Where to Measure a Heatelator Pre-Fab Fireplace

Q: We have a 2 sided fireplace on the ground floor of our house and a 3 sided fireplace in the basement. Our house was insulated from 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.- JU

A: Dear JU, Correct me if I am wrong, but it sounds like you have two separate chimneys for your two fireplaces they just share the same chimney structure? (like in the picture on the left)

If the answer is Yes, you do have two separate fireplaces and the flues are separate. Then the Chimney Balloon will stop the smoke from crossing over and re-entering the house through the other chimney. – Jason