I have installed a new boiler - ( taking into account everything posted on the news group :) ) The old boiler rating plate was 100,000 BTU, fed from 22mm pipe. The new boiler 80,000 BTU, is fed in exactly the same way. ( except small run of 15mm at the last 1/2 m )
The hob pressure now fluctuates when the boiler lights, whereas it didn't with the old boiler. I can't think why.
The input gas pressure is well over 20mbar (minimum required by boiler) The burner pressure is 13mbar ( as spec ). The boiler takes the correct amount of gas ( from timed meter reading ).
The boiler is about 4m from the meter. The hob is fed from a T down to 15mm over a run of about 12m. T is about 1m from the boiler.
I cant obviously re-fit the old boiler and see whats going on. One thought is that the old boiler simply never switched off and we never noticed it.
Presumably the hob has no local regulation - I havent checked if there's any pressure test points on it. Hints welcomed.