Posted by: @andehh

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However even setting the offset to +5 degrees, after 4 hours of operation the outflow temp is still only 29 degrees, and the rooms are all a couple degrees colder then I would like.

4 hours isn't very long, houses have a large heat capacity (ie energy stored in the fabric) and can therefore take a day or more to reach a steady temperature.  Is it possible that you have hit the maximum output of the heat pump which, if it were the case, would explain what you are seeing?  ASHPs are generally sized on the assumption that they are run full time; if you are only running part time you could quite easily be hitting max output even though 12kW is quite a large ASHP.  What was the design loss at what OAT and for how many hours are you running?  

If you have in fact reached the max output capacity then there is not much more you can do with the heat pump.  At this point its either run it for longer (as intended) or deploy some supplementary heating.  Remember its only this cold for a few days of the year so if you hammer your electricity bill for a few days its not so much in the grand scheme of things.  You do have to think about any heating, but particularly ASHPs, over a season since load varies quite dramatically.

@andehh, do you have a way of monitoring the power being produced by the ASHP when the UFH is on or, failing that, can you check the Instantaneous Usage shown in the Energy information display?

With your Water Law settings and the +5C set point, the target output temperature should be 48C when the outside air temperature is 3C or lower. If the output temperature is only 29C then I would expect the ASHP to be running up to its 12kW maximum to achieve the target (with regular defrost cycles given the low outside temperature).

With my Samsun 8kW Gen 6 it is very obvious from the noise when it is running at maximum during the DHW cycle. How does your 12kW unit sound during heating and DHW operation?

The reason I am asking about the power output is that my 8kW unit is regularly failing to achieve the Water Law target temperature but it is running well below full power, see this topic.

Highest draw I've seen from it is about 5kw input via the instantaneous power menu , though I don't check it too often. From octopus half hourly it seems to only pull 2 to 2.4kw over a 30min window on average..... Ie 4am to 7am slot. DHW is the same.

I presume this is ok? 2.4kw x my Cop of 3.5 ish comes close to the 12kw which I imagine is the OUTPUT up to?

And yes.... It could be 29 degrees is as hot as it can get if the whole slab is relatively cool from a period of 3-5 hours of a heat off cycle I guess?

I have added an extra 2 hours a day onto the schedule and will monitor, maybe add a couple more each time until 'cold' winter passes and then I'll reduce it back to cosy periods for the shoulder months.