I dont understand the reason why not to use a fga on a cast iron heat exchanger, I have come across many boilers over the years with knackered burners / flues which looked fine
on initial inspection.
I had an 15 year old Classic a month or so ago that sent my analyser of the scale, burner pressure and gas rate were spot on.
Client saw what was happening and decided straight away to have a new boiler rather than start messing about with it.
If you have the equipment surely you would use it?
B Gas developed the machines when floor standing boilers and netaheats were nearly 70% of the market.
When I did my training on the analysers (around 1996) we were even taught how to use them on back boilers.