Some super interesting findings. Not surprised about heating DW to 45C being the most efficient way to go. That's what we set ours to.

Thanks Mars I am getting 3.7 Scop with an oversized Mitsubishi 8.5 kw heat pump . Still
Trying different settings to try and improve it. Have installed open energy monitoring and
It’s under Dalkieth. Set hot water 47c . Does anyone know if switching down pump speed helps presently 25.61 l/m
Cheers Andrew Scotland

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Does anyone know if switching down pump speed helps presently 25.61 l/m

Pump speed needs to be sufficient to carry away the energy, any less wont do, any more is wasting energy.

The energy carried away is flow rate (l/s) * specific heat capacity of the transfer medium (water or glycol as appropriate) * deltaT at the heat pump (normally designed for 5). 

Assuming water and 5C DT thats 25.61/60*4200*5 =8.9kW, so just abut right for an 8.5kW pump!

If the heat pump is oversized then there may be the potential to reduce the water pump speed.  Some heat pumps modulate pump speed according to energy demand.  If yours does or can then thats an option.  Otherwise you could try turning it down on the water pump itself but the heat pump may complain.  The energy saving from turning down the water pump is small however so this isnt likely to make a massive difference.

I have tried my 8.5kw Ecodan at various pump speeds its currently at pump speed 2 that gives 14L/min, the recommended flow rate for 8.5 Ecodan is 15.4L/min, to achieve the optimum dT of 8, not 5 that you see quoted.

Using the mass flow calc James quoted you get to 8.6kw at dT8 so that makes sense as well.

The advantage of using a lower flow rate is that the Ecodan doesn't have its own internal pump so ours is in the airing cupboard so the slower the pump the less noise it makes too

Two other things that greatly effect SCOP but havent been mentioned are the climate where you are living and what you deem to be a comfortable internal temperature.
Someone living in London who is happy with a 19C indoor tempearture will have a much better SCOP than someone with exactly the same system living in the North of Scotland who likes a 22C room.

Posted by: @david-stanford

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Aim for 15+ l/kW System Volume: This seems to be a reasonable rule of thumb for optimising efficiency.

Yes I have room temperature 20.5 and live in central belt . Perhaps I was overthinking scop it’s gone back to daily cop of 4.5 . Will try lower pump speed as heat pump comes off and on 2 or 3 times an hour
thanks for commenting

Thanks I will try that and look at results

A heat pump is only as good as the desired room temperature. Bearing in mind that optimal room temperature for cognitive function sits between 21 and 24 degrees, with average around 22/23 degrees. 45 flow temperature relates to around 42.5 mean water temperature (MWT). Make sure to use that with your heat loss calculation, desired room temp and radiator sizes for best outcome.

You can even go as low as 40c flow temp, if you want a high SCOP, as long as your radiators are sized correctly and you use the correct desired room temp, nothing is impossible. I don't think you will find any radiators that can do 40c flow rate and room temp higher than 21c, unless going to silly 3 meters K3 rads or very very expensive multi chamber rads (£10k per radiator).

Posted by: @drei

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You can even go as low as 40c flow temp, if you want a high SCOP, as long as your radiators are sized correctly and you use the correct desired room temp, nothing is impossible. I don't think you will find any radiators that can do 40c flow rate and room temp higher than 21c, unless going to silly 3 meters K3 rads or very very expensive multi chamber rads (£10k per radiator).

We run a standard flow temp of around 32C all year round (certainly between 30-35C) and achieve a comfortable (for us) 20C room temp. Our radiators are all of standard K2 types. Other than on the very coldest of nights, even running a flow temp of 30C overnight puts out more heat than required as we like a setback of 18C for sleeping.

As long as your emitters are large enough to dissipate the required amount of heat at reduced flow temps, very low flow temps are possible. The limiting factor for us is actually dissipating the ~3.5kW of heat produced by the heat pump when running at minimum during milder weather, hence the required 32C flow temps. Any lower and the radiators cannot dissipate that 3.5kW output flowing from the heat pump.