Thursday, March 11, 2010



In a 6 month old building, metal surfaces were rusting, and mold was everywhere. Vinyl wall coverings were coming unglued. What was the cause?

The air conditioning system in this building employed small (5 tons or less)direct expansion units. The fans units ran continuously,and included fixed outside air intakes. All of the equipment was sized for a peak daytime cooling load with high outdoor temperatures and sun. In many cases two small cooling units served a single space.

We took temperature and humidity measurements, and found that the indoor humidity level was well above 60%. I theorized that during moderate weather, the compressors would cycle on temperature, but the high humidity outdoor air would continue to be introduced to the space, as the fans ran all the time. When a compressor would come on, the cold air from the system would cause condensate of the highly humid air.

Our solution was to disable half of the cooling units. This allowed for longer operation of the remaining equipment, allowing for better dehumidification. After about an hour in this operational mode, the indoor humidity dropped from 63% to 51%, solving the problem.

After confirming the solution, we made some duct and temperature controls modifications, and the humidity and mold problem was solved.

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