How to use this guide
Rooftop units and air handlers serve most commercial suites in the northern metro. They move air, filter it, cool it, heat it and mix in outside air. A smell in the supply air can come from any one of those stages, or from air that was already carrying the smell when it entered the unit. This guide gives a rough sorting, not a diagnosis. A qualified HVAC technician should open the equipment and confirm.
From smell to suspect
- IfMusty or sour smell when cooling startsCondensate pan, coil or drain. Ask for an inspection.
- IfBurnt or dusty smell on the first cold morning of the seasonDust on heat exchangers or electric elements. Often brief. Ask for service if it persists.
- IfBurnt rubber, hot electrical or smoke smellShut the unit off if safe and call a technician right away. Treat as a possible fault.
- IfSewer smell pulled in by the unitCheck for a plumbing vent stack near the outside air intake.
- IfCooking or exhaust smell from neighborsCheck the shared plenum and the roof layout for discharge near an intake.
- IfExhaust or fuel smell from vehiclesLook at intake placement near docks, drive-thrus or idling areas.
- IfSmell is present with the fan offThe source is probably in the room, not the unit.
Inspection requests to match each suspect
| Suspect | What to ask the technician to look at | Who to involve |
|---|---|---|
| Condensate pan or drain | Standing water, slime, float switch, line slope | HVAC contractor |
| Filters and coil | Filter condition, coil face, belt wear | HVAC contractor |
| Outside air intake | Location relative to vents, exhaust, docks | Landlord and HVAC contractor |
| Return plenum | Openings between suites, unsealed penetrations | Landlord and contractor |
| Duct liner | Moisture, dust or film | HVAC contractor |
Terms used above
- Rooftop unit
- A packaged heating and cooling unit on the roof that supplies air to one or more suites.
- Plenum
- A space above a ceiling or within a duct system that carries air.
- Condensate pan
- A tray under the cooling coil that collects moisture removed from the air.
- Outside air intake
- The opening where fresh air enters the unit.
Common questions
- Can staff change filters to fix a smell?
- Sometimes, if filters are loaded. Follow the building's maintenance arrangement and the equipment manufacturer's guidance.
- Does a smell in the vents mean the ducts are dirty?
- Not always. Source location, intake placement and moisture are more common explanations than duct dirt.
General information for Westminster, not a quote or professional advice. In an emergency, call 911.