Q: Why does the metal connector between our gas furnace and the chimney keep rotting?
A: I’m going to guess that you have a newer furnace. If that’s true, you’re probably getting condensation in the flue. Older furnaces didn’t have that issue. The hot flue gas they produced literally shot out of the chimney – too hot and moving too quickly to condense. Although that method kept the flue clean, there was a glaring problem: all of that hot gas was wasted energy. So furnace manufacturers, obeying mandated fuel-efficiency standards, now build appliances that use far less fuel.
This results in a smaller volume of exhaust at a cooler temperature, giving the water vapour formed in the combustion process a greater chance of condensing in the flue. If a chimney is on the end of the house, with exposure to cold air on three sides, the problem is even worse. The resulting condensate can erode the chimney flue, rot the metal connector, and even damage the furnace.
“Diagnosis requires a competent contractor to check for condensation, combustion efficiency and adequate combustion air,” says HVAC expert Pat Porzio. If the furnace is designed for direct venting, the solution may be to bypass the chimney, either by venting out the sidewall or through another roof vent. In some cases, the contractor can install a power vent – a blower to move the exhaust gas out before condensation can form. An insulated chimney-flue liner may solve the problem, too.