How a forced-air system spreads a smell
A forced-air system has two sides. Supply registers blow conditioned air into rooms; return grilles, usually larger and often in hallways or on a stairwell wall, pull air back to the furnace. Everything the return pulls in passes through the filter and the blower and gets mixed with air from the rest of the house before it comes back out of the supply registers.
Many newer LoHi townhomes are tall and narrow, with a garage on the ground floor, living space in the middle and bedrooms or a rooftop deck stair at the top. Returns are often placed in the stairwell, which makes the stair a highway for air from every floor. When a guest smokes in a top-floor bedroom with the door open, or cooks something pungent on the main floor, the return can carry that air through the system and spread it everywhere.
Smoke and cooking residue also settle inside the return duct and on the blower and coil, so the system can keep releasing a faint smell for days after the room itself has been cleaned.
Tracing where the smell starts
- Turn the fan off and close the house up.Set the thermostat fan to off for an hour with windows closed. Then walk each floor. The room that smells strongest with the system off is the likely source.
- Turn the fan on and sniff the registers.Run the fan only. If every supply register smells the same, the odor is in the system or the air it pulls from the source room.
- Check the return grilles and the filter.Look at the filter's date and color, and wipe the return grille with a damp white cloth. Yellow-brown staining points to smoke residue.
- Close the source room's door.If closing one bedroom door with the system running makes the rest of the house smell better, that room is feeding the return.
In the room or in the system?
Odor mostly in the room
- Strongest in one room even with the fan off
- Clings to that room's bedding and curtains
- Improves when the door stays closed
- Handled by cleaning that room's surfaces and textiles
Odor in the HVAC system
- Equal at every register when the fan runs
- Returns each time the heat or air starts
- Filter or return grille is stained
- May need a new filter, coil and blower cleaning or duct cleaning
Common questions
Will an ozone machine fix a smelly duct system?
Ozone can damage rubber parts and is a lung irritant, and it does not remove residue that keeps releasing odor. Cleaning the source and the system works better and is safer for the next guest.
General information for LoHi, not a quote or professional advice. In an emergency, call 911.