In another post, @jamespa wrote:

"In summary, if I had the choice of a lower flow rate and opening up the emitters more, or a higher flow rate and them being more closed, which should theoretically be better for efficiency / reducing flow temps / cycling?"

Im not sure you really have the free choice or what you are assuming is kept constant/varied in the two scenarios

Im assuming that you want to keep the heat output from the emitters constant, to preserve the room temperatures.

Well, I guess slightly-more nuianced - see below.

The heat output from the emitters is determined by the difference between the average emitter temperature and IAT, not directly by flow rate.

However if you can increase the flow rate through the emitters you will reduce the deltaT across the emitters which means that the average temperature is closer to the flow temperature, which in turn means that the flow temperature can be lower (but only by a degree or so) for a given average emitter temperature.

This will be more efficient and is one of the reasons we operate heat pumps at a lower deltaT (across emitters) than boilers.

This answer is independent of the emitter type.

Does that answer the question?

or even: My logic says, if all UFH loops are more closed, less heated water is going round the loops and therefore the returning water is of a higher temperature and is therefore being 'wasted' / bypassing the emitters.  If the dT were larger, the heater has to work harder to heat the LWT but more is being 'used'; which should be most-efficient?

Posted by: @rhh2348

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Noted.  I have read elsewhere that rads generally require a different flow rate to UFH loops (and if so, why - if they are the same diameter pipework?) - is this just in essence the same as flow-rate-to-emitter, whereby each UFH loop openness is adjusted to allow for required IAT?

I guess that depends on where you measure it.  If measured at the heat pump then the flow rate for any given heat transfer at any given deltaT (across emitters) is the same.  If you measure it at the emitters then not so, because you generally have 10+ radiators and maybe 4-6 UFH loops.  That said I dont know if you typically operate UFH at a lower deltaT even than radiators (it cant get much lower)!

Posted by: @rhh2348

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"If the IAT can be maintained (or even raised?) by a higher flow rate, reduced flow temp (by up to 1deg C), and adjustment to emitter 'openness', which should result in the higher CoP/better system operation?"

As always lower flow temperature = higher COP.  However once you get down to UFH temperatures the effect gets smaller

.

Posted by: @rhh2348

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My logic says, if all UFH loops are more closed, less heated water is going round the loops and therefore the returning water is of a higher temperature and is therefore being 'wasted' / bypassing the emitters.  

Qssuming you keep the flow temperature the same:  If UFH loops are more closed the flow rate through the UFH will be less so the deltaT will be higher so the average temperature of the UFH will be lower so you emit less.  The returning water is of a higher temperature but thats because the UFH emits less so the room will be colder.

Posted by: @rhh2348

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If the dT were larger, the heater has to work harder to heat the LWT but more is being 'used'; which should be most-efficient?

In principle the former for the reason stated

However, at low FTs the effect can be small and if its also a low loss house and mild weather than things like the consumption of the water pump become significant in comparison to the energy for actual heating.  Also the effect of cycling may be important at this low output.  At this point the simple thermodynamics-based arguments above break down and the engineering starts to matter.  Whether anyone really cares at this level is a moot point, since the cost of heating is now very small!