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Why Buffer Tanks & Low-Loss Headers Can Sabotage Your Heat Pump's Efficiency

 
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Air Source Heat Pumps (ASHPs)
Last Post by CH_18 8 months ago
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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 6 years ago
Posts: 4884
Topic starter 02/05/2025 8:38 am  

In light of the recent buffer tank/warranty comments in this topic, I’ve written this piece below. It’s a 10 minute read, but I think it warrants a dedicated video/podcast, because there’s a lot of ambiguity and grey areas here that need clarification.

https://renewableheatinghub.co.uk/fact-or-fiction-are-buffer-tanks-really-necessary-to-protect-your-heat-pump-warranty/


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 Heatgeek
(@heatgeek)
Estimable Member Member
Joined: 2 years ago
Posts: 117
02/05/2025 9:55 am  

@potatoman As no one seems to be obliging, I attach a diagram. The idea is that only bypass flow goes through the tank when necessary so it is undisturbed by main flow. Connection to tank must be closely coupled.

 

3 Port Buffer Tank

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 ASHP-BOBBA
(@ashp-bobba)
Prominent Member Member Professional+
Joined: 2 years ago
Posts: 495
02/05/2025 10:02 am  

@heatgeek is it that this design satisfies the manufacturers? its still (although balance by bypass) a double hydronic volumiser/buffer with 2 pumps


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.


   
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 Heatgeek
(@heatgeek)
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Joined: 2 years ago
Posts: 117
02/05/2025 10:14 am  

@ashp-bobba  I am afraid that you are talking to the wrong person about manufacturers preferences. However, as far as I can see, it functions the same as a 4 port buffer except that flow doesn't go through the top of the tank causing mixing, therefore I would have thought that it is a perfectly acceptable alternative to a 4-port.


Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 ASHP-BOBBA
(@ashp-bobba)
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Joined: 2 years ago
Posts: 495
02/05/2025 10:57 am  

@heatgeek Oh agreed, its a slightly better and slightly less resistive solution, perhaps even the middle ground of no buffer V buffer.


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.


   
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 Potatoman
(@potatoman)
Trusted Member Member
Joined: 2 years ago
Posts: 57
02/05/2025 12:53 pm  

@heatgeek Thank-you for your reply, can you clarify the circulating pump situation, would you have to have 3 circulating pumps in operation, one in the heat pump and two in the design. Thanks

 



   
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 ASHP-BOBBA
(@ashp-bobba)
Prominent Member Member Professional+
Joined: 2 years ago
Posts: 495
02/05/2025 12:59 pm  

@potatoman hi, the scheme detailed is 2 pumps, one for the ASHP (primary) and 1 for the secondary circuits.

 

 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.


   
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 Heatgeek
(@heatgeek)
Estimable Member Member
Joined: 2 years ago
Posts: 117
02/05/2025 1:04 pm  

@potatoman  No, the installation is exactly the same as a 4 port buffer with primary and secondary pumps except that the output flow bypasses the top of tank so as not to disturb the stratification of the tank. This removes some of the inefficiency problems associated with 4 port buffer tanks, like output temperature being reduced by mixing.


Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 Heatgeek
(@heatgeek)
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Posts: 117
02/05/2025 1:35 pm  

Note a key point here though. The primary pump is connected in the return pipe while the secondary pump is connected in the flow pipe after the buffer tank to provide good hydronic separation between the primary and secondary pumps.


Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 Heatgeek
(@heatgeek)
Estimable Member Member
Joined: 2 years ago
Posts: 117
02/05/2025 1:48 pm  

Only the difference in flow between the heat pump and the emitters passes through the tank. This lower flow helps to preserve the stratification of the tank so that dilution of temperature to the emitters is minimised.


Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 Heatgeek
(@heatgeek)
Estimable Member Member
Joined: 2 years ago
Posts: 117
02/05/2025 1:59 pm  

Apologies. Poor choice of words. Delete "dilution of temperature to emitters is minimised".


Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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JamesPa
 JamesPa
(@jamespa)
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Posts: 5476
02/05/2025 2:56 pm  

Posted by: @heatgeek
↑

Note a key point here though. The primary pump is connected in the return pipe while the secondary pump is connected in the flow pipe after the buffer tank to provide good hydronic separation between the primary and secondary pumps.

That all sounds very believable.  Presumably mixing of return water with flow water to the emitters (and thus a reduction of ft to emitters) will however still occur if the primary pump throughput is less than the secondary, so the requirement to set up properly remains albeit that it is less sensitive than a 4 port.

 


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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