Many thanks for contacting the brand manager, that's very useful to know. Thank you to everyone for their comments on this thread.

@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!  

That certainly make sense. Is DT a parameter that one can set on the ASHP controller, and the ASHP then adjusts other parameter(s) such as the flow rate in order to maintain the target DT?

Might work better… Toodles.

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Regards, Toodles.

Mine was commissioned running at DT5 and still is with Homely control. Toodles.

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'.

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!