A Hidden Problem in a “Clean” Home
During my assessment, I found a single-stage, 2-zone, 5-ton AC system paired with a gas furnace. A 14" bypass duct was in place, and both the supply and return ductwork were mostly well sealed and insulated with R-8 insulation - though I did note some minor gaps in insulation and air sealing. Interestingly, the microbial growth was limited to the supply ducts, with no signs of it on any return surfaces.
The system’s total external static pressure, with all zones open, was 1.3" w.c. - more than double the rated 0.5" w.c. Even more concerning, the zone board’s air temperature sensor was installed in the return duct rather than the supply duct, meaning it wasn’t accurately monitoring supply air temperatures. The fresh air damper was also disconnected and manually closed, cutting off fresh air ventilation altogether.
In the master bathroom, I noticed the main bath fan was positioned right next to the supply vent, while a second 50 CFM fan was tucked into the separate toilet room. When I asked if the vanity mirror fogged after showers, the homeowner admitted that it did occasionally, despite using the fans.
What caused the microbial growth in the supply vent and what are possible solutions to remedy the problem?
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