roger wrote:

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

For our non-condensing Worcester Bosch 350 the difference is the other way
round:

71% efficient at 9 Kw
79% efficient at 25 Kw
80% efficient at 35 Kw

--
Mike Clarke