Smoke Entering House Through Unused Fireplace

Q: We read your explanation of why the smell of smoke can enter a house through an unused chimney, but our problem is a little different. We have two fireplaces that share the same chimney, one upstairs and one downstairs (which we never use). Whenever we have a fire in the upstairs fireplace, smoke (not just the smell) comes billowing in through the downstairs fireplace, even when we close the damper! Can you explain this one?

A: Actually, the explanation is based upon the same principle: whenever air travels to the outside of a house, an equivalent amount of air attempts to enter somewhere to replace it, and an unused fireplace flue is often the path of least resistance.

In houses where two fireplaces share the same chimney, each has its own flue, running side-by-side inside the masonry housing. Whenever you have a fire in one of the fireplaces, that flue becomes "charged" with rising hot wood exhaust, and vacuums large amounts of air out of the house. If the adjacent flue is the path of least resistance to the replacement air, smoke from the chimney in use can be drawn down the unused flue and into the house with the makeup air. As discussed in the smoke smell answer, most fireplace dampers don't close tightly enough to stop this flow. This phenomenon is called cross-drafting, and there are several possible solutions:

1) Open a window near the fireplace that is being used. If you can provide a path of less resistance to the incoming makeup air, you can eliminate the backflow down the unused chimney. Drawback: you won't want to be sitting in the cold draft between the open window and the fireplace.

2) Add an extra flue tile extension to the upstairs fireplace flue, so the smoke exits at least 18" above the downstairs flue. This won't stop the downstairs chimney from acting as a makeup air return, but should stop it from vacuuming smoke into the house. Possible drawback: if the downstairs fireplace has ever been used, the replacement air might still carry a smoky smell from the chimney into the room. Another possible drawback: this trick probably won't keep the upstairs fireplace flue from vacuuming smoke down into the house if you ever decide to have a fire in the downstairs fireplace.

3) Consider a pair of top-sealing dampers. These mount at the top of the chimney, and are opened and closed via stainless steel cables running down the inside of each flue. Caution: these are much tighter sealing than standard fireplace dampers, but are not airtight: we have experienced cases where some smoke was still drawn into the house through the closed top damper.

4) The ultimate fix: provide a source of combustion air from outside the house to both fireplaces, and install good, tight-fitting glass firescreens to cover both openings. This will stop the vacuum effect from whichever fireplace is being used, and will also inhibit the flow of heated air OUT of the chimney when you use the fireplaces. This fix will require the services of a Mason to install the outside combustion air intakes, and usually can't be accomplished unless the fireplace/chimney structure is on an outside wall.