Is it okay to use silicone to seal gas furnace heater supply ducts? If so, do you know how long the silicone will need to cure before the system is useable?

Some background: I've been trawling around my basement recently and found that a lot of my duct connections have exceeded my wildest dreams of what poor craftsmanship looks like. Lots of connections were "sealed" using literal duct tape - which has since crusted off and is not holding on to anything. The joints where circular ducts attach to box ducts are the worst offenders.

Thanks for your time.

  • So your question seems to start with asking about combustion gas vents, but then confuses the issue by talking about heated air ducts at length. Which is it you are asking about? – Ecnerwal Feb 17 '16 at 1:55
  • It seems I am not sure of the terminology or the difference. I am asking about the ducts that supply the heated air from the furnace to the home. – Ulad Kasach Feb 17 '16 at 1:56

Silicone or the Duct Sealant are fine. But, either need to be protected, faced, supported & sealed with Aluminum Foil Tape, the real duct tape. Before anything though, you need to solidify your duct joints (supplies & returns), they can't slip apart or slide & deflect in any direction. Just sink short self-tapping duct screws (sharp pointed ones, not the drill-head ones) through the joint's 2-pieces to lock everything together.

  • Why do they need protection after the sealant? – Ulad Kasach Mar 31 '16 at 12:19
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    The "protection" is from the surrounding air. The goo's are good for filling any gaps, but the aluminum tape will be your permanent seal. Whatever goo will eventually dry-out & release, the tape won't since it's air-tight...the goo does the air-tightness from the inside. – Iggy Mar 31 '16 at 12:24

Duct sealant

If your question is about the air ducts, why not use the correct material for the job, rather than silicone caulk. Duct sealant is, literally, made for the job. It also costs less, on a quick look.


Duct mastic or metal tape is the recommended method for sealing forced air supply ducts. All the cool kids are using mastic these days.

  • Do you know if there is there anything wrong with silicone? – Ulad Kasach Feb 17 '16 at 1:57
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    I suspect this one of those services where it won't adhere all that well due to differential expansion. I know rather better that it's a poor choice on metal roofs for that reason – Ecnerwal Feb 17 '16 at 2:01

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