I have a gas boiler from 1987. The main burners go out.
That is, with the electric and pilot on, I turn the thermostat up. The burners come on and are on for a couple of minutes and then shut off. To me this seems as if the automatic vent damper is not opening. The pilot stays if I do not turn on the mail burners but shuts off when the main burners suddenly turn off.
Here is the funny part, I cannot find an automatic vent damper on the flue in the furnace room (see attached photos). I don't hear a motor sound when I turn up the thermostat. Is it possible that there is no automatic vent damper? If that is the case, what else could cause my recently-working (last week) boiler to turn off?