had a few of these that as they get up to temp make a droning sound, both occassions replacement of the aav did the trick, if that is the problem in your case the sound will becoming from the righthand side of your boiler, other reasons on other boilers can be swollen o rings on divertor valve which was sometimes due to problems with certain brands of inhibator, system debris inside the boiler could be at fault, sometimes on new boilers when you get high pitched sound you have to slacken of nuts on parts inside the boiler and gentley retighten, however as your boiler is coming to temp is there a kettling sound coming from heat exchanger? sounds like a job for an RGI as you don't want to be poking around inside the boiler, have you had anywork done to the system recently?
Hello
Because I've changed both, I'll gas rate it today but you'd think
if it was a worn injector it would be a problem on full demand and not
when it's nearing temp and racking back wouldn't you?
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