Boiler efficiency query

[XTiffy]

Thanks for the update Deema.
Hopefully the heat-exchanger of our Vaillant will be replaced on Wednesday as this was the part affected by its fault code of F769 Overheat).
Unfortunately having a physio appointment at the same time I am unlikely to be able to quiz the Vaillant engineer. My wife is resolutely non-techie and is not likely to ask qustions (or understand the answer!). I might try to print out your conclusions to give to the engineer to see if there is a response. I also have a gas safe engineer friend (too far away) that I can ask the same questions. All in all good information.

 

[Spectric]

What I’d clearly missed out considering is WHAT the boiler is designed to operate at.

That is why an oversized boiler is not efficient, a smaller one working harder delivers more efficiency but you still get installers fitting larger boilers "just in case" .

The return temperature to your boiler is simply the main flow temperature minus all the heating outputs plus some loss in the pipework. The low return temperature allows the water in the flue gases to condense on the secondary heat exchanger and we recover some heat whilst the condensate now runs down the condensate pipe. In exchange for this efficiency we have reduced the lifespan of our boilers though.

 

[XTiffy]

Edit fault code was F76 (don't know where that "9" came from).