Quick answer
Vents smell like sewage when sewer gas gets into the return air, most often from a basement floor drain whose trap has dried out beside the furnace, or from a condensate line piped into a drain with no trap. Pour two litres of water into every floor drain. If the smell could be natural gas, leave and call Enbridge Gas at 1-866-763-5427.
When your vents smell like sewage, the ductwork is almost never the source: it is the delivery system. The furnace blower pulls basement air in through every gap in the return duct and the cabinet, and whatever is in that air comes out of each register in the house. There are three usual ways sewer gas gets into that air, and the first one costs nothing to fix.
Is it sewer gas, natural gas or a dead animal in a duct?
Sort this out before anything else, because one of the three is an emergency. Sewer gas and gas odourant can both read as rotten eggs, so any doubt goes to the gas company.
- Natural gas or propane: a sharp sulphur smell from the odourant added to the fuel, strongest near the meter, the gas piping or a gas appliance, and present whether or not the blower is running. Leave without touching a switch and call Enbridge Gas at 1-866-763-5427 or 911 from outside. On rural lots that heat with propane, call the supplier's emergency line or 911; propane is heavier than air and settles in basements. The full steps are in what to do if you smell gas.
- Sewer gas: a septic, outhouse smell that is strongest at a floor drain or at the furnace, arrives a few seconds after the blower starts, and is worse after a dry spell.
- A dead animal: a sweet, rotting smell from one or two registers rather than the whole house. It builds over a few days, peaks, then fades over two or three weeks. The usual find is a mouse in a floor boot.
A dry floor-drain trap beside the furnace is the most common cause
The floor drain near your furnace has a U-shaped trap that holds a few centimetres of water, and that water is the only seal between the house and the sewer. If nothing drains into it for a few weeks, the water evaporates and the pipe is open. It happens most in spring and fall, when neither the air conditioner nor a high-efficiency furnace is dripping condensate into the drain.
The fix is a two-litre jug of water poured slowly into the drain. A tablespoon of mineral or cooking oil on top slows the next round of evaporation. Some newer houses have a trap-primer line that tops the floor drain up automatically, and those lines can clog with scale in Halton and Hamilton hard water.
A condensate drain tied into the plumbing without a trap
The second cause is at the furnace itself, where the cooling coil and a high-efficiency furnace drain their water through a plastic tube. That tube should end in open air above a trapped floor drain, or run to a condensate pump. When it has been pushed deep into the drain, glued into a stack or tied into a standpipe past the trap, it is an open pipe from the sewer to the side of your furnace.
Sewer gas rises up that tube and comes out beside the blower compartment and the return drop, the two places under the strongest suction. On an air handler whose drain pan sits on the suction side of the fan, the blower draws it straight through the drain line. Re-piping with a proper trap and an air gap is a small job: condensate drain cleaning or trap replacement typically runs $150–$350, and the $90–$150 diagnostic visit is credited to the repair when you go ahead.
A cracked plumbing vent near the return or the fresh-air intake
The plumbing vent stack carries sewer gas up and out through the roof. In pre-war brick houses in Hamilton and west Toronto that stack is often cast iron, and with age the joints loosen and the pipe cracks. A crack in the basement section leaks gas a few steps from the return duct.
Outdoors, the HRV or fresh-air intake hood can sit too close to a plumbing vent, and the fan pulls the smell indoors when the wind is right. The clue is a smell that shows up only on some days, or only while the HRV runs: switch the HRV off for a day and see if it stops. A cracked stack is a plumber's repair, usually confirmed with a smoke test of the drain system.
Check this yourself first, and call now if the smell does these things
- Rule out gas. If the smell is at the meter, the water heater or the range as well as the vents, stop here and call from outside.
- Pour water into every floor drain, unused shower, laundry tub and spare sink, then note whether the smell is gone by the next day.
- With the blower off, smell at the floor drain and at the end of the condensate tube, and see whether that tube stops above the drain or disappears into a pipe.
- In a 2000s or newer house, find the basement bathroom rough-in. Those capped pipes in the floor are open to the sewer if a cap was knocked off or never glued.
- If only one register smells, lift the grille and look inside the boot with a flashlight.
Call now if any of these is true:
- You cannot tell the smell from natural gas: call Enbridge from outside the house, not us.
- Anyone in the house has a headache, nausea or dizziness along with the smell. Get outside and air the house out.
- The smell is back days after you refilled the traps, which means a trap is leaking or siphoning, or a vent has failed.
- Sewage is backing up through a floor drain, which is a plumber's emergency call.
Sewer gas from a dry trap is mostly a nuisance, but it carries hydrogen sulphide and methane and should not be left for weeks. ZK Mechanical checks the condensate drain, the return duct leaks and the fresh-air intake on an indoor air quality visit, fixes the HVAC side and names the right trade when it is a plumbing fault. If the odour is closer to damp socks, see why an AC smells musty. To book, call (647) 801-1252 or send us the details; related questions are grouped under air quality and ducts.