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Do heat pump defrost cycles impair heat pump efficiency (COP values)?

 
Air Source Heat Pumps (ASHPs)
Last Post by cathodeRay 15 minutes ago
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cathodeRay
 cathodeRay
(@cathoderay)
Famed Member Moderator
Joined: 5 years ago
Posts: 3129
Topic starter 15/11/2026 11:16 am   [#3227]

The short answer is no, as far as I can see from data from my Midea heat pump. I plotted mean hourly OAT against COP for that hour from 11 Oct 2026 (when the heating went on) to date which has a fullish range of OATs from below zero upwards and got this:

 

image

 

Firstly, the plot suggests OAT itself has a major effect on COP values, causing it to halve from around 6 at 19°C OAT to around 3 at 5°C OAT (usual caveats about association is not causation apply, but I am not aware of any obvious biases in play). I can't explain the volatility of COP values at higher OATs, but that said, there is a tolerably linear relationship between OAT and COP value, without any apparent down tick at defrost OATs (~4°C and below). 

While the lower OAT values are in the plot, they influence the position of the regression line. I therefore did the same plot, but excluded OATs less than 5°C, with a view to extending the regression line to the left to predict what the COP should be, assuming the OAT/COP relationship is linear (yes, I know this is mild whatiffery):

 

image

 

It seems that removing the low OAT values in effect makes no difference. The gradient and Y intercept at 0°C OAT is effectively the same in both plots, y = 0.2122x + 1.9968 (1st plot) vs y = 0.2233x + 1.8608 (2nd plot), if anything, the defrost plot values are marginally better than expected (COP 1.9968 at 0°C OAT vs 1.8608 at 0°C OAT, but I am sure that is just a quirk of the data.

Bottom line for me is that defrost cycles appear to have minimal effect on my heat pump's COP values. The precipitous drop in COP values at lower OATs is caused by the low OATs themselves, not the accompanying defrost cycles.

Any comments as always very welcome.

 


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


   
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