it seems HVAC forums have a thing against helping DIY’ers. Anyway, heres my issue:
I’ve got an old Crane boiler, model number 70-266, that only runs when the viewing door is open. Best i can tell its from the 60s? Its a dinosaur. Anyway, its an oil boiler with a coil in it for domestic hot water.
A few days ago we noticed there was no more hot water. Went into the basement, hit the reset button and it fired right up. It will run for 20-40 seconds, then it shuts off, hit the restart and it does it again and again and again. If you open the door and watch the flame it looks great, and the boiler will continue to run. As long as the little viewing door it is OPEN it’ll run and run and restart and work exactly like it should. If you shut the door it’ll shut off and not restart until you hit the button, then run for 20-40 seconds and shut off again.
When its running and the viewing door is open it is clearly pulling air in through the opening. If you hold a flame near it would suck the flame right in. Obviously its dangerous to run with the door open all the time. My first thought was something plugging the flue but its pulling all kinds of draft. Even with the boiler off the draft control in the flue is opening itself and you can hear it drawing air up.
Im having a hard time understanding why it shuts off within 20-40 seconds of shutting that little door. Its there a air pressure sensor that could shut the boiler down because of TOO MUCH vacuum inside? Seems odd to me.
If that has a separate combustion air intake- check to see if/where it is plugged. Some older ones pull from around the roof jack, others pull from a separate intake hose isolated from the actual furnace compartment. If the inspection window is open and it pulls intake air from there, the flame would be fine until you close it and the flame would starve out without intake air.
Could also be a near-dead/clogged burner motor, also, I guess?
(Not a HVAC pro by any means, but have spent some time with it.)
Had the same thing happen to me the day I moved into my new (to me) house. The old homeowners must have used to leave the viewing door open…and you could see where the flames had gone on the outside of the unit. Instead of throwing parts at it and since it was older we opted to get a new one and I’m glad we did.
Best I can tell it pulls air right on the front of the furnace, i’ll check closely for anything that might be obstructing flow. The way the flame burns (like a torch) it doesnt seem like it would be starving for air from there though.
Hmm, not sure what to tell you really without looking at it. I’m guessing the combustion air is been blocked somehow. Generally there’s some sort of intake on the combustion motor, be it ducted from outside or wherever, or just draws from atmospheric- either way, it sounds like there’s no combustion/intake air being fed to the combustion chamber.