I would suggest that if the heat loss characteristics are the same, the only difference that matters is the system volume and therefore energy required to move the water. I’d expect all other areas where energy is lost to eventually end up as that energy being translated into heat within the building.

@majordennisbloodnok, so you reckon they'll both be equally efficient in terms of energy consumption, heat generated and, essentially, comfort? Interesting.

Except for the extra energy used for more or larger pumps, yes, @editor. I can’t get away from that equal heat loss point and that energy in equals energy out. Unless I can see another way more energy can be consumed and converted into something other than heat, that’s my overriding factor.

The larger house might turn out slightly more efficient due to increased system volume, but I'm basically on the same page as @majordennisbloodnok.

One thing that did strike me, though, was that if two otherwise identical households living in identical neighbouring houses with identical heat loss characteristics heated by identical heat pumps and kit differed only in how warm they prefer their internal temperature to be, the household wanting the warmer temp would be turned off later into spring that the other. That, I suspect, would return an overall slightly better SCoP whilst actually costing more over the year to run.

Posted by: @majordennisbloodnok

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One thing that did strike me, though, was that if two otherwise identical households living in identical neighbouring houses with identical heat loss characteristics heated by identical heat pumps and kit differed only in how warm they prefer their internal temperature to be, the household wanting the warmer temp would be turned off later into spring that the other. That, I suspect, would return an overall slightly better SCoP whilst actually costing more over the year to run.

But the house with the warmer IAT, and hence higher flow temperatures (everything else being equal), will have a lower average COP during the heating season. Likely more than offsetting any SCOP improvement from running at milder OATs for longer. 

Regarding @editor's scenario, if the heat loss and flow temperatures are the same then the emitters will presumably have the same output and the same volume? So only the longer pipe runs will increase the system volume of the larger house. 

I would agree with your view that the larger house will be less efficient.  The basis for this is:

1. There might be longer pipe runs outside the heating envelope of the house, though this is not necessarily the case.
2. A larger house will have bigger rooms or more rooms.  This implies more radiators or longer runs of underfloor heating pipes.  You need more energy to pump water through more radiators, longer connecting pipes, and longer UFH pipes.

On the other hand, the larger house might have some characteristics that increase efficiency:

1. The greater water volume might mean that you no longer need a volumiser tank, which might increase efficiency.
2. Greater water volume might also improve the efficiency of defrosting.

On balance, I would expect the smaller house to be more efficient.