I'm really sceptical about flue gas heat recovery - offering far more energy into the hot water than can possibly come out of the last few percent on heating combustion efficiency (maybe it works with really inefficient boilers and high heat loss houses - but there are other fixes for those).
The point about weather compensation is that this will ensure the boiler is already operating at maximum efficiency, with flue gas temperatures down at 40C say, and there is no heat left to be recovered from the flue gas.
