August 5, 2013
Mixed Humid

This document covers a description of the need and applied solutions for supplemental dehumidification in warm-humid climates, especially for energy efficient homes where the sensible cooling load has been dramatically reduced.

Building designers, builders, HVAC designers, HVAC system contractors, above-code building program managers, utility energy efficiency program managers, and building researchers will be able to use this information to plan and deliver better residential buildings in warm-humid climates.

In older homes in warm-humid climates, cooling loads are typically high and cooling equipment runs a lot to cool the air. The typical cooling process also removes indoor moisture, reducing indoor relative humidity. However, at current residential code levels, and especially for above code programs, sensible cooling loads have been so dramatically reduced that the cooling system does not run a lot to cool the air, resulting in much less moisture removed. In these new homes, cooling equipment is off for much longer periods of time especially during spring/fall seasons, summer shoulder months, rainy periods, some summer nights, and some winter days. In warm-humid climates, those long off periods allow indoor humidity to become elevated due to internally generated moisture and ventilation air change. Elevated indoor relative humidity impacts comfort, indoor air quality (including allergen production and chemical offgassing/interactions), and building material durability. The HVAC and homebuilding industries are responding with supplemental dehumidification equipment, either stand-alone or integrated with central space conditioning systems, but that effort is really in its infancy compared to what will be needed to respond to the growing need to control indoor relative humidity in homes throughout the year in warm-humid climates.

Available supplemental humidity control options are described and discussed, with application guidance. Some options are less expensive but may not control indoor humidity as well as more expensive and comprehensive options. The best performing option is one that avoids overcooling (cooling below the requested set point) and avoids adding unnecessary heat to the space by using waste heat from the cooling system to reheat the cooled and dehumidified air to room-neutral temperature.

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