Expert: James Ball - 11/29/2010

Question
I have two fireplaces-main floor and bsmt level- they use same flue chase. When burning fire on main floor, smoke comes down second flue to basement. How can I stop this?  

Answer
Hello Rudy, when you have 2 fireplaces the one you are using needs to draft, lets say 10 gallons of air per minute, then it needs 10 gallons of air per minute from the home and in most cases it will pull that air down an other fireplace flue this is called “make up air”, 1st make sure the damper is closed on the un used fireplace, close off doors to the other fireplace area (basement doors), leave a window open on the lower level, install top mount damper caps on both flues and close the one you are not using and open the one you are.
There are many other reasons a fireplace will smoke, negative pressure, that is hot air rising in your home (and getting out through the attic vents, doors, lights) your house is a better chimney than your chimney. Also kitchen/bathroom exhaust fans, Radon systems, cloths dryers and the furnace all take house air out of the house (mechanical negative pressure)
I would have a local Certified Chimney Sweep www.csia.org, (there you can look up by zip code to find one near you) take a first hand look at what you have they should be able to help.
Hope this helps
James