Is it enough air?

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New Member
Jan 13, 2024
Grande prairie Alberta
Might be simple might not be. I have a double sided fireplace upstairs and a single sided downstairs. I lit a fire upstairs one time since owning the house ( about a year and a half now) and the basement started to fill with smoke. I found that birds had came to make home in the outside air vents but have recently got that all cleared up. Both chimneys are at the same height on exit. My question is is the air vents having a birds nest in it enough to mess up the houses pressure when having a fire? Since there’s no air vent inside the actual fireplace. Just on the outside. It’s that enough air getting in? I’ve added a few photos and the air vent is the small one on the base. There’s one on each side. The house also has a boiler. No actual forced air vents. Not sure if that makes a difference.
Thanks for the help.

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Do the two chimneys terminate side by side at the same height? If so, that is the problem. Have the upstair's flue extended 12-18" so that the smoke does not siphon down into the lower fireplace.

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