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Underfloor heating: Diagrams with COP 'per water flow'

 
Air Source Heat Pumps (ASHPs)
Last Post by rhh2348 1 year ago
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 wvtienen1
(@wvtienen1)
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Topic starter 29/01/2025 5:39 pm  

Anyone ever seen diagrams or tables that give COP per water flows; something like L/min versus COP (and of course for a certain temperature of water and Power...)

Just wondering how water flow impacts performance.



   
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JamesPa
 JamesPa
(@jamespa)
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29/01/2025 6:47 pm  

Posted by: @wvtienen1
↑

Anyone ever seen diagrams or tables that give COP per water flows; something like L/min versus COP (and of course for a certain temperature of water and Power...)

Just wondering how water flow impacts performance.

Unless its 'out of range' I dont think it materially affects COP (directly) which is why you wont see it plotted.  The essentials of water flow are

  • there must be sufficient volume flow to carry the required heat to the emitters at a deltaT of 5C, or the figure specified in the heat pump documentation.  Your heat pump will probably complain if it isn't, unless the installer has fitted a buffer tank or bypass valve to mask the problem
  • the linear speed of water in the pipes must be sufficiently low that you dont get noise in the pipes or worse still pipe erosion ~1 m/s max in noise sensitive areas, up to perhaps 1.5m/s in less noise sensitive areas.  (In some circumstances these limits can be exceeded).  I dont know the recommended flow rates through the actual heating pipes in UFH (which is in the title) as opposed to the feed pipes.

Some heat pumps adjust water flow to achieve a constant deltaT whatever the thermal load, others don't, it doesn't matter much. 

There are a few more constraints if you are unlucky enough to have an LLH, buffer or PHE

COP varies mostly with OAT and Flow temperature.  You cant do anything about the former, you want flow temp as low as possible.

 

 

 


This post was modified 1 year ago 3 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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 rhh2348
(@rhh2348)
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11/02/2025 2:32 pm  

I was in two minds whether to comment here or start a new post - I decided the latter as this was specifically about reference material for CoP vs flow temps, but it is probably related, given @jamespa response - see https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/flow-rate-vs-emitter-openness-and-cop#post-42153



   
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JamesPa
 JamesPa
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11/02/2025 2:52 pm  

Posted by: @rhh2348
↑

I was in two minds whether to comment here or start a new post - I decided the latter as this was specifically about reference material for CoP vs flow temps, but it is probably related, given @jamespa response - see https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/flow-rate-vs-emitter-openness-and-cop#post-42153

I read the post you referenced and Im not wuite sure what the question you are trying to answer is.

 

If its this "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.

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?

 


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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 rhh2348
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11/02/2025 3:09 pm  

(Answering in the referenced post)



   
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