"Tom" wrote in message
...

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most
efficient
setting for heat transfer?

It is the most efficient for the heat exchanger design which will be a low
priced one. Having the heat exchanger within a tight temp range is cheaper
and quicker to produce. That is why the expensive condensing boilers have
25C across the heat exchanger as the it is made to a higher spec and with
superior manufacture and materials. They can stretch the temp range across
it.