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I live in York Region (Vaughan Ontario). The gas company came by and performed a visual inspection. They identified my furnace exhaust pipe (black APS 2") is starting to crack at the coupling (its about 2 feet above the actual furnace) and said I should replace the pipe with the white PVC (Ipex or Royal).

I did some research and based on the recommendation, I should prime, then glue the new pipe.

I'm assuming I can cut the black pipe with enough room to add a coupling just above where the pipe comes out of the furnace and then continue with the new white pipe.

Can I also add a coupling at the other end near the exterior wall of the house? Do I have to replace the full length to the white pipe to make it go outdoors? Obviously its easier to just add a coupling near the wall since the exterior is all caulked and already sealed. In essence I would have about 2 inches of black pipe coming out of the furnace, then about 8 feet of white pipe, another coupling joined to about 1 foot of black pipe which is already going out doors.

Thanks.

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  • I was not aware of your codes being different I always use pvc and have not had a failure in over 20 years of high efficiency furnaces ripping it all out sounds extreme to me if it was originally installed to code and the code changed.
    – Ed Beal
    Oct 7, 2020 at 13:44
  • @EdBeal -- yeah, Canada had a rash of failures of plastic appliance venting, so they tightened up their specs. Furnaces, luckily, are fairly kind to vents (low bonnet temps and few issues with HX fouling = low stack temps, and if your air filter clogs up that the HX is impacted, you'll trip on high limit), unlike boilers/water-heaters, which have higher stack temps to start with due to the higher operating temperatures involved and inevitable HX fouling/scaling concerns as well to jack the stack temp up further)
    – ThreePhaseEel
    Oct 8, 2020 at 0:02

3 Answers 3

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You need to look at the markings on the white pipe. Standard DWV PVC should not be used for flue gases, but unfortunately it is used all the time. If the entire run is not rated for flue gases, then you should replace it.

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    @isherwood -- furnace manufacturers kind of are getting away with a no-no when they recommend PVC for venting, since the fuel gas and mechanical codes don't permit such a material for such a service. (It's not as bad with a furnace, as the risk of HX fouling is lower, but it can be quite serious with condensing water heaters/boilers, and I know of no condensing appliance that has integral stack overtemperature protection.)
    – ThreePhaseEel
    Oct 5, 2020 at 23:46
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    @isherwood -- also, they'd need ULC-S636 vent piping in any case since they're north of the Canadian border, and those systems can't be mixed-and-matched at all
    – ThreePhaseEel
    Oct 7, 2020 at 1:46
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No, you can't do this

Since you're in Canada, your special gas vents (used for condensing appliances) fall under ULC-S636, and must be installed according to their accompanying listing instructions as a result, which strictly prohibit the mixing of vent systems from different manufacturers. So, you'll have to rip all the old ABS out and put in a new vent system outright. (While you could use ULC-S636 listed PVC or CPVC if it's available to you, the general recommendation would be to use something higher-temperature-rated, such as a high-temp polypropylene at a minimum or better yet stainless steel venting, as the margins on PVC especially are quite slim when used in venting applications.)

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There is a special glue that will work on both pvc and abs I found it a few years back for making transitions this would allow you to use a coupler like you want. I think it was called abs to pvc glue made for transitions.

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  • Your advice would be fine for DWV (or in the US, which is a bit scary frankly) but falls flat in Canada -- they actually got on top of regulating the use of plastic pipe for gas appliance venting, unlike the US situation which is sad
    – ThreePhaseEel
    Oct 7, 2020 at 1:45

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