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R32, "optimum" delta T

 
Air Source Heat Pumps (ASHPs)
Last Post by madbilly 4 weeks ago
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 madbilly
(@madbilly)
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Joined: 3 months ago
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Topic starter 18/03/2025 10:34 pm  

Hi everyone,

I heard on the podcast recently that Daikin have changed to a 10C delta-T, and that this could reduce the flow rate needed which has intrigued me as I think our house is pipework limited (also heat pump power output limited but that's another story). I then read around a bit and saw that natural refrigerants can operate with a much larger deltaT than synthetic refrigerants like R32, e.g. R290 can operate happily with more than 10C deltaT I believe and CO2 (R744) actually needs a very large deltaT to operate effectively.

This made me wonder, what's the optimum deltaT for an R32 heat pump? I see most default to 5, Daikin just started using 10, and I'd read but without explanation that 8 is the magic number. I'm sure there's a good scientific calculation for this, something to do with PT curves?

Cheers 🙂


   
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 robl
(@robl)
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Joined: 3 years ago
Posts: 203
18/03/2025 11:54 pm  

There’s no definitive magic deltaT for a heatpump, just pragmatic ‘generally optimum’. If you put more power into the water pump, the deltaT will reduce, and so then does the refrigerant gas-liquid transition temperature, making the heatpump more efficient.  This lower deltaT requires a faster water flow though - so the pump needs more power.  There’s an overall minimum power - and it’s generally found somewhere around deltaT=5 ish.  If your pipes are thin, it’s likely higher.

This post was modified 4 weeks ago by Mars

   
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 madbilly
(@madbilly)
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Topic starter 19/03/2025 10:54 pm  

In our case the pipe run for the heat pump is short and large bore to a heat exchanger, so it's a bit more complicated. Interesting that a smaller deltaT is more efficient, I had thought that a higher deltaT would be more efficient, because it would mean the water was extracting more energy from the refrigerant per m3 of flow. Anyway, for us I think it's irrelevant as I don't think our Midea heat pump allows the deltaT to be changed, it seems to be fixed at 5C.


   
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