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The asbestos cement pipe here can't be changed, but how can I make a robust fix for this junction to the a heater flue pipe?

The gas utility is suggesting flue tape, but that's crazy. Tape won't last and when it is eventually torn off, bits of asbestos will just be released into the air.

Transition to asbestos cement pipe

The fit problem here is the old and newer pipes have different diameters.

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  • Have you thought about getting a custom piece made to order?? It could fit over the cement pipe and then reduce down to the duct. Wouldn't be cheap but would be solve your problem for good.
    – JACK
    Oct 25 '19 at 19:08
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    find metal pipe that fits snugly inside the concrete pipe ... fill any gaps with fire clay
    – jsotola
    Oct 26 '19 at 2:16
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What I did. I purchased a new flexible elbow and borrowed a five blade HVAC crimper from my local Tool Library.

5 Blade HVAC Crimper from Tool Library

I carefully spray painted the transite asbestos flue pipe, just to make it a bit less powdery if touched. Then crimped the standard 5" pipe down to about 4.5", and sent it home with improvements to the straps holding the entire pipe up. I wore a P100 purple mask the entire time.

transite asbestos flue repair

This should hold things for now. It's impractical to replace the pipe itself, but one day a heat pump or direct vent gas furnace will come, and this pipe will be obsolete.

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That cement pipe is called Transite and hasn't been available in many, many, years. I ran into this stuff when I started in the residential HVAC trade in about 1965. According to my boss, it was OK to use on natural gas but not on oil fired equipment. I would find a way to replace it if possible or do as @JACK said about having an adapter made.

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