@editor That's an interesting idea, I am not sure how they work, do you have any more details on them

@potatoman as you already know a four-port buffer can cause heat pump inefficiencies due to uncontrolled blending of hot and cooler water, which reduces the supply temperature and forces the heat pump to work harder.

In contrast, a three-port buffer helps maintain stratification, reduces unnecessary mixing and ensures that the heat pump isn’t cycling. This improves efficiency and keeps return temperatures lower.

A volumiser, while not technically a hydraulic separator, can also satisfy most manufacturers’ minimum volume requirements. Its primary role is to provide thermal mass to prevent short cycling, but because it’s integrated more seamlessly into the system than a four-port buffer, it avoids distortion issues.

Ultimately, both the three-port buffer and the volumiser offer better solutions than a standard four-port buffer by reducing mixing, maintaining flow rates and ensuring the heat pump operates within its optimal parameters.

Posted by: @potatoman

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@jamespa surely there must be a third way to improve efficiency, not just buffer or no buffer, but to balance a buffer, especially if you are using an open loop system.

Yes there is, provided the buffer is tall and well designed so there is stratification (forget 50l buffers so commonly fitted)

With one of these you have two temperature sensors and then you can detect where the thermocline (the division between cold water at the bottom and hot at the top) is.  This information can be used to switch the secondary pump on or off to ensure its always in the middle section.

Alternatively you can attempt to do it 'open loop' by adjusting the pumps either side to run at the same speed.  This only works fully with heat pumps that don't modulate the water pump speed to keep deltaT constant, unless they can modulate both water pumps, and with no valves that can switch on and off on the secondary side.  Effectively you are adjusting the pumps to operate as if the buffer tank weren't present at all (underlining the fact that it is unnecessary!).

You can also adjust the secondary pump to be slightly slower than the primary, ensuring the thermocline is at the bottom and thus the flow to the emitters is at the same temp as the flow from the HP.

Any of these are likely to be better than doing nothing.

Posted by: @judith

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they do allow a volumiser to be used, so the obvious question is could a 4 port buffer be configured as a volumiser and stay in warranty?

 

if that is the case then re-configuring it as a volumiser is the best thing to do.  The answer to the question 'can this be done?' is yes.  

@editor Thank you for the information,do you have any information or a link of how to pipe up a 3 port buffer

Posted by: @potatoman

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@editor Thank you for the information,do you have any information or a link of how to pipe up a 3 port buffer

@heacol do you have any info to assist potatoman with this?

I have an update. While the buffer to volumiser change (and removal of the extra circuation pump) was certainly a good thing, it seems that our main issue was simply the oversized heat pump. The installer (BG) now thankfully also swapped the 7kW for a 5kW ASHP (also Vaillant aerotherm plus) and our COP so far jumped up massively. See graph with the daily heating COP for different outdoor to indoor T differences. green: 7kW and buffer, red: 7 kW and vollumiser, yellow: 5kW and volumiser. 
The flow rate of that 7kW unit was high, so there was also a big difference between the ASHP side and the circulation pump side on the buffer, so probably lots of mixing. Perhaps with the lower flow rate of the 5kW unit, the buffer configuration would not have been an issue, however it would have still been completely pointless in our case, and simpler is better.

Many thanks to British Gas, for actually sorting this out for us! I am overall very happy with their support.

THe ASHP has been really great to set up and provided very comfortable heat, and now finally we also have the good  efficiency to save loads of CO2 emissions. I would boldly project that we might get a SCOP around 4 now.

We replaced our gas boiler, which wasn't too old and was still working, so it could be argued we shouldn't have swapped it, but we had a few recurring issues with leaks and defect parts at one point the leak also led to destroying our dishwasher below it. Now it also seems that it has been very inefficient indeed. We used around 10,000 kWh of gas to keep our house around 19-20degC. Now according to the Vaillant app we actually used around 6000 kWh of generated heat to keep the house around 20-21degC, so only around 60% efficient. 

Assuming 75% boiler efficiency, head demand of 6000kWh per year and a SCOP of 4 (also assuming 0.309 kg CO2 per kWh electricity, and 0.185 kg CO2 per kWh gas), I calculate, we now save around 1000 kg CO2 per year!
Even at our worst SCOP with the previous ASHP unit, we still saved around 700 kg CO2 emissions per year. 

Cost-wise, I would very roughly estimate that in around 12 years the expensive ASHP will have overtaken any initial savings if we had gone with a new gas boiler, however I think this calculation is a bit pointless. We would have saved 2000 GBP if we had had the current BUS grant, I don't know what a new boiler installation costs, etc. Mainly for us it was to reduce our CO2 emissions, and have a warm house with a better conscience also with regards to the war in Ukraine. Anyway, a decently efficient ASHP (SCOP 3.5) costs the same or less money to run compared to even a highly efficient a gas boiler (95%). In our case it seems we should now spend around 180 GBP less for heating per year. 

Spreadsheet with my calculations (cost and CO2 emissions) attached.