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Flow temperature- At what point?

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 10 months ago
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 Westkent
(@westkent)
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Joined: 2 years ago
Posts: 58
Topic starter 27/11/2024 12:02 pm  

Hi, I’m sure this would of been asked before but a question for the more knowledgeable on these matters.

At what point does it start becoming a false economy by lowering the flow temperature as to attempt to increase the COP.

I ask as I presume there’s a point at when  the flow temperature is lowered to a certain point the Heat pump will simply be forced to work to hard to reach the room target temperature.

Thanks in advance.

Barry

 



   
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 Gary
(@gary)
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Joined: 3 years ago
Posts: 350
27/11/2024 12:12 pm  

If you are running on weather compensation then room target temp doesn't factor into it, the heat pump will be running 24/7 and your house won't reach target temperature as it will be losing heat faster than its being put into the house as the flow temp will be set too low.

IMO, COP isn't a good measure of anything other than instantaneous efficiency, depending on TOU tariffs etc the best COP may not even be the most financially economical.  I run at a higher temp at a COP of 3 overnight to defer the need to run at a COP of 4 during the day as electricity is 1/4 of the price overnight, worse efficiency but 1/3 the cost. 

The primary function of my heating is to keep me warm, secondary factor is how much it costs to run, appreciate that may not be the same for everybody.


This post was modified 10 months ago by Gary

   
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 Westkent
(@westkent)
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Posts: 58
Topic starter 27/11/2024 12:47 pm  

Thanks for the reply Gary.

Just to note I’m running on  auto adaptation.

I had read on here somewhere that running on A/A the set flow is immaterial as the system finds itself the best flow temperature but from what I can see with my ecodan that seems not the case.



   
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 Gary
(@gary)
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Joined: 3 years ago
Posts: 350
27/11/2024 1:29 pm  

@westkent Ah your question makes more sense now, auto adapt does use the room temp and uses the weather comp curve set as a starting point but then will adjust the flow temp from that base curve as it learns how the room heats up at various outside temps.

Is there a specific issue you have?



   
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 Westkent
(@westkent)
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Joined: 2 years ago
Posts: 58
Topic starter 27/11/2024 3:19 pm  

@gary

I have been running my curve at 45c to -2 but last night moved it down to 38c to see if it helps efficiency wise.

I guess my original question was how low would I be able to lower the curve before it either wouldn’t be able reach the desired (20c) or the H/P ends up running continuously hence affecting efficiency??

Im still on somewhat of a learning curve myself on these situations.



   
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 Gary
(@gary)
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Joined: 3 years ago
Posts: 350
27/11/2024 3:33 pm  

@westkent I'm not that familiar with autoadapt as it for use with radiators and I have UFH.  I think there may be limitations within the service menu on how far auto adapt can move the curve, if that's the case if you move to too low then it won't be able to maintain the temperature, but this is purely speculation on my part.  It may be that autoadapt just ignores any changes you make and will do its own thing.

I think you will just have to experiment, once it gets to 2C or lower mine runs continuously anyway as it has to have down time for defrosts and needs to run continually to keep the house temp at the desired level



   
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 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 2982
27/11/2024 4:06 pm  

Posted by: @westkent
↑

I have been running my curve at 45c to -2 but last night moved it down to 38c to see if it helps efficiency wise.

I guess my original question was how low would I be able to lower the curve before it either wouldn’t be able reach the desired (20c) or the H/P ends up running continuously hence affecting efficiency??

Im still on somewhat of a learning curve myself on these situations.

You have to experiment, reducing it a degree at a time until it fails to heat the house up sufficiently.  TRVs (if you have them) all fully open and and external (or internal) thermostat set at least a couple of degrees above desired temperature while you do this (which most should be anyway), and don't change it more than once per day

Heat pump running continuously at lowest possible flow temperature is exactly what you want, certainly when it cold (say 10C or less).  Thats the most efficient operating point.  For every degree you can notch down the WC curve you should save 2-3%.  That soon builds up!

 

As you have discovered Mitsubishi features an auto adaption mode, its not entirely clear how this works but the basic concept is that you dont have to get the WC spot on, so it may be that turning it down further manually doesn't in fact help because the machine will do it for you.  I don't know, maybe those more familiar with Mitsubishi 'in practice' do.

 


This post was modified 10 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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