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/

@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.

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

@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.

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

@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

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

@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.

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.

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.

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

Posted by: @heatgeek

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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.