OK I did some very crude energy balance calculations attached.  I had to make lots of assumptions, some of which matter to the outcome, many of which dont.  I also had to do a bit of 'interpretation' of your numbers.

In summary, with all the assumptions made, the calculation estimates that, when the OAT is 0C you would use about 5% more energy than you currently use if you heated both zones 22 hrs per day, at a single flow temperature just sufficient to keep the 'upstairs' warm enough, throttling the downstairs flow so that its not too warm.  using the same assumptions, when the OAT is 7C, you would save about 5%.  The single flow temperature comes out at 35C (at 0C OAT) in my assumptions.  These are all well within the error margin of the calculation and the assumptions, so to the accuracy of the calculations simplifying it makes no difference to the cost (but perhaps improves the comfort)

Given all the uncertainties in the modelling I dont think its worth taking the analysis any further, its something you could try or not if you want to!

WOW - thank you so much for the time you spend working this out, had a Quick Look and I def need to look super close to try and understand all those numbers, but much appreciate that you summarised above.

Considering how close the figures are it def seems worth a try, how do you suggest to set this plan?

Select a heat curve which does 35c @ 0 which I think is 0.45, and then set the stats to inactive? setback? remove any program times? All the settings so confusing 🤣 

Posted by: @jeegnesh

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All the settings so confusing

They definitely are.  Part of the problem is that some settings apply only when others have certain values.

There are probably some mistakes in the working somewhere so please don't take it as anything other than a very general indication.

I'm terms of implementation, you need to think about the heat pump and the external controls.  So both zone valves need to be open and the relevant thermostat(s) calling for heat.  I think you said both zones were open when the downstairs was on, so it may be you engineer this by setting the thermostats appropriately, one high the other low.  Then if you change the downstairs zone heat curve and set heating modr to manual, hopefully that will result in the heat pump staying on, on a single curve, except when it's doing dhw (dhw does take priority).

Id probably start off with a curve somewhere in between, say 0.6, and adjust from there.  You somehow need to throttle the ufh down to balance it.  Hopefully there is a valve that does that.  This is just the same concept as balancing radiators (which is a bit of a pain).

And do make a note of all settings/take a screenshot before you change things!

No guarantees what this will do but if it ends up about the same cost it will be easier to manage and optimise and also probably give better comfort.  Obviously if it ends up being more expensive you may want to revert.

Just a quick update, during the cold spell I went full WC but I ran into a different issue, the flow temp would max out at around 36c when it asking for 42c -- you will see one curve that went up to 40c this is when I turned the UFH part off so half the system running.  The figures are 1.1c lower than reading at the control itself this I don't know why (spikes is hot water boosts).

The mad part for me, you can see that its pretty much uses close to 4kw all the time, so 4*20=88kw per day (thats surely insane by any standard?)

To then throw a spanner in the works today I tried test again with the whole system and it got up to 38c when target was 41c but it was warmer outside then previous test -- I have reported back to my installers see if they can come up with any reasoning or something is not working right.

Becoming a challenge finding that happy medium of comfort and cost, totally not convinced pure WC is going to tick the cost box although it ticks the comfort box.

Posted by: @jeegnesh

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The mad part for me, you can see that its pretty much uses close to 4kw all the time, so 4*20=88kw per day (thats surely insane by any standard?)

Consumption is not that much different to mine when it was very cold.  You wont be getting a COP of 4 when its -3~-5 outside, more like 3 or possibly a bit less.  88kWh per day when its very cold is not unbelievable, most of the time it will be one third this amount.

Based on what you say and the graph you have posted it looks to me like its not fully recovering from defrost (and thus not reaching target FT) and I can see that the FT during defrost is going pretty low.  Unless you have a sizeable volumizer (do you?) you are almost certain to have trouble with defrost if you have only got part of your heating 'in circuit' at the time the defrost takes place which, given the way it is operated is inevitable.

Why is the FT going up to 55C 2-3 times per day, is that DHW.  How is the DHW set up?

Sadly I am not convinced your installer will understand it properly based on what you have said so, If I were you, I would post any advice you get here for a complementary view.

Unfortunately I think you might have to go back to basics if you are to get to the bottom of this.  I dont think anything is fundamentally wrong, you are just suffering from a system which has been set up by someone who perhaps hasn't thought about it as much as might have been helpful, possibly with a fossil fuel boiler rather than a heat pump mentality.

You can help things by putting the DHW on to a timed operation so it’s only topping up before you need it, not continuously, then your system can be used just for heat.

As James says you are basically running two half systems so your pump may not have all the water volume it needs.

Is your kW shown on your image heat or electricity?