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.

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.

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.

We are on octopus cosy, and for the last 12 months average about £160 a month, inclusive of Elec cooking, daily dish washer, daily washing machine, twice weekly tumble dryer etc.we go through a lot of hot water. We get away with the cheap heating periods, albeit it for cold snaps we do need to extend the hours noticeably.

We have a Samsung gen6 12kw ASHP,  powering two UFH manifolds (one with 13 zones, other 5 zones). No buffer tank. From what I can tell we don't short cycle?

However  the bungalow does have an inefficient design... . Very very high ceilings, sprawling a bit, lots of glass. Children's rooms have small floor space, but then a Mezzanine in them to give them two stories under a 4.5m bungalow roof. It's cool, but means they are tricky to heat die to surface area if UFH vs volume of room.

Now everything works well, however during these sub zero the children's rooms struggle  to heat behind 18 degrees due to the design.

I am keen to see if I can force increase the UFH temps. Especially as the pays are nicely cheap in my mind, and the cosy periods at 11p mean I want to hammer them vs extending the hours excessively into the 24p tariffs.

My Samsung water law settings are....

Outdoor temp for water law:

15 degrees ........3.0


Water out temp for WL1 Heat (WL1 floor)

36 degrees..... 43degrees high target


Water temp out for WL2 Heat (wl2-fcu)

36 degs....... 43 degrees

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.

Welcome any recommendations. I know I could run the ASHP for longer... But the 11p vs 24p tariff change means I am keen to loose efficiency to make more usage of it.

Thank you!