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Heating · VentingFurnace Vent Pipe Repair & Replacement
Furnace vent pipe repair and replacement covers the pipe that carries flue gas outdoors: re-supporting sagging runs, re-making separated joints, correcting the slope and replacing cracked plastic or corroded metal. If you can see a gap at a joint or water stains below one, or a CO alarm has sounded, switch the furnace off and call; a gurgling vent that causes the odd lockout can be booked as a regular visit. The diagnostic is $90–$150 and the vent work is quoted in writing on site.
On a condensing furnace the vent is plastic, and Ontario's gas code has required that plastic to be a certified ULC S636 system on every new and replacement installation since 2007. S636 pipe is stamped along its length, and the pipe, fittings, primer and cement must all come from one certified system. Older ABS and unstamped PVC installed before the rule may stay in service only while they show no cracks, deformation, heat discolouration or separated joints. Once a defect appears we replace the run end to end in S636, because a certified system cannot be spliced into uncertified pipe.
Four defects account for most of the repairs. A run that sags between hangers holds a pool of condensate, which narrows the pipe until the pressure switch opens and the furnace locks out, often with a gurgle first. Joints that were dry-fitted or under-cemented pull apart as the pipe heats and cools, and a horizontal run pitched the wrong way sends condensate to the outside wall, where it freezes at the termination, instead of back to the furnace's drain. On mid-efficiency furnaces the vent is metal, and flue gas that cools too far rusts through the connector or the B-vent from the inside.
Where the pipe leaves the house matters as much as the run indoors. Under CSA B149.1 a sidewall termination sits at least 30 cm above grade, and in practice above the snow line; 30 cm from any window or door that opens for furnaces up to 100,000 BTU/h and 90 cm for larger ones; 1.8 m from a mechanical air intake such as an HRV hood; and at least 90 cm clear of the gas meter regulator's vent. The furnace manual adds its own spacing between the intake and exhaust pipes. A termination that fails any of these is moved or extended as part of the repair.
This page is the repair job. Which vent type your furnace uses, and why, is explained in furnace venting types, and a termination buried by a drift is a different problem with a shovel-first answer: furnace vent blocked by snow. If a vent defect has already drawn a warning tag, the steps are on our red tag repair and clearance page. Anything else wrong with the furnace is handled under regular furnace repair.
Signs the vent pipe needs repair
- Gurgling or sloshing from the plastic pipe when the furnace starts, followed by a pressure-switch fault code
- White crust, brown streaks or drips at a joint — condensate is getting out, and flue gas can follow
- A joint you can turn or pull by hand, or a visible gap where pipe meets fitting
- Rust holes, white powder or loose sections on the metal flue of a mid-efficiency furnace
- Ice building on the outside wall around the termination, or exhaust staining the siding beside a window
How it works
Gurgling, a fault code, a stain, a loose joint. If flue gas may be escaping indoors we will tell you to shut the furnace off at its switch and leave it off until the visit.
The technician follows the pipe from the furnace collar to the outside wall: material and stamping, slope with a level, hanger spacing, every joint, and the termination clearances. A pressure reading at the switch shows whether the vent or the switch is at fault.
Defective sections are rebuilt and supported, and the furnace is run through full cycles with a combustion analyzer in the flue. You get the readings and photos of the finished run; book a vent inspection to start.
Common questions
Can I glue a separated vent joint myself?
No. A flue joint is part of a gas appliance's venting, and the cement has to be the one certified for that pipe system; ordinary plumbing cement on S636 pipe is not an approved joint. A joint that has come apart also usually means the run is unsupported or under stress, and that cause needs fixing along with the joint. Turn the furnace off and have it repaired properly.
My furnace still has black ABS vent pipe. Does it have to be replaced?
Not automatically. The S636 requirement was not made retroactive, so ABS installed before the rule can remain while it is sound. It must be replaced when it shows cracks, sagging, heat discolouration or loose joints, and it must be replaced along with the furnace: a new appliance always gets new certified venting. Home inspectors often flag ABS, so expect the question when you sell.
Why does my vent pipe gurgle?
Water is sitting in it. Condensing furnaces produce condensate inside the vent, and the pipe is meant to slope back to the furnace, roughly 2 cm per metre, so the water drains through the trap. A sag or a reverse slope leaves a pool that the exhaust has to bubble through. Re-hanging the run at the correct pitch cures it.
Is a rusted metal flue on an older furnace dangerous?
It can be. Holes in the connector let flue gas, including carbon monoxide, into the basement, and a flue that is rusting is usually one that runs too cool, often into a chimney that is too large for it. We replace the connector and check the chimney; if a liner is the real fix, see chimney liner installation.
Does a new window or deck affect my furnace vent?
Yes. A window added beside the termination, a deck built over it or a heat pump set in front of it can put a compliant vent out of compliance. Tell us before the renovation if you can: moving a termination on paper costs nothing, and moving it after the deck is built means opening finished work.