https://renewableheatinghub.co.uk/forums/postid/48532/

Posted by: @pash44pump

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So now the question has changed completely. What should my new system look like? Obviously, bigger ASHP, no buffer tank just a system volumiser, pipework inside rather than outside (I've never liked it outside), different location for the ASHP? I really want to make sure the revamp is 100% right. Anyone have "what an ideal ASHP system should look like"?

If I were in your shoes I would absolutely want to get an independent handle on loss based on actual measurement given that, from what you have said, the value could drive significant design decisions which you would kick yourself if you get wrong.  Can I suggest you refer back to this post and the few either side to remind yourself.   Presumably you have 4 years of at least partial logs you might refer to.  Otherwise what evidence do you have that the new survey is more accurate than the old?

Then in summary: No buffer, PHE or LLH, heat pump connected directly to emitters, volumiser in flow, no external controls not designed specifically for heat pumps (meaning only homely, havenwise or adia), 3 way diverter and antifreeze valves, no glycol.  FRVs if you can, TRVs if you want but with the intention of opening up the vast majority so they have no effect.  Heat pump ideally able to do programmed flow temperature set backs/set forwards natively (otherwise homely, havenwise).  If its possible given your loss, avoid a cascade (2 heat pumps).  In short, KISS.

Posted by: @pash44pump

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As you know, there was also a new Heat Loss Estimate at their expense as my calcs (thanks to the Freedom Heat Pumps model that @cathoderay shared, came out with 12.9kW). Today, I got a call from the installer as I chased the new HLE up and lo and behold. The new HLE shows a requirement of 14kW and I have a 12kW Midea.

Posted by: @pash44pump

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Another point, I'm thinking that if I've been running an underpowered ASHP for 4 winters, I have probably used way more power than I should have as my COP has probably been well below what it should have been. Any reasonable way I can quantify that?

Some things worth mentioning:

(1) the Headroom Heat Pump Heat Loss calculation is still just a calculation based on whatiffery. In the case of Midea heat pumps there is even some hidden whatiffery, depending on which version you use, because the Midea outputs were guesses, and the guesses tended to over-estimate output. You need to unhide the hidden parts of the spreadsheet to see all this. 

(2) a 12kW Midea is also a 14kW Midea and even a 16kW Midea, or has been until recently, because the hardware is the same for all three, output is limited in the 12 and 14kW versions by dip switches. You mat already have a heat pump that can supply the necessary amount of heat.

(3) the last four winters with an underpowered heat pump: I don't think you will have used 'way more power', the problem is more that you can't use enough power (because the heat pump was too small) and the consequence is too cool a house, rather than too low a COP (though having to run at a high flow temps may affect that, as in higher flow temp = lower efficiency, all other things being equal)

(4) finally, and perhaps most importantly, amd this is what @jamespa was getting at above, if you also have four winters of ASHP data, then you can, if it is detailed enough, do an empirical ie measured heat loss assessment. In almost all cases, this will always trump a whatiffery based assessment. The basic idea is simple: when the IAT (indoor air temp, room temp) is stable, then the building is in heat balance, ie energy in = energy out. Thus by taking periods when the IAT is stable, and plotting OAT (outside air temp) against energy out (ie into the building), you have the heat loss over a range of different OATs. The relationship is effectively linear (for energy out), and once you have the plot, if it doesn't already reach the design OAT, say -2°C, then you extrapolate the line so it does.

But this does need at a minimum daily data for IAT (to check it is stable) plus mean OAT and energy out, which you may not have...but if you do, we can take you though how to do the plot. If you don't have the data, start collecting it now!