I am a home inspector. I saw a Lennox 8 year old VAV system with three zones. It had a bypass from supply to return. The outside mastic on the evaporator / supply plenum was very moldy. The evap case cover was also becoming moldy. The VAV by pass appears to operate via air pressure (a weight balanced damper). The inside of the ducts were very clean; no mold. The attic did not show poor ventilation although I am sure it gets humid. What is causing the evap coil exterior and mastic to become so moldy. Taking a wild guess, I thought perhaps the occupant uses only 1 of the 3 zone routinely therefore the air recirculating in the bypass becomes much colder thereby allowing condensation to occur on the outside case. Thank you.