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Replacing my 18 month old Hitachi Yutaki ASHP

 
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Air Source Heat Pumps (ASHPs)
Last Post by RobS 28 minutes ago
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 RobS
(@robs)
Estimable Member Member
Joined: 1 year ago
Posts: 58
27/11/2025 10:27 pm  

Posted by: @jamespa
↑

The defrosts look normal except that the flow temp doesnt get back to 31.  Yet the gradient of the rising edge is the same as immediately after the defrost and your expanded plot where you explain whats going on shows it level at the top not continuing to rise until the next defrost.  Also the temp it reaches starts rising again for the last three of the defrost cycles, which wouldn't obviously be the case if something is 'wrong'.

It is usual for the flow temp to start to drop as a heat pump approaches a defrost, here's a 7kW Vaillant like yours doing the same (if less pronounced):

Screenshot 20251127 220607 1

Also the last three defrosts happened after sunrise and so the OAT would have been higher and that would have allowed the heat pump to achieve the target flow temp, where before the lower OAT prevented it. 

The question of why a seemingly oversized heat pump is unable to achieve the target flow temperature could be due to humidity or its handling of the defrost condensate. Both can greatly effect the performance of a heat pump. 

 

 



   
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