Power burner and chimney dynamics

ChrisL

I have an apartment building with a 1,000,000 btu steam boiler with power burner. Chimney is approx 70' high and masonry with clay lining. The chimney has tremendous draft, so a barometric damper was installed to prevent the chimney from pulling too much on the boiler combustion. Standby draft is now .5" . During the very cold recently (-20 degrees), the damper was wide open, and chimney was at 1" w.c. Should another draft damper be installed to bring that down?

Also, I was having a problem on startup where the flame would get all turbulated, and high vibration. My suspicion is that even though the chimney was drawing strong, the initial entry of hot flue gases into the cold chimney reached the bubble of slower colder air going up, thereby putting some back pressure on the exhaust. Is there any solution to this? Would a chimney cap help?

STEVEusaPA

Chimney cap couldn't hurt, but it's not your problem.
What is the make/model of your boiler?
Where were these reading taken?
Most large boilers will have an actual damper on the outlet used to set the proper over fire draft, and a baro damper may not be needed.
Proper consistent steady draft over the fire is the most important factor for proper combustion.
Also a pre-purge, or longer pre-purge will help.

ChrisL

Its a Rayes boiler, and it doesn't have a damper. Uses a Beckett cg-10 gas power burner. The readings are taken in the vent maybe 3' from outlet of the boiler. Now it has a 30 sec pre-purge, but I don't think it has much effect on this problem, as it seems its effect is dwarfed by the chimney draw. I can increase that if you think it will help. The fire is fine for about 4-6 seconds until the flame vibration occurs. So I'm guessing the flue gases move up the chimney and reach a point where the cooler air above holds it back. The barometric damper shuts closed for a second when this happens, and flame gets all messed up. Seems like a boiler damper would help a lot, but I'm not sure I can retrofit something like that. I'll have to ask the manufacturer. I will check a reading of the draft when boiler is running tomorrow, but I think its still slightly negative, indicating the chimney is still pulling too much on the boiler exhaust.

EBEBRATT-Ed

With a 70' chimney a second barometric damper would not be unusual.