Behind the Screen
Words: Dan KamysWhen an architect designs and a contractor builds a home with two fireplaces, one on the first floor and another on the second floor sharing the basic chimney system, there might be a phenomenon called crossover smoking. This sometimes occurs in masonry flues that run parallel through the same chimney. Crossover smoking, essentially, is when the homeowner starts a fire in their upstairs fireplace and notices that there is smoke coming into the house through their downstairs fireplace opening.
According to information from Bernard Dalsin Manufacturing (BDM), "There are two primary reasons why smoke from one fireplace enters the house through the other fireplace. First, winds, down drafting, and/or deflection can cause smoke to come down the other flue; second, smoke could be communicating from one flue to the other through cracks in the flue tile joints. It is this second situation that is called crossover smoking from one flue to the other."
BDM makes the Chim-A-Lator, a stainless steel top-sealing fireplace damper system that easily mounts on top of a masonry fireplace flue and is operated by an adjustable regulator installed on the side wall of the fireplace. It is far more efficient than conventional cast iron dampers and the ideal replacement for old defective dampers. However, BDM admits, "The Chim-A-Lator will solve the first problem but not the second one. In fact, installing a Chim-A-Lator may make the problem more noticeable. The Chim-A-Lator does not cause the problem, but by sealing off the second flue, the crossover smoke cannot escape up that flue. The smoke instead is drawn down the second flue, thus making the existing problem more noticeable."
If your work shows signs of this problem, follow this test that Dalsin recommends: "Cover the unused flue at the top with a piece of cardboard and a weight. Start up a fairly hot fire in the other fireplace. Then open the damper in the other fireplace and observe in the flue with a flashlight whether or not any smoke is crossing over. Also, turn on a kitchen or bathroom fan and observe under extreme conditions. If there is a power attic ventilator, it should never run when using the fireplace. It pulls too much air out of the house."
Dalsin explains, "There is a very simple, practical, cost-free method of dealing with crossover smoking once Chim-A-Lators are installed: Open both Chim-A-Lators, reverse the draft in both chimneys in the usual manner, and then start your fire. Also the opening of a window or the installation of a fresh air intake to reduce the negative pressure in the home would help.
"This is a solution but does not solve the problem with your fireplace. Try sealing up any cracks or holes, including the expansion crack just inside and above the fireplace opening. Use high temperature fireplace caulk or crinkled foil to seal holes and cracks. If this does not correct the problem, the leaks are most likely in the joints of the flue tile."
Many masons are relining, repairing and insulating chimney flues today with modern and proven methods. It might be a good idea to check it out.