sturharv said:
What size header would you suggest for linking the 22 mil f & r's into.... 42mm?
I would split the heating system. Divide and rule. Or if they need two on the one system, for power and back up, then a low loss header. 42mm should be fine, even the next size down. There are various methods of doing this. One is a vertical pipe with the CH flow taken off the top using a pump - return at the bottom. At the top and sides of the pipe a tee each side for the flows of the boilers, same at the bottom. Make the pipe as long as possible. The CH circuit pump must "generally" be more powerful than the two CH pumps, so the heat is immediately taken out of the header and does not immediately return to the boilers lowering condensing efficiency.
The boilers must never be on with the CH circuit pump not running, as they will just short circuit.
Others may have another take on headers.
Re: the basins. The likelihood all are on at the same time is remote. Even a cylinder based system will have problems providing full flow on all at full flow.
If the basins are a way from the combis then you will have the dead-leg lag. Insulate the DHW outlet pipe to all the taps to keep the water warm.