Expert: James Ball - 11/20/2010

Question
We had a 1970 gas fire when we moved into our 1930s house.  We removed this and put in a fire grate with back plate.  The chimney does draw and we can get a fairly decent fire going with a lot of attention.  However we have two unused fireplaces upstairs and even though the dampers are shut upstairs can fill with smoke whether we burn any wood type or coal.  

Answer
Hello Chris, it is called Negative Pressure, if 10 gallons of air are going up the chimney then it needs 10 gallons of air from the house and it will pull it from the path of least resistance, another fireplace and that will grab the smoke at the top and drag it down.
They make a "top mount damper" that can install on top to close off the the chimney completely when not in use and will stop the smoke/air from coming down.
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