Q: Jason, We have a 2 sided fireplace on the ground floor in our house and a 3 sided fireplace in the basement. Our house was insulated for electric heat. When we have a fire in one fireplace we get smoke coming back into the house through the other one. Would a chimney pillow stop the smoke from coming back in? Thanks very much for you help. - JJ
A: Dear JJ, Correct me if I am wrong, but it sounds like you have two separate chimneys for your two fireplaces and the two chimney flues go up one main brick chimney structure?
If the answer is Yes, you do have two separate fireplaces. Then the Chimney Balloon will stop the smoke from crossing over and re-entering the house through the other chimney flue. This is called smoke crossover. It is when the one chimney is breathing out air from the home and smoke from the fire mixed together, the other chimney is breathing in outside air to equalize the pressure in the home but is also sucking back in smoke.
Many people assume closing the damper on the unused fireplace will stop this crossover, but that is not the case. Dampers are not tight enough to stop this air draw. That is why a Chimney Balloon is needed in the unused fireplace to stop the air draw inward through the second flue. This forces the house to find another location to draw air in at where there is no smoke.