Again it will depend on control strategy if it’s compensated the getting a delta t of 5c may be impossible 

take a room temp of 19.5, a set point of 20 and a calculated flow of 24c..  then the delta t cannot be 5c

on off - balance for delta t

compensation  controls  - balance for heat distribution

Balancing is significant though 

Posted by: @saf1973

@alec-morrow

So I'm going to take a different approach and balance the rooms and not worry about the Radiator Delta.  My installer has said the rads don't need upgrading (mostly T2s) but I'm thinking they do.   As it happens this weeks forecast is 7 degrees, which is where my ASHP is rated at for 4.3 COP, so its a good week to look at room temperatures and the HP flow temp.   I will start high and check all the rooms can get up to 21C, those which overshoot i will throttle back on the rads and those which undershoot i will open up the rads.  I will continue to do this while dropping the HP flow temperature until I cannot hit the desired room temps.   At the end of the week I should be able to see what my lowest flow temp is and if that is nowhere near 35C at 7Deg outside then i will know my rads are undersized.

There is a buffer in the system which reading through the forum will degrade performance, I've opened up all the EVOhome TRVs to high temperatures which wont be reached.  The Danfoss system pump is set to Constant Press II, I have it set there as it was the recommended setting for TRVs with a buffer but I may look at a different setting as I have the TRVs open.

Hi Saf1973,

Keeping your home warm is very much a balancing act, between keeping the family happy and keeping the bank manager happy.

ASHP's are more efficient, and more economical, when operating at the lowest water flow temperature that meets the heat demand. But the heat demand will not be uniform throughout your home, some rooms will have a low heat demand compared to the capacity of the heat emitters, but there will be at least one room that will require a higher water flow temperature to achieve the desired air temperature. Since a heat pump can only produce warm water at one temperature at a time, the coldest room therefore sets the minimum water flow temperature required to heat all the rooms in your home. When the coldest room is up to temperature, the other rooms will be above the desired temperature, so will need to be scaled back in some way. This can be done by using the shield lock valves on the radiators or merely by using TRV's, though temperature control may not be that accurate.

If you are finding that you are having to scale back the heating in the warmer rooms by a considerable amount compared to the coldest room, then if space and money allow, it may be prudent to replace the heat emitter in the coldest room, which of course would probably create a new coldest room, but should improve the overall efficiency of your system.

@derek-m thanks for the input and you have confirmed my thoughts on how I am proceeding.    The installation is due to be finished this week and the installers said I don't need radiator upgrades, however I'm finding otherwise.  The HP is operating (heat only) and they are to finish the DHW side this week.   I am already seeing that some rooms cannot get up to temperature even with a 50C flow and lockshields fully open.   I am like you say throttling back those rooms which are meeting the heat demand, so in effect balancing the house rather than the delta across the rads which was the traditional way.   This week is a perfect opportunity to test temperatures as we are at 7C outside which is the temperature at which the highest COP is given by Daikin at 35DEGC Flow.   I am betting that by the end of it i will see that all rads will need changing.

I have reduced the flow temp today and will see which rooms cannot get up to temperature, as i go I open up the lockshield in rooms where needed to see if the temperature can be achieved and obviously those that couldn't meet the demand at higher temperatures will fail even more miserably.  I will post my results when i'm done.

Posted by: @saf1973

@alec-morrow

So I'm going to take a different approach and balance the rooms and not worry about the Radiator Delta.  My installer has said the rads don't need upgrading (mostly T2s) but I'm thinking they do.   As it happens this weeks forecast is 7 degrees, which is where my ASHP is rated at for 4.3 COP, so its a good week to look at room temperatures and the HP flow temp.   I will start high and check all the rooms can get up to 21C, those which overshoot i will throttle back on the rads and those which undershoot i will open up the rads.  I will continue to do this while dropping the HP flow temperature until I cannot hit the desired room temps.   At the end of the week I should be able to see what my lowest flow temp is and if that is nowhere near 35C at 7Deg outside then i will know my rads are undersized.

There is a buffer in the system which reading through the forum will degrade performance, I've opened up all the EVOhome TRVs to high temperatures which wont be reached.  The Danfoss system pump is set to Constant Press II, I have it set there as it was the recommended setting for TRVs with a buffer but I may look at a different setting as I have the TRVs open.

Hi Saf1973,

You have to be careful how you interpret the manufacturers data when it comes to COP values and water flow temperature. Since your system can only operate at a water flow temperature around 35C, if the overall heat demand and total heating capacity of your heat emitters allows it to do so.

If you have heat loss calculations and details of your heat emitters, it should be possible to estimate the required water flow temperature at different outdoor air temperatures using the modelling tool that I created. I have attached a copy of the one I created for Kev, who has a 14kW Ecodan.