A property manager near Roswell called me convinced her building had a sewer gas leak, based on an odor that showed up every afternoon like clockwork in the lobby. Two plumbers had already checked the trap seals and found nothing. The actual source turned out to be a condensate pan in the mechanical room upstairs, growing a biofilm that released its smell most strongly when the afternoon sun heated the roof and the mechanical room along with it — nothing to do with the sewer line at all, and nothing a plumber would have caught by checking traps. By the time we got involved, the building had already spent two service calls and a fair amount of tenant goodwill chasing the wrong system entirely.
Why odor complaints get routed to the wrong trade first
Sewer gas, mold, and mechanical-room biofilm smells share enough overlap — musty, faintly sulfurous, worse in humid weather — that a facilities team’s first instinct is almost always to call a plumber. Plumbers are good at finding actual sewer gas leaks, which usually involve a dry trap or a cracked seal, but a biofilm smell from a condensate pan or a clogged drain line inside an air handler produces a similar odor with a completely different fix. The overlap is close enough that even an experienced facilities manager can be forgiven for guessing wrong the first time, especially in an older building where both systems are aging at roughly the same rate.
The pattern that actually points to the mechanical room
A sewer gas leak is usually constant or worsens steadily; a mechanical-room biofilm smell often has a daily or weather-linked pattern, because the growth releases more strongly when temperature and humidity inside the equipment room rise, then recedes overnight. If a facilities team notices the smell is worse in the afternoon, worse after rain, or worse when the building’s air handlers are running hardest, that pattern points toward the mechanical equipment rather than the plumbing stack. Keeping a simple log of when complaints come in — day, time, weather — for even a week or two often makes this pattern obvious before any equipment gets opened up.
What’s actually growing in there and why cleaning alone doesn’t always fix it
Condensate pans, drain lines, and even the interior of an air handler housing stay damp enough, constantly enough, to support biofilm growth in almost any commercial system that isn’t on a strict maintenance schedule. A one-time cleaning removes the visible growth but doesn’t change the conditions that let it grow back within weeks if the drain line stays partially clogged or the pan continues holding standing water. The lasting fix usually pairs the cleaning with clearing the actual drain restriction, which is mechanical work, not a cleaning-chemical problem. Recommending a cleaning-only visit when the drain is actually the culprit sets a client up for a recurring complaint that eventually gets blamed on the cleaning company rather than the real mechanical cause.
Getting the right trade in the room the first time
Once I started asking whether the odor tracked with a daily or weather pattern before quoting any cleaning scope, I stopped sending crews to treat a symptom that a mechanical contractor needed to actually fix at the source. On the Roswell job, once the mechanical crew cleared the clogged condensate line and we treated the pan itself, the smell didn’t return, where a cleaning-only visit likely would have bought the building three or four weeks before it came back.
Frequently asked questions
How can you tell mechanical-room odor from an actual sewer gas leak without special equipment? A time-of-day or weather pattern is the strongest early clue; a true sewer gas leak from a dry trap tends to be more constant regardless of temperature or humidity.
Is biofilm in a condensate pan a health concern? It can contribute to indoor air quality complaints and, in some cases, microbial growth that affects sensitive occupants, so it’s worth treating even when the smell alone seems like a minor nuisance.
Does cleaning a condensate pan require special certification? Basic pan cleaning doesn’t typically require HVAC licensing, but clearing a clogged drain line or diagnosing airflow issues inside the unit does, which is why the two trades often need to work the same call together.