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I discovered my furnace’s fresh air intake pipe had disconnected where it enters the furnace. How screwed am I? How do I prevent from happening again? The pipe just sits snugly in furnace hole, nothing else holding it in. Not sure how it popped out on its own.

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    In our setup the fact that it isn't screwed into place is on purpose, so it can be removed to get at any debris that might have been pulled into the intake from outside (in our case, small leaves)
    – Joe
    Feb 23 at 12:47

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Assuming that the pipe seats deeply enough in the collar you can secure it with three screws driven in 120° apart through the collar into the pipe. Running the furnace without the pipe connected shouldn't have caused any damage. If the pipe is used to bring in outside air for combustion then you may have lost some efficiency for the duration. If it includes a counterflow heat exchanger with a concentric exhaust pipe then a little more efficiency was lost. So it goes. Hopefully no one crawled in.

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  • You should vaporize your redundant comment yourself instead of leaving it for the community to clean up.
    – isherwood
    Feb 23 at 19:28
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I agree with HABO that no harm has been done. I have a different take on the right solution, though. Since it's in intake duct, there's no outward pressure. And since it's a top-mounted vertical installation, gravity isn't a factor. The only thing that could really result in it coming loose is a poor fit from the installation--it's too short, and tension on the pipe from above is pulling on it.

Consider cutting the pipe, installing a coupler (cemented or no-hub), and extending a half inch or whatever is needed so it fits securely with only gravity. Screws are fine, but this is how it's intended to be.

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    I’ve added a second photo showing that there used to be a zip tie pulling it close to the exhaust pipe. Wondering if that was to keep it from popping out but someone cut it for some reason.
    – user216096
    Feb 24 at 2:22
  • What's even more strange is the foil tape. That doesn't make much sense there. I would check and see if those elbows are cemented.
    – isherwood
    Feb 24 at 21:55
  • I checked and can’t really tell if that particular exhaust elbow is cemented. All the other ones are noticeably cemented/glued. When I put my hand up to the suspect one while furnace is running, I do feel a very weak air flow. What product do you recommend for sealing?
    – user216096
    Feb 25 at 2:37
  • I have a carbon monoxide detector down there and it’s not leaking enough to set that off thankfully.
    – user216096
    Feb 25 at 2:40
  • Pipe cement, for PVC in this case. You're not "sealing" them. You're installing them properly by welding them together (if they aren't).
    – isherwood
    Feb 26 at 13:45
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A hose clamp might also be sufficient to compress the black collar onto the white pipe.

Not necessarily applicable to this case, but for dryer vents, screws intruding into the air flow is a no-go because they pick up lint and become a fire hazard.

hose clamp

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    Those plastic fittings aren't terribly conducive to compression clamps. I'm not sure you'd add enough grab to overcome whatever pullout force caused the problem here.
    – isherwood
    Feb 23 at 19:32

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