The Cold Air Return
The return side pulls air from wherever it is leaking, and on a Toronto house that is frequently a damp basement or a crawlspace.
Short answer
A cavity return — a joist space or a stud space used as duct — is leaky by nature and pulls air from the basement, the crawlspace or a wall cavity. If any of those are damp, the return is drawing that air and the smell into the supply stream. The fix is sealing and converting the return, not cleaning it.
Why the return side matters more
The return is under negative pressure, so any leak pulls air in rather than pushing it out.
A cavity return is formed from building materials rather than sheet metal, so it leaks at every joint by design.
Where it runs through a damp basement, crawlspace or wall cavity, the return is drawing that air directly into the system.
That air then goes through the furnace and out to every room.
A panned joist return has the subfloor as one of its surfaces, so anything in the floor cavity is in the airstream.
And an unsealed filter slot or cabinet does the same thing at the furnace itself.
The test
Smell the air at a supply register with the fan running, then smell the basement or the crawlspace. If they match, the return is drawing from there. That is a five-minute test and it points the work at the right place, which is sealing rather than cleaning.
Where the damp is coming from
A damp or wet basement, which on a Toronto house is frequently roof water and grade rather than groundwater.
A crawlspace with bare soil and no vapour barrier.
A wall cavity with a water entry problem.
A cold joist space against an exterior wall where warm moist air condenses.
An attached garage, which is not damp but brings its own air quality problem into the system.
And the furnace room itself, where a floor drain, a laundry area or a water heater pan contributes.
The fix
Find and fix the moisture source, which is the only thing that actually ends it.
Seal the return: the filter slot, the cabinet, the plenum joints, and every joint in a cavity return that can be reached.
Convert cavity returns to sheet metal duct where access allows, which is the full answer and is a sensible thing to do during any basement work.
Seal the panned joist return edges, which is a real improvement even without full conversion.
Clean the furnace, the blower and the coil, which have been receiving this air.
And then clean the ducts if they need it, which at that point is finishing rather than fixing.
Common questions
Why does the supply air smell like the basement?
A leaky cavity return is drawing basement air into the system.
Will duct cleaning fix it?
No. Sealing the return and fixing the moisture source will.
What is a panned joist return?
A joist space with sheet metal across the bottom, using the subfloor as the top surface of the duct.