You are basically correct in what you state.

A buffer tank may cause lower efficiency due to mixing unless the water flowrate both in and out is kept balanced.

A volumiser vessel, as the name suggests, just adds volume to the system. If you look in the heat pump manual it should state the minimum required volume for the system. If the heat emitters and pipework do not provide sufficient volume then a volume vessel would be required.

I suppose it also depends where the volumiser is located, since it will lose some thermal energy, which could add to the overall energy loss if located outside the buildings thermal envelope.

There will also be added cost.

@derek-m Thanks Derek. Apart from the cost issue, would you say it was better not to have a internal volumiser if there were sufficient volume of circulating water in the radiators or is cost the only downsize?

Posted by: @bontwoody

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@derek-m Thanks Derek. Apart from the cost issue, would you say it was better not to have a internal volumiser if there were sufficient volume of circulating water in the radiators or is cost the only downsize?

I don't know if anyone has actually performed any tests to see if having extra water volume within the system, could improve the defrost cycle. I suppose a further possible downside would be if the system contains anti-freeze, since more anti-freeze would also increase costs.

Are 'low-loss headers' in the same category? And what do they do?

As far as I am aware, LLH's perform like a very thin buffer tank. How many water pumps are installed in your system?

@derek-m two pumps- one on the flow on the radiators side of the header and the other on the main flow from the ASHP.

Unless the flow rate of each water pump is matched, then there could be mixing occurring within the LLH.

Probably the easiest way to optimise the system operation, would be to try to ensure that the primary water pump (the one at the heat pump side), is always supplying slightly more water flow than the secondary water pump.

@bontwoody I went with an inline volumiser. All you say is correct, it cannot detract anything from efficiency, whereas a buffer or LLH always does - you can only seek to minimise that inefficiency with 1ary/2ary side flow rate balancing but you'll never get to zero loss.

I believe tests have been done that show that the effect defrost with a volumiser can be a bit better because there is that higher volume of hot water circulating. I have the volumiser on the flow side of the CH circuit, between the HP and the rads, inside the house in the airing cupboard. my reasoning for this is that it stores water at the LWT when the HP shuts down, meaning that at the next startup there's 50L of water already in the circuit at the last LWT, which I think smooths startup a little bit. and any heat loss is into the house.

the main reason to have one though is if your system volume is too low . I was just over the min but decided to add one to be sure (and I wasn't sure about which TRV's I'd keep at the time either), no real downside other than buying it / fitting it / finding room for it .

@iancalderbank @derek-m

I found this by Graham Hendra which is interesting around the siting of a volumiser.

@bontwoody yes thats one of the ones I read when making my decisions.

Apologies for re-introducing this topic. I was persuaded  by contributers on this forum, that I should remove the second CH pump and change the function of the buffer tank to use it as a volumiser.

I have forgotten how it was suggested that I should use the four connections on the tank in this configuration.

Existing connections are LHS: Top - From ASHP; Bottom - Return to ASHP RHS Top- To CH: Bottom - Return from CH