2 years ago, I had an exterior 3 flue brick chimney built on the side of my house. Chimney is around 30' tall. One flue is used for a wood stove on the first floor. A second is used for the oil boiler (basement)and the last is going to be used for a wood boiler (basement). During the time of construction, I blocked the thimbles & cleanouts to the unused downstairs in the basement and repiped the oil boilers flue piping to include a barometric damper. I've used the woodstove numerous times without any smell in the basement. I just happened to be installing the stovepipe to the wood boiler in the basement while I had the woodstove on the 1st floor operating. I removed the thimble covering and immeditely smelled smoke. I quickly installed the stovepipe and sealed to the thimble temporarily with silicone. It's obvious I have a strong cross draft scenario. Is there any reason why this didn't occur to where smoke escapes the barometric damper to the oil boiler?