Question: I have a wood furnace and an oil furnace side-by-side venting into a common 8" connector pipe and then dumping into a 6" steel flue. The wood furnace was an add-on unit and is located in-between the chimney and the oil furnace. There is a barometric damper (draft-o-stat) up-stream from the oil furnace but down-stream from the wood furnace. I have recently been getting oil fumes and wood smoke leaking into the house through the barometric damper. Any answers why? Any solutions? Answer: A 6" flue in most cases is too small to accept the combined BTU input from a furnace and a wood heater. Your reduction from 8" to 6" is a reduction in almost 1/2 the flow rate. You need a separate chimney for each or a much larger flue. 11/2007 It should be noted that it is against IMC and NFPA code for two separate units to share flues.