crankcase heater. Then I can shut the system down fully, keeping it warm myself until it’s actually needed. If I also leave one fully off until I really need both heat pumps, my calculations show that I can reduce my standby power use by nearly 80%. That will be my next project then…

My question is - is anyone else similarly outraged about crankcase heaters and the standby power use of their heat pumps…? Has anyone else tried any other drastic solutions…? Does anyone know of any other reason not to hack my system this way (apart from a wide range of electrical safety concerns that is…)


my heat pump has nothing like that. – I have solar water heating for me when the hotwater is over 50c a relay that turns off all the main power to my heat pump so none of the circulation pumps run ( one runs all the time and one pulse ran saving about 300w on background consumption ) so basically they are skimping on robustness of compressor pump design. to make a smaller lighter cheaper pump… that is similar to what they did to squirrel cage blower motors used in furnaces too. older motors had full winding and were very efficient motors and operated 30- 250 watts depending on speed selection. more modern ones then took out most winding the motor gets super hot and it consumes the about the same amount of energy irrelevant of what speed setting you have it on…( generally 500 watts).

For high current use, a contactor as the relay, you’ll need a low power relay like a Sonoff Basic (flashed with Tasmota) to control it as well. For temp it is hard to beat a WemosD1 Mini flashed with Tasmota for simplicity.

Well I would first double check what the real vs apparent power is being consumed here for these crank case heaters.

I know that on some of the systems I support, the inverter is used to excite the motor windings to act as a crank case heater. On these heat pumps, the current drawn however is not in phase with the voltage (low power factor) whilst in “standby”, so a clamp meter might register a couple of hundred watts, whereas in reality the actual power recorded by the electricity meter is rather minimal.

On other commercial heat pumps I work with (3-phase >16kW to 130kW fixed speed & staged), a large jibilee-clip style crank case heater is often wrapped around the compressor body and may have an inline bi-metal stat & contactor arrangement to switch it as needed.

I personally would have the crank case heater running whenever the manufacturers software decided it was necessary, rather than try to hobble the crank case heating and have a system fail due to oil issues. A replacement compressor + refrigerant recovery + re-gas is not cheap. If the damage is bad enough to cause burn-out, the refrigerant system could become acidic too, which increases the repair costs.

So yes, it is annoying to see a waste of energy like this - but to put it into perspective - I would imagine the reverse cycle defrost needed on most ASHPs would use more energy than the crank case heater…