I have an old (1980s?) Carrier horizontal gas furnace in the
crawlspace next to my basement. It has an electrically lit pilot
light. Lately it has developed the following problem:
When the furnace is cold, and there is a demand for heat, the pilot
light is lit (with a click) and the little fan over the transformer
starts running. I'm not sure if there is an inducer fan; I don't
think I hear a second fan. After a short time the main burners light
(with a few clicks) and burn fine for a few seconds. Then they go out
for a few seconds and relight with a few clicks and a small bang.
This repeats several times over the course of a couple minutes.
Throughout this the pilot light continues to burn without
interruption. Finally the main burner stabilizes and burns fine.
Shortly thereafter the blower fan comes on and the furnace completes
the rest of its cycle OK. If the furnace has been run fairly
recently, there there is less or no problem with starting it up again.
What can cause this behavior? I'd like to keep the furnace working
for a year or two, then I'll be replacing it. It actually works OK
now, but all the bangs at the beginning are disconcerting. It sounds
a lot like a guttering flame, but the air is still in the vicinity of
the furnace. When the burners are lit, it looks like the draft is OK,
in that the jets of flame are blue and are pointed into the heat
exchanger as if something is sucking them in.