Mold in Ductwork Caused by HVAC Operation

case_leg_waterdamageMold was found growing inside lined ductwork on one floor of a high-rise office building in New York City.  We were asked to determine the cause of the mold growth, and recommend corrective action.

Our initial investigation found that the HVAC system was operating properly, with no apparent reason for mold growth in the ductwork, aside from the fact that the system operated twenty-four hours a day, seven days a week, so that there was never a drying out period.  We monitored temperature and relative humidity of outside ventilation air, mixed air entering the cooling coil, air leaving the cooling coil, and air in the ductwork.  Examination of this data found that the temperature of air leaving the coil gradually increased during the day.  This resulted from a temperature control scheme that was intended to reduce operating costs by sub-cooling the conditioned spaces in the morning and allowing them to warm gradually during the day.  Unfortunately, thermal mass of the ductwork resulted in the walls of the ductwork being cooler than the air coming from the HVAC unit.  In fact, the walls of the ductwork were consistently at a temperature below the dew point of the supply air, and as a result, moisture would condense on the cool duct walls, and they remained damp all day and night long, thus supplying the moisture that supported the mold growth.

Correction of the problem was accomplished by simply changing the temperature control cycle to provide a constant daytime temperature, and constant setback temperature at night.