My guess would be that the unit is measuring combined COP for heating and hot water. Therefore because now you are only heating water at a lower COP than heating your average drops. 

@ecobaker,

Posted by: @ecobaker

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I don't have any advanced monitoring, just the energy use and generation output by the unit itself.

How do the energy use and generation output differ between the good COP and bad COP weeks? Is it that the energy used has increased or the energy generated has decreased to give you the lower COP?

My Samsung Gen6 8kw had a bit of brain fade earlier in the year when the recorded energy generation figures became ridiculously high - 1,398kWh for one week in February. The energy consumption was recorded as a (more believable) 90kWh, so a fantastic COP of over 15 🤗 . I knew something was really wrong when I saw that the recorded weekly operation time for that week was 524 hours - around 75hrs a day. Samsung Technical Support talked me through a reset of the Wired Remote Controller and that appeared to sort the issue.

@bontwoody The thing is I have been using it for only DHW for a month or two, i was getting higher cop just on the hot water, so it can’t be that I’ve turned off the heating.

@sjn Okay so it was a software issue. It appears my energy use is going up, but my energy generation is staying about the same according to the numbers.

@ecobaker Yes, it must have been a software bug disrupting my historic energy numbers. I haven't looked at them recently so I should check to see if they are still ok.

Your generation staying about the same makes sense for a consistent use of DHW so it is the increased usage that is unexpected.

One thing I have noticed is that there is some rounding going on with the displayed energy numbers - the sum of the energy figures for each day does not add up to the displayed total for the week. I wonder if with DHW only the usage and generation will be quite low so the rounding errors may have a bigger significance to your COP calculations, possibly amplifying any week to week changes due to different outside air temperature 🤔 .

@sjn I've seen this on other brands as well. Rounding for small consumption (for instance only hot water) really messes up the accuracy. 
Another thing we've seen is that some manufacturers just add on a flat energy use per hour (during periods when the heat pump is not running). Maybe Samsung did a software update recently and started doing this as well?

@sjn i've actually been doing calcualtions on a daily basis, so a rounding error would make a big difference.

@hcas Thing is I've not updated the software, that's actually quite hard to do with Samsung as its not directly connected to the internet.

SOLVED

So it turns out the problem is, I am a massive Idiot. 

A few weeks ago, I had used most of my hot water, and needed a quick shower as I was in a rush and had to go out. So I put on forced mode for DWH and turned on the immersion heater, which I normally have turned off, to turn on after 15 mins. When I had enough hot water I turned off forced mode but forget to turn off the immersion in the field settings. 

So my sudden drop in COP has been down to my immersion firing up and helping heat my hot water for the last few weeks 😭 

Turned it off and I got 4.1 cop for DHW today

Glad to hear it's resolved, and wasn't a major issue.

@ecobaker May I ask a question of you please. I too have a recent install of a gen 6, one of the 16kw units. As it’s warm, only been running DHW cycles and trying to get my head round how it can be tweaked to run optimally. 
With my legionella cycle off, and a hot water set point of 45C I’m getting a COP around 3.6. But it’s not behaving as I’d expect, mine seems to ramp up the circulation pump inverter to 100% and ramps up the heat pump power input to about 6kW and then ramp it down again, it finishes the cycle in less than 20mins (a 250L tanks) and uses approx 1.5 kWh. I was expecting it to behave less bruitishly, tickling up the flow temp slowly to get better efficiency. 

Does yours go flat out too please?

thanks