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Observation about radiators and weather compensation

 
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Air Source Heat Pumps (ASHPs)
Last Post by cathodeRay 3 years ago
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cathodeRay
 cathodeRay
(@cathoderay)
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24/11/2022 8:46 pm  

@derek-m - yes, it does, thanks, apart from one question/observation: what happens if the secondary pump isn't running, because the room stat isn't calling for heat? Unlikely if things are cold enough to need defrost cycles, but not impossible. At that point there will be nowhere for the heat pump to draw heat from to effect the defrost cycle.

Midea 14kW (for now...) ASHP heating both building and DHW


   
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 Derek M
(@derek-m)
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Joined: 4 years ago
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24/11/2022 9:05 pm  

Posted by: @cathoderay
↑

@derek-m - yes, it does, thanks, apart from one question/observation: what happens if the secondary pump isn't running, because the room stat isn't calling for heat? Unlikely if things are cold enough to need defrost cycles, but not impossible. At that point there will be nowhere for the heat pump to draw heat from to effect the defrost cycle.

I'm afraid that is one question that I cannot answer, because I don't know how your system is wired and configured to operate. All I can say is that if you have followed previous advice, and have your system set in WC mode with the thermostat set high as an over-temperature stop, then the secondary pump should be running.

I suppose that you could always ask your installer, Freedom or possibly Graham Hendra who used to work for Freedom.

 


   
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cathodeRay
 cathodeRay
(@cathoderay)
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25/11/2022 9:17 am  

@derek-m - I currently have the system running mostly but not exclusively in WC mode. The room stat is set high-ish (19 degrees) and most of the time the system runs in WC mode (as can be seen in the Midea app) but every now and then the room stat does trip (stops calling for heat) and so the secondary circulating pump stops. But this tends to happen when it is mild and there has been solar gain, ie conditions in which a defrost cycle is not going to happen.

The only time I can see when a defrost cycle could happen when the secondary circulating pump is off is when the system is in DHW mode, but presumably that isn't a problem, as the defrost cycle can get its heat from the hot water in the tank, though the water may not be that hot (which is why the system is in DHW mode). I also have the DHW on a timer (only runs between 1300 and 1400 daily) so it generally will be a warmer part of the day.

Today I am going to go over to 100% weather compensation ie set the room stat very high (30 degrees) and adjust the curve on the Midea controller and perhaps slightly close some of the upstairs TRVs to keep the rooms at design temps. I expect it will take a few days to get it just so, if that can ever be achieved. Once things are good enough, I will then do another monitoring run and see how the COP looks - it may not be 100% accurate, but running the same methodology on the same system should be a good enough relative indicator - to check I have a tolerable COP.      

Midea 14kW (for now...) ASHP heating both building and DHW


   
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