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  1. #1
    Join Date
    Sep 2012

    VAV mold question

    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.

  2. #2
    Join Date
    Nov 2007
    Central Ga.
    My thoughts are you are correct on your wild guess. With a bunch of air bypassing, but with the system still calling for cooling, the supply air temperature is likely to drop well below normal conditions. With a little balancing this condition might be corrected. I would try 'pushing' more air to the zone that has the system T-stat, in an attempt to equalize the zone loads.
    This situation is common in smaller bypass type VAV systems served by D/X cooling coils. Even with a complex control system (perhaps looking at return air temperature and maybe even by pass flow) there is always a compromise in system performance. System balancing is critical, but, as the zones go from low load to high load and then back again there will always be a compromise in these type systems.
    All that being said, I think your wild guess was an accurate one.

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