In an earlier contribution to this discussion,
wrote:


If its nothing more than soot in the exchanger, can it just be poked
clean?

I imagine that a certain amount of dismantling will be required.

I used to have to clean an old Baxi Bermuda from time to time. That had a
cast iron heat exchanger, in two halves - like chunky chocolate bars facing
each other. You had to slide out all the control bits - complete with burner
rail - from under the heat exchanger, and then use an over-size bottle brush
up the middle of the heat exchanger to remove all the soot.

Yours is almost certainly constructed differently, but the general
principles will be similar.
--
Cheers,
Set Square
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