Andy Dingley wrote:
On 12 Nov, 22:06, The Natural Philosopher
wrote:

A balanced flue IS a primitive heat exchanger..;-)

Not since second-generation condensing boilers - they extract all the
sensible heat they can at the condenser, then try to avoid cooling the
flue gas any further, so as to avoid visible pluming. AIUI, there are
even some using double-wall construction to reduce heat transfer in
the flue.

Trouble is they all quote efficiency based on 50 degree or so return
temp. And mine is a lot more than that - I'd guess 65 at worst case -
which will take off a lot of efficiency.

Andy