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Cementing Furnace Exhaust Ductwork and Chimneys


News Flash – Protect your Home from Fire by Cementing Furnace Exhaust Pipes to the Chimney


By Mark J. Donovan



During a recent home inspection of my vacation home, the home inspector discovered something that I had missed before with my hot air and hot water furnaces. The tin exhaust ductwork associated with each furnace was not properly seated into the chimney flue holes. Moreover, there was no furnace cement applied to the chimney and exhaust pipes were the pipes connected to the chimney flue holes. As a result, my home had been at risk of fire. If either exhaust furnace ductwork pipe had separated from its respective chimney flue hole, a fire could have ensued in the lower level of my home.


To resolve the problem I first pushed each exhaust ductwork pipe into its respective chimney flue liner hole at least one inch. I then applied a thick layer of furnace cement around each exhaust pipe where they connected into the chimney.


To apply the furnace cement I first used a wet sponge to clean the furnace ductwork and the surrounding chimney mortar. I then used a putty knife to apply the furnace cement to the furnace exhaust ductwork and chimney. I made sure to apply a good and thick one inch diameter bead of furnace cement around both the ductwork and chimney.

I also inspected the rest of the furnace ductwork for missing screws. Surprisingly I found a few missing screws in the furnace ductwork.


With both furnaces, the screws to the ductwork were missing where the ductwork pipe directly connected to the furnaces. Apparently the furnace service technician removed the screws and ductwork pipes when cleaning the furnace but failed to reattach the ductwork pipes using the screws. Again, this was another major fire threat averted. I used small self tapping screws to properly attach the furnace pipe sections together, and to attach the pipes to the furnace flanges.

So the next time you have your furnace cleaned by an HVAC service technician make sure to check his work before he leaves your property. Otherwise you may be putting your home and life at risk!


Furnace exhaust ductwork cemented into chimney flues.

Furnace cement and self-tapping screws used to resolve furnace ductwork safety issues.

Photos by Mark Donovan


For information on maximizing a wood stove’s home heating efficiency see the “Installation of Hood over Wood Stove Ebook”.


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