Ah that’s cool, I didn’t know this was possible. Thanks 
Hello Sarah. After a summer of tweaks and installer revisits to fit normally off valves to the supply and return, the hp working fine in what was a warm, dry autumn here in Slovakia, we’ve now had a few weeks of freezing nights (-1 - -6) and + 4-6 in the day. And now snow. As I’ve agreed previously with you, the manual has a steep learning curve, and it really takes something not to be working as expected to get an appreciation of what a particular FSV or set of same really do.
Anyway, things seem to be working ok, except for the dhw. This was fine until the colder weather, but the key issue seems to me to be the defrost function. Has anyone gained any understanding of how this function is controlled? Because the key issue is that very soon after a scheduled dhw demand and the hp gears up, it often goes into defrost mode, extracting heat from exactly the place in our heat store tank (800l) where we’re trying to put it! Now clearly the hp cannot know what future demand might be, so can’t defrost before the change to dhw function, but it would make sense to me, if a defrost is called for, that the diverter valve was turned back to heating mode for the period of the defrost, and then returned to dhw for continuing heat to the water. I can’t find any mention of defrost in the manual. Any ideas?? Thanks, Andrew
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
Thanks Sarah! Well I hadn’t spotted a thread on this exact subject! 
. A little light reading ahead…
But on your point 3 - yes, that’s exactly my view, that it provokes a defrost. I should say that the hp runs pretty much constantly during the day (5.30 to 22.00 or thereabouts) when the setback on the ufh kicks in and elec use falls back to a low rumble. So when the scheduled dhw demands occur, I think the hp starts increasing output (55° setting from heating water law of around 34-36) the fins cool rapidly, frost up and bingo, it needs to defrost.
I’ll report back anon, after a lie down to read. Andrew
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Hmm, not that simple, Andrew, I fear.
The evaporator inlet temperature (downstream of the expansion valve) tells the controller what the maximum compressor suction pressure should be to ensure 100% evaporation (by adjusting compressor speed and expansion valve position - one of the more important controller algorithms) but it (the evap inlet T) doesn’t know whether this is because DHW has been running, unless 
the controller knows that a DHW cycle has operated because of the 3-way valve position, and initiates a defrost just in case…
One of the things on my hit list to look at over winter…
Sarah