TL;DR
Musty odor from vents is microbial growth on the evaporator coil or in the drain pan. Clean and disinfect the coil and pan. Install a UV-C lamp to prevent recurrence.
What you might see
- musty or mildew smell from supply diffusers
- odor strongest on first startup after a shutdown
- visible biological growth in the drain pan
- occupant complaints of stale or sour air
Likely causes
Microbial growth (mold, bacteria) on the wet evaporator coil surface from persistent moisture and organic load
Algae and biofilm in the condensate drain pan releasing volatile organic compounds into the airstream
Dirty air filter holding moisture and supporting microbial growth upstream of the coil
Dead animal or debris in the ductwork or air intake louver
Required tools
- EPA-registered HVAC coil cleaner (antimicrobial)
- Diluted bleach solution
- Wet-vac
- Replacement air filter
- UV-C germicidal lamp kit (optional install)
- LOTO kit
Safety first
- Lock out the unit before accessing the evaporator section.
- Chemical coil cleaners are skin and eye irritants. Wear gloves and safety glasses.
- Never look directly into an energized UV-C lamp. UV-C radiation causes severe eye and skin injury.
Procedure
- 1
Lock out the unit at the disconnect.
- 2
Replace the air filter. A heavily loaded filter holds moisture and is often the primary source.
- 3
Access the evaporator coil and drain pan. Clean the drain pan with a diluted bleach solution and flush the drain line.[1]
- 4
Apply an EPA-registered HVAC coil cleaner with documented antimicrobial action to the evaporator coil. Follow the product dwell time. Rinse thoroughly and allow to drain.[1]
- 5
Inspect the air intake louver and ductwork near the unit for debris or animal ingress. Remove any organic material.
- 6
Consider installing a UV-C germicidal lamp in the supply plenum adjacent to the evaporator coil. UV-C exposure continuously inhibits microbial surface growth.
- 7
After cleaning, run the fan-only mode for 30 minutes to dry the coil and duct surfaces before returning to cooling mode.
Sources
Trane Voyager Commercial Rooftop Service Literature, Trane (Trane Technologies)
Trane Voyager Commercial Rooftop IOM, evaporator coil and drain pan cleaning procedures (general)
View source
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