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Heat Pump SCOPs – The Truth Might Not Be What You Think

 
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Air Source Heat Pumps (ASHPs)
Last Post by HCas 6 months ago
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 Old_Scientist
(@old_scientist)
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Joined: 2 years ago
Posts: 359
04/09/2025 9:00 am  

Posted by: @mike-h
↑

Posted by: @david-stanford
↑

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

Can you explain the mechanism for this effect? Could it just be coincidental rather than causal? System volume may prevent short cycling, but there are other ways of achieving that. 

Sufficient system volume is also essential for ensuring there is enough energy in the system to perform effective defrost cycles (unless there is some form of additional heater present). Without sufficient volume, constant defrost cycles will quickly kill efficiency in colder weather, and leave the house cold.

 

 


This post was modified 6 months ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 Judith
(@judith)
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Joined: 2 years ago
Posts: 486
07/09/2025 9:37 pm  

Posted by: @drei
↑

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).

I think you’ll find it depends on the room heat loss. A low loss room can be heated by a much smaller difference between mean water temperature and required room temperatures with a moderate sized radiator. Of course we usually all want the smallest radiator that will work in the room. They’re not works of art despite the marketing of some designs.

 


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC


   
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 Scalextrix
(@scalextrix)
Estimable Member Member
Joined: 1 year ago
Posts: 127
08/09/2025 6:48 pm  

Posted by: @david-stanford
↑

Aim for 15+ l/kW System Volume

Is that heat loss, or maximum output of the ASHP?  Understand they should be fairly close in a good system, but both vary by outside temperatures.



   
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 HCas
(@hcas)
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Joined: 2 years ago
Posts: 132
09/09/2025 12:53 pm  

@scalextrix Name plate capacity


CEO and co-founder at HavenWise


   
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