"Doctor Drivel" wrote in message
...

You have a "buffer" only on the return pipe to the heat pump to ensure
cool water is fed back to the heat pump. This give the large delta T
needed for heat pump operation. This is not a thermal store.

If this buffer cylinder had a large coil in it and the cold feed to the DHW
cylinder run through it, it would pre-heat the DHW and ensure a cooler
return to the heat pump. It is enhances condensing boiler operation and
creates a larger temperature differential if the heat pump requires it. I
am surprised this was not done.

A large tall heat bank/thermal store using a plate heat exchanger for DHW
probably would have been better and using off-peak electricty could be
heated up during the night.

A heat bank/thermal store can ensure a large flow and return temperature
differential for a heat pump or condensing boiler, act as a buffer for the
heat pump and CH system and provide DHW - all off the same cylinder.
Wonderful things.