Cross-Drafting

[SIERRADMAX]

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?

 

[brian89gp]

Sounds like the damper on the oil boiler is doing its job.

Any height difference between the top of the 3 flues? You shouldn't have to rely on a damper (which may break) to prevent a cross-draft sucking in smoke.

(broken link removed to http://www.chimneysweeponline.com/xdraft.htm)

 

[SIERRADMAX]

No height difference with the 3 flues. However, they share the same covering with one stainless chimney cap. My concern is the requirement for a barometric damper on the wood boiler and when not in use, smoke from the wood stove (adjacent flue) will be sucked down and vented through the barometric damper.

 

[woodgeek]

IF one flue is firing, all three are hot and drafting upward. If an inactive (warm) flue has a damper, any leaks will go 'up' rather than down.

If the house is very tight, negative pressure could reverse the other chimneys in principle. Or wind maybe. But gravity and passive draft is on your side.