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Air Source Heat Pumps (ASHPs)
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 RHH1
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Topic starter 15/09/2025 4:07 pm  

I saw this article that mentions that Daikin recommend delta T =10 for radiator circuits. Does anyone know which of Daikin's ASHPs this applies to? (I asked Daikin, but they said to ask an installer, which didn't help much...)

 

https://renewableheatinghub.co.uk/heating-technology-heat-pumps-2025/

 


This topic was modified 5 months ago by RHH1

   
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Mars
 Mars
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15/09/2025 5:44 pm  

That's hysterical that a manufacturer would defer a question like to this to an installer.

@grahamh do you have any words of knowledge here?


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Toodles
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15/09/2025 6:05 pm  

@editor Well, it is not the Altherma range, they are definitely Delta T = 5. Toodles.


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JamesPa
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15/09/2025 6:51 pm  

Posted by: @editor
↑

That's hysterical that a manufacturer would defer a question like to this to an installer.

That was Daikins answer to every single question I asked them during my discovery phase.  

...Which is one of the reasons I discarded them from consideration.


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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Mars
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15/09/2025 9:27 pm  

@rhh1, I’ll contact Daikin’s brand manager tomorrow and see what they say.


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GrahamF
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16/09/2025 6:41 am  

That seems like a large number.  Our Grant Aerona 290 15.5kW is running with a Delta T between outlet and inlet of 2.7C at a current flow temperature of 28C.  We have 30 radiators.  During the hot water Legionella cycle the flow temperature of 61.3C has a Delta T between inlet and outlet of 4.1C.


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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JamesPa
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16/09/2025 7:31 am  

Posted by: @grahamf
↑

That seems like a large number.  Our Grant Aerona 290 15.5kW is running with a Delta T between outlet and inlet of 2.7C at a current flow temperature of 28C.  We have 30 radiators.  During the hot water Legionella cycle the flow temperature of 61.3C has a Delta T between inlet and outlet of 4.1C.

Crazy large and unexplained so far as I know, I had heard Daikin recommend this. 10% efficiency penalty compared to DT of 5 (because average emitter temp is 2.5C lower for any given flow temp) unless somehow the Daikin pump works better with larger DT.  Perhaps daikin fit undersized water pumps so need to run slower hence higher DT?

 


This post was modified 5 months ago 2 times by JamesPa

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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Toodles
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16/09/2025 8:24 am  

@jamespa Ours is a Daikin EDLA03 8 kW and the Delta T = 5, as I mentioned before; the pump starts a cycle at ~28 l/m and later drops to ~15 l/m. The DT seems to stay pretty close to 5 at all times. We have an LLH and secondary pump which I have set at ~15 l/m. Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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 RHH1
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Topic starter 17/09/2025 5:17 pm  

@jamespa That's an interesting point! I was thinking that having a delta-T of up to 10C would be a useful degree of freedom to avoid having too high a flow-rate, and hence a high pressure-drop, through microbore pipe-runs. But if their reason for supporting a delta-T of 10C is to circumvent an undersized water pump then I'd be back to square one...



   
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JamesPa
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17/09/2025 6:41 pm  

Posted by: @rhh1
↑

@jamespa That's an interesting point! I was thinking that having a delta-T of up to 10C would be a useful degree of freedom to avoid having too high a flow-rate, and hence a high pressure-drop, through microbore pipe-runs. But if their reason for supporting a delta-T of 10C is to circumvent an undersized water pump then I'd be back to square one...

So far as I can tell designing for DT5, which is 'the norm', is a convention not an absolute.  I have seen people argue that a higher DT is preferable or that, due to some unspecified mechanism in the heat pump, no lower than 3C DT is desirable, but none of these arguments have been supported by any coherent explanation at least IMHO. 

Thus, in the absence of a coherent explanation I fall back to the physics which is that:

  • lower DT  =  Higher average deltaT (emitter-room) for any given flow temp = good (because of heat pump efficiency)
  • higher DT = lower flow rate for any given heat delivered to the emitters = good (for water pump efficiency, pipe erosion and pipe noise)

Thus its a trade off which in principle is specific to any given house, but one where DT about 5 is reasonable in many cases, which does not mean that you shouldn't depart from that.  Openenergy monitor did once have a thread on the trade off which suggested 2-3C was optimum for the specific conditions in question (but not for other conditions)

So you are right that a higher DT does avoid high flow rates and if thats a problem feel free to accept a higher DT.  However if it isn't keep DT as low as you can within reasonable parameters.  Or, put another way, dont get too hung up about it if departing from the norm or a manufacturers recommendation makes the design work!  


This post was modified 5 months ago 2 times by JamesPa

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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Mars
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18/09/2025 12:18 pm  

@rhh1, got a reply from Daikin. There brand manager was way on holiday. He's replied saying, "The delta of 10 for the radiators applies to all our air to water heat pumps. Please let me know if you require more info on this."


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JamesPa
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18/09/2025 12:51 pm  

Posted by: @editor
↑

@rhh1, got a reply from Daikin. There brand manager was way on holiday. He's replied saying, "The delta of 10 for the radiators applies to all our air to water heat pumps. Please let me know if you require more info on this."

I think, based on what others have said here, that the Daikin adjusts its water pump to get the required DT.  Others have said that you are asked somewhere in the menu to tell it what type of heating you have (UFH/Rads) and it decides the DT to control for based on this.  If thats all true then the recommendation has surely got to be tell it UFH no matter what you have unless there is some other reason not to or the setting has other 'effects'.

 


This post was modified 5 months ago by JamesPa

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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