Hi Glyn
Schematic here.
DHW doesn’t go via a buffer, but once the diverter valve turns back to ‘B’ if the pump is running on, it will push water around buffer and primary loop.
I suppose the only ‘bypass flow’ from cylinder to buffer(&primaries) would be whilst the diverter was travelling between ‘A’ and ‘B’ which might only be a few seconds. If the valve isnt fully closing that might explain it.
Where is the supply line sensor located compared to the heat meter hot sensor? I would have expected these two sensors to read the same?
Externally; on the Heat pump flow I expect; which is a few meters from the heat meter which is in the plant room. Youre right they shoudl read the same. I should check the supply line sensor is properly in position.