Thank you @bobflux , @ashp-bobba , @f1p , @old_scientist , @grahamf and @jamespa for replying with such a wealth of knowledge. I didn't realise so many people had responded as I only had maybe 3 email notifications, so I've gone back to the beginning and re-read the lot.

Really good information, and everything explained in terms even I can understand, so thank you all for your contributions.

Regards 

Craig 

It is simply not practical to set the flow temp to 20C for a desired room temp of 20C.

Er thats exactly what the weather compensation curves on my Vaillant do doubtless because, as Graham F says, its a logical thing to do.  You can overlay a minimum flow temperature, but you dont have to.

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The radiators (emitters) cannot emit heat into the room if they are the same temperature as the room.

er thats the point I think, when the room is 20C you dont want them emitting heat into the room (assuming 20C is your target temperature)

Start from the amount of heat (kW) your heat pump will generate/output when running on minimum. That is the minimum amount of heat you need to dissipate into the room through your emitters. Knowing the rated heat output of your emitters, you can then calculate the flow temperature that will be required to emit that amount of heat. It is pointless trying to set the flow temp lower. The heat has to go somewhere. If the system cannot dissipate the heat, then the heat pump will simply cycle off.

Once the OAT is sufficiently high so that the demand is less than the minimum output, either you must allow the room temperature to rise above target or accept that cycling is inevitable, whether controlled by the heat pump or controlled by some temperature sensor somewhere.  Why is it better to impose cycling externally to the heat pump than it is to let the heat pump optimise its own cycling?  Do we think that somehow we are better at designing an optimum cycling regime than the heat pump manufacturers themselves are, and if so why?

The radiators (emitters) cannot emit heat into the room if they are the same temperature as the room.

Start from the amount of heat (kW) your heat pump will generate/output when running on minimum. That is the minimum amount of heat you need to dissipate into the room through your emitters. Knowing the rated heat output of your emitters, you can then calculate the flow temperature that will be required to emit that amount of heat. It is pointless trying to set the flow temp lower. The heat has to go somewhere. If the system cannot dissipate the heat, then the heat pump will simply cycle off.

Obviously, the WC curve has two ends.  You need to get both of them set correctly.  Otherwise, the house won’t keep to a steady indoor temperature when the outdoor temperature changes.

Setting it to 20C flow temperature when it is 20C outdoors has a clear logic to it, as I explained.

It will depend on the DeltaT, typically determined by the medium you are heating. If you are heating a swimming pool (like another user in this forum was) or a large, concrete floor from a low temperature then a sustained flow temperature of 25⁰C may well be possible and efficient.

Agreed, it's totally dependent on the emitters being able to dissipate the heat being produced. Imagine a very large concrete floor with large underfloor heating system, heating a high heat loss poorly insulated space in winter. You could happily pump 25C water around that circuit all day with a RWT of 20C or lower (dT of 5C or greater). 

In my own home (real life example), with large K2 radiators (no UFH), the lowest flow temps we can achieve without cycling (continuous running) at OAT of 5-10C is 32C. This is a simple equilibrium between the minimum output of the heat pump (~4kW) and the flow temperature required to dissipate that 4kW of heat at a room temp of 20C. Every system will be different depending on the minimum output of the heat pump, the heat loss of the property and the size of the emitters attached to the system.

Is any lower than 25⁰c not worth it as your just pumping luke warm water around and just costing yourself and lowering your cop by paying for the heat pump(and in my case a secondary pump) to run and the extra wear and tear?

very unlikely that emitters will stay at a point where they need to operate on a low MWT of 17.5 (20F - 15R) as this is very low.

It will depend on the DeltaT, typically determined by the medium you are heating. If you are heating a swimming pool (like another user in this forum was) or a large, concrete floor from a low temperature then a sustained flow temperature of 25⁰C may well be possible and efficient.

Is there also a chance the heat pump will use the booster heater too as I've read it can do this?

A booster heater is an electric element typically found inline with the flow, in the Ecodan these are only fitting in split designs (where the heat exchanger is located in a hydrobox unit inside the property) and located in the outdoor unit. 
In monoblocs, most common in the UK, they are not fitted. The rest of Europe and Ireland almost exclusively use split systems, although R290 monoblocs are gaining popularity. 

lwt of 25⁰c can trigger the immersion heater

Immerson elements are submerged into a domestic hot water tank, not a space heating operation mode. The logic to start these is not exactly known, but often are used when the heat pump is unable to sustain a positive rate of temperature climb in the DHW tank.

The final part of critical information is that the Ecodan's minimum compressor frequency, and therefore minimum flow temperature is hard coded in their logic.
For example in my R410a 8.5kW monobloc unit (this does varies unit to unit), that at OAT 4⁰C the minimum compressor frequency is 32Hz and therefore the minimum output power is ~4.0 to 4.7kW (47% to 55% of rated output)
Now, at an OAT of just 1 degree more, 5⁰C the minimum frequency is allowed to drop to 26Hz and the minimum output power is ~3.1 to 3.9kW (~36% to 45% of rated output)

It's also said with Daikins that anything below an lwt of 25⁰c can trigger the immersion heater, I don't know if anyone knows if this is true or not? Is that the case with the Ecodan? 

Cheers 

If you mean the immersion heater in the cylinder to heat the hot water tank then No, that is very unlikely or the system has something set up wrong like boost priority. The ASHP has 2 sets for settings, usually a heating settings in your case something like 35@-2, 20@20 and another setting for hot water where the ASHP will run at (R32-58 Deg C flow - R290 Up to 60/70 Deg C) to heat the water)

If you mean the Daikin has a 3 or 6kW electric booster for the heating circuits, unless your in Northern Scotland or Northern Europe you should never need this or have this option in your ASHP for our mild climate, these options are designed more for our Northern friends in Europe where the winters get to -15 or more and a month of boosting helps not to have to have an oversized ASHP the other 5 months of winter. 

If you mean that you have a high temp system that uses a second refrigeration module like Hitachi to boost the heating from 50-70 or heaters to boost then you have the wrong system completely to run at 20 Deg C, this would be designed to run at 65-80 like a boiler. Its not that this system would not just run the ASHP section only at 20-35, it would but you would have wasted a fair bit of money purchasing a hi temp system without needing it.  

Let me know which you were referring to.

Indoors temp wiggles between 19-20°C.

Underfloor heating with a 10cm thick concrete slab, 20mm PEX pipes, 125mm spacing, so 25°C flow is already "high temperature" 🤣 

Thermostat is necessary due to enormous south facing windows letting in a ton of free heat. When the sun shines, running the heat pump open loop would overheat the house. In fact you can look at the plot above, see how long the heat pump runs each day, and that's pretty much the reverse of how sunny the weather was.

It is possible to use higher flow temp, up to 35°C, but this heats the core of the slab too much, by the time the heat reaches the thermostat, the slab is too hot, and it will dump the heat into the room making it way too hot for the rest of the day. So it's important to control the flow temperature to "as low as possible" which also suits the heat pump. A low flow temperature reduces system open loop gain and allows a simple control strategy to keep the house comfortable. 

There is so much thermal mass that it's fine to run the heat pump only during the day when PV provides free electricity and COP is also higher.

It's the opposite of the usual "intermittent heating regime" ASHP Bobba talks about. It heats when it's warm outside, and stops heating when it's cold outside 🤣 

I'm currently redoing the whole installation, so this is WIP.