Things You'll Need
High temperature RTV silicone caulk
If you have an older furnace, it likely vents into a chimney with rigid metal duct. The connections on the flue slip into each other secured with screws. Look closely at these connections and you will likely see gaps. Through these gaps, if left unsealed, combustion gases may leak into your home and build up to unhealthy levels. Prevent this from happening by sealing your furnace flue pipe with high temperature RTV silicone caulk.
Cut the tip off your high temperature RTV silicone caulk tube with a knife. Ideally, you want a 45-degree angle at the tip of the caulk tube.
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Insert the tube into your caulk gun. Repeatedly squeeze the trigger until the plunger seats tightly against the caulk tube. You might even notice a bit of caulk begin to work its way up into the caulk tube's tip.
Squeeze the trigger with firm, even pressure as you move the tip around the seam on the furnace flue. Ensure you are getting an even, consistent bead of high temperature RTV caulk around the seam.
Wet your finger and tool the bead by gently dragging your finger along its entire length. This will push the caulk into the seam and also flatten it a bit.
Caulk and tool each additional seam on your flue pipe. Allow the high temperature RTV silicone caulk to cure fully before operating your furnace.
Silicone caulk will begin to skin over within a few minutes. Always tool your most recent bead before applying a new one.