Is it enough air?

[AGPBurning]

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.

 

[begreen]

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.

Chimney Flue Extension | Chimney Extension Kit - Rockford Chimney

The Specialty Flue Extension will increase height and draft of your chimney and features an attached chimney cap. Easy to install and ships free.

www.rockfordchimneysupply.com

HomeSaver 1' Stainless Steel Flue Extender - Choose size

1' Stainless Steel Flue Extender by Olympia

www.fireplace-chimneystore.com

 

[AGPBurning]

Thanks for the reply. They do come out at the same height side by side. I will definitely look into the flue extension