Also for condensing boilers to actually achieve their claimed efficiencies they need to be used in the right way, namely allowed to run hot so that they spend most of their time in condensing mode ...
A correction on that.
Non-condensing boiler have to run hot (return above about 55˚C*) as condensation will kill them quickly.
A condensing boiler works best when the return temperature is below about 55˚C* and they can recover the extra energy from condensing much of the water vapour in the exhaust.
As you say, many condensing boilers are installed and gain nothing over the old reliable one they replaced - simply because the system isn't set up properly to allow them to work. Combi's make this worse due to the need to grossly oversize the boiler (to give something approximating to an adequate hot water supply) relative to the size needed for efficient space heating.
* 55˚C (or thereabouts) is down to the chemistry of the gas burning.
Biting the hand that feeds IT
