You still haven't explained how your imaginary friend calculated boiler efficiency without measuring flue gases and condensate volume/distribution.
Why don't you speak up and give the actual efficiency.
The calculation of true efficiency requires only :-
(1) Heat energy available in the fuel consumed
(2) Heat energy transfered into the fluid ( the water )
and is best expressed as Coefficient of Performance ( COP ) ,
Calculation is COP = (2) divided by (1)
The perfect boiler would have a COP of 1
The results of boiler testing. The boilers being used in the way that the boilers would be used in a domestic hot water sytem. Testing by the Building Research Establishment on a range of different boilers.