She noted that for the past couple of days the boiler had sounded "different" when it first fired, although it is located in an outhouse, so not too easily heard.
I took the front cover off the boiler, and turned it on. It went through the usual pre-fire purge, then fired. It then "pulsed" three times, before settling down to a steady and normal sounding fire. The pulses occurred at approximately one second intervals, and with each "whoosh" a cloud of unburnt fume exited the air intake of the burner, though no flame was apparent.
I went outside and looked at the top of the flue, and there was a fairly thick cloud of black fume exiting same. It struck me that the whole problem was likely related to over fueling, although it had come on suddenly, with no outside intervention.
I opened the air damper in several stages, until I got to a point that the black fume disappeared, other that when it first fires, when the fume is apparent for five seconds or so.
We are hoping that the service guy will be available tomorrow, so that it can be set up properly with the flue gas analyser, fuel pressure gauge and what ever else is required, but does anyone have any thoughts on what has happened to the burner? It is an Electro-oil Sterling burner.
There is some more history to this installation, in that the new boiler was installed by a pucker oftec guy, with no problems - initially. After a short while, it began to lockout, fairly frequently. The installer returned, checked everything, seemed ok, so left. This occurred several times, so the installer summons the manufacturer. Manufacturers guys attend on numerous occasions, find no fault, eventually change, one at a time, photocell, capacitor, control box, and ignition module, finally giving up and fitting complete new burner. There has been no further problems until now, some ten weeks at a guess. As an aside, I see that they left the old burner on-site, not even taking it back to HQ for analysis...