Hi again Andrew, and glad to hear that you have managed to tame your system somewhat 
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- I’m afraid I can’t really help you with defrost diagnostics (I don’t continually monitor my heat pump). I’m sure you will have read the Defrost cycle Samsung Gen 7 8Kw thread, which includes the Samsung controller logic I posted (but which makes no sense to me).
- There are n! references to “defrost” in the OEM Search function, some of which (e.g. Daikin: Defrost cycle pulled heat from DHW) indicate similar observations on other vendors’ HPs. It might be worth trawling through these for clues.
- The one thing I do know about Samsungs (courtesy their UK Helpdesk) is that “During defrost energy is taken from the part of the circuit in operation at that time, heating circuit during heat operation, DHW cylinder during DHW operation”. I can only speculate that in your setup, the DHW cycle is actually provoking a defrost (though I can’t explain why this might be - I’ve had a look though my MIM-C02N outputs for space heating runs and DHW runs and can’t see any obvious difference in Outdoor Unit operating conditions for each that might explain the defrost kick-in.)
- Now that it’s getting colder I’ll try to gather some more Outdoor Unit data using the MIM-C02N, hopefully catching a defrost cycle, to see if I can reverse engineer the defrost algorithm. (The C02N outputs really useful info, but uses prodigious laptop memory so I can’t leave it running for long - maybe an hour at most - and catching a defrost cycle will be a matter of luck…)
Sarah