AC Systems And Humidity Control

January 6, 2008

If the indoor humidity in a building is above 60%RH this may be a concern. If not corrected high humidity can contribute to mold, dust mites, and also condensation problems.

Maintain humidity between 30%RH and 60%RH.

In hot humid climates around the world the reason for humidity problems is not always, but is often related to improper AC use or improper AC design.

1) Holes in the AC closet walls, AC return walls, ceiling, or floors, are extremely common and are often ignored. AC closet holes are often made during construction or AC instillation to provide openings where AC plenums, AC refrigerant lines, condensation drain lines, and wires and wire conduits can pass through. In some climates holes may allow the AC to suck hot humid air in from the attic, crawlspace, or outdoors. When AC units pull in unwanted air we call this entrainment. Any holes in AC closet walls, ceilings, or floors, should be sealed.

2) Thermostats set to the on setting can cause a problem because the on setting causes the AC units blower fan to pull humidity in and distribute it constantly, even when the evaporator coils are not cooling and drying the air brought in by the constantly running AC fan. This is even more of a problem in commercial buildings because most commercial building AC systems are designed to intentionally pull air in from outdoors. In homes where the AC is in the garage, attic, or in an outdoor closet this is also a possible problem because the AC will pull air in through loose panels. The on setting is ok in some areas but in hot humid regions if the fan runs constantly because the thermostat is set to the on setting the amount of humidity drawn in by the AC can be more that the AC is designed to dry.

3) A buildings AC system should not provide more cooling capacity than the building was designed to have. In other words if you have to many AC units or your AC unit is to large for your home or building it can create a problem. Buildings in general should have roughly only one ton of cooling capacity provided by the AC system for each 600 square foot of conditioned area. To much cooling will cool the building and shut down the compressors long before the buildings air has had a chance to dry, thus contributing the the buildings humidity problem.

When humidity is a problem condensation often forms on the AC registers. One of the first places for mold to show up following humidity problems is in the AC ducts, and on the air handler’s blower fan, and insulation. In addition humidity related mold very often grows as small dusty white or green spots of pen asp mold only visible on black clothes and shoes in the closet.

Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.



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