Firstly welcome to the forums.

I hope you are running your heat pump 24*7 with any TRVs and thermostats set well above the desired temperature so they have no effect other than when there is excess solar gain.  Assuming we are over that hurdle then:

With 2104 set to 0 the FT (and therefore your emitter output) will flatline at 0.  However your house demand will keep increasing as the outside temperature falls so inevitably the house will cool because the output of the emitters depends on flow temperature so, even if the heat pump can generate more heat, the emitters cant transfer it to the house.

Its impossible to be prescriptive on temperatures because it depends entirely on your house but 2104 needs to be set to (or below) the minimum OAT at which you want the heat pump to heat the house and 2102 adjusted accordingly.  I dont know how cold it gets where you are, but if you were say to set 2102 initially to 50 and 2104 to -10 this would give the same curve at OAT above 0 but extend it when OAT falls below zero.   Based on your statement that it works well at OAT>2 this shouldn't be far out.  You can then tweak 2102 up/down (slowly, no more than one degree per day) to get it exactly right.

When it is mild expect radiators to feel cool at most, 35 doesn't feel hot to the touch!  If however they are stone cold (and assuming no active TRVs) then you may need either to bleed one or more of them or balance them.

@jamespa Thanks for the guidance.  Yes all trvs are fully open and the thermostat set at 25Deg.

I spent a lot of time balancing the radiators.  takes a long time to get them somewhere near a delta t of 7 degs.  

when everything had settled down and the house was feeling very good it got out of kilter when we had  sunny days which got the house too warm.  I then  set the set the thermostat to the temperature of the house on dull days to , in our case, 23 degs so the heat pump would stop heating the radiators unnecessarily on the days the sun shone.

I live in the very north of Scotland.  my back garden overlooks the north Atlantic about a quarter of a mile away.  due north from my lounge window the next house is in the Bering strait Aleutian islands.

I run 9.4 kw of solar panels and i have four 5.4kw batteries (21.6kw) on a good day my batteries will run the house and the ashp for about 12 hours using 17,4 kws but today with the oat at -5deg the batteries ran out after 4 hrs. 

ill let you know how i get on.

Posted by: @chinkoa

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@jamespa Thanks for the guidance.  Yes all trvs are fully open and the thermostat set at 25Deg.

I spent a lot of time balancing the radiators.  takes a long time to get them somewhere near a delta t of 7 degs.  

when everything had settled down and the house was feeling very good it got out of kilter when we had  sunny days which got the house too warm.  I then  set the set the thermostat to the temperature of the house on dull days to , in our case, 23 degs so the heat pump would stop heating the radiators unnecessarily on the days the sun shone.

I live in the very north of Scotland.  my back garden overlooks the north Atlantic about a quarter of a mile away.  due north from my lounge window the next house is in the Bering strait Aleutian islands.

I run 9.4 kw of solar panels and i have four 5.4kw batteries (21.6kw) on a good day my batteries will run the house and the ashp for about 12 hours using 17,4 kws but today with the oat at -5deg the batteries ran out after 4 hrs. 

ill let you know how i get on.

All sounds good and like you know what you are doing. Radiator balancing can be tedious and the poor control that LSVs give really doesn't help.  I have the same 'problem' with solar gain.  I run on pure WC for most of the season but at the extreme ends (where solar gain is highest in relation to house loss) I use a limit function provided by my controller, basically what you are doing.

I have one query and one comment.  The query is triggered by DT of 7 which is quite high.  I assume at least one LSV is fully open (otherwise you are unnecessarily restricting flow).  If you have scope to increase flow rate (pump speed) and reduce the DT you will be able slightly to reduce your FT which will save a bit.  However its only about 1.5% per degree you can reduce the DT so barely worth the effort.

My comment relates to 2103.  There is a school of thought that setting the low FT to 35 means that radiators always feel warm so its a way to avoid call outs, which is why installers often do it.  You don't need a FT of 35 when its warm outside so it may be better to set this to 30 or even a bit lower.  That said this is a finesse which may or may not help so if I were you Id concentrate on the other end first!

changed the settings last night.  this morning the house was warmer but could have been a little more warmer. outside temp -5c

I  looked at the flow rate gauge on the asap. and it is at 20 litres which is too low in my opinion.  it is set at max flow so i cannot increase the flow rate    The service engineer said he had lowered the flow rate setting to low on the outside unit, no reason other than he thought it too high.  i will set it back to medium when the weather improves.  what flow rate should i be looking at? 35 feels right?  What ct do you suggest as you think 7deg. is a bit too high. temperature at the thermometer reads 19c with a target of 25c,

Power consumption is going from 3kw to 5 kw.  temp outside now at 2.00pm is -5.5c.

Posted by: @chinkoa

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  what flow rate should i be looking at? 35 feels right?

What's the capacity of the heat pump.  Flow rate (l/s) x DT (say 5) x 4.2 = heat transferred in kW which needs to be greater than or equal to capacity.  4.2 assumes water not glycol/water mix, downrate by 10% or thereabouts for glycol.

Sounds like a bit of tweaking of WC will do the job, or it may just be a matter of time for your fabric to warm up fully and stabilise

Grant ashp capacity is 17.5kw. bear with me, whats WC

sorry WC weather comp.  so when you say tweak up to increase heat?

@chinkoa 

Sorry there are two separate thoughts going on which I didn't make clear:

"What's the capacity of the heat pump.  Flow rate (l/s) x DT (say 5) x 4.2 = heat transferred in kW which needs to be greater than or equal to capacity.  4.2 assumes water not glycol/water mix, downrate by 10% or thereabouts for glycol.  " -> so with 17kW you need a flow rate of 0.7l/s = 42l/min to get DT of (say) 5

"Sounds like a bit of tweaking of Weather Compensation will do the job, or it may just be a matter of time for your fabric to warm up fully and stabilise" -> refers to your comment that the house is warmer but perhaps not quite warm enough.  You seem to basically have sorted the problem by extending the WC curve, maybe you will need to tweak 2102 up or down a degree or two to get it spot on.

Hopefully thats now clear.