What to know
Deal with this one first
If you smell gas, leave the building and call your gas utility or the fire service from outside. If you smell burning plastic, hot metal or anything electrical, shut the system off at the thermostat and the breaker and call an HVAC technician or the fire service. Do not wait to see whether either one fades, and do not try to diagnose it from the vent. The only smell on this page worth waiting out is the dusty one described below, and only when the heating has just come back on for the season.
Everything below assumes you have ruled that out.
Dusty or slightly burnt, at the start of the season
The most common one by a distance. Dust settles on the heat exchanger over the months the heating is off, and burns away over the first few cycles when it comes back on.
It should fade within a few hours of running and not return. If it is still there after a day, that is worth following up.
Musty, and strongest when the system starts
Usually points at damp somewhere the air is passing. Common sources are a condensate drain that is not draining, a pan holding water, insulation that got wet and has not dried, or ducts running through a crawl space that is damper than it should be.
This one is worth having looked at, because whatever is holding the moisture will keep doing it, and the smell is only the part you can detect.
Something died in there
Unmistakable, and more common than anyone would like — rodents get into crawl spaces and attics, and sometimes into ductwork. It gets stronger over several days and then slowly fades.
Waiting it out works but takes weeks. Finding it is faster, and it usually turns up a way in that is worth closing.
Cooking, damp, or pets, only stronger
Sometimes the vents are not producing a smell at all — they are distributing one. A return in a kitchen, near a litter tray, or in a laundry will pick up whatever is around it and deliver it everywhere else in the house.
The test is whether the smell matches something in the room where the return is. If it does, the ductwork is doing its job rather than failing at it.
What you can check
- When it happens: at start-up only, all the time, or seasonally.
- Whether it is at every register or only some, which narrows it to a run.
- What is near the main return, since that is what the system is drawing from.
- The filter, which holds smells and is cheap to eliminate as a cause.
- Whether anything nearby has been wet recently.
What we would do
Look at the accessible ductwork, the condition of the connections, and what is actually inside the runs, and tell you what we find. A smell with a physical cause in the ducts is usually visible once somebody can see in there.
If the cause turns out to be a drain, a wet crawl space or a dead animal behind a wall, cleaning the ducts is not the answer and we will say so rather than book it.