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Same Heat Pump Size, Different Sized Homes: Would One Be More Efficient?

 
Air Source Heat Pumps (ASHPs)
Last Post by GrahamF 2 months ago
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Mars
 Mars
(@editor)
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Topic starter 31/01/2026 10:15 am  

Here’s a thought experiment I’ve been mulling over.

Take two identical heat pumps: same model, same output (say 10 kW), same controls, same design flow temperatures. One is installed in a small home, the other in a much larger home.

Both systems are appropriately designed for the properties they serve (not oversized, not undersized) but the physical scale of the homes is very different. Both have the same heat loss.

The question I’ve been pondering is would the heat pump in the smaller home be inherently more efficient and consume less electricity than the same heat pump serving a much larger home?

Not because of poor design or incorrect sizing, but purely because of system scale.

My rationale is that a smaller property typically means:

  • shorter pipe runs
  • fewer emitters
  • lower system water volume
  • fewer branches
  • reduced distribution and pumping losses

A larger property inevitably introduces:

  • longer and more complex pipework
  • more emitters and hydraulic branches
  • higher system volume
  • greater reliance on pumps, balancing and controls

Even with the same heat pump operating within its design envelope, the larger system has more places for energy to be lost or diluted before it does useful work.

So is efficiency purely a function of the heat pump itself and its operating temperatures or does property size and system complexity erode efficiency as scale increases?

In other words, can two identical, well-designed, identically sized systems still deliver materially different efficiency figures simply because one is serving a compact home and the other a sprawling one?


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Majordennisbloodnok
 Majordennisbloodnok
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31/01/2026 10:41 am  

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.


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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Mars
 Mars
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Topic starter 31/01/2026 10:45 am  

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


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Majordennisbloodnok
 Majordennisbloodnok
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31/01/2026 1:14 pm  

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.


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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JamesPa
 JamesPa
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31/01/2026 1:22 pm  

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


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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Majordennisbloodnok
 Majordennisbloodnok
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31/01/2026 1:37 pm  

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.


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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 RobS
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31/01/2026 11:31 pm  

Posted by: @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.

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. 

 

 



   
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GrahamF
 GrahamF
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Posts: 120
01/02/2026 8:35 pm  

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.


Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation


   
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