Variable-air-volume (VAV) systems provide effective, indirect dehumidification over a very wide range of indoor load conditions (that is, sensible heat ratios). As long as any zone needs cooling, the VAV air handler supplies dry (low-dew-point) air to all terminal units. The dry supply airflow, modulated to control the sensible indoor load directly, removes the latent indoor load indirectly by absorbing space-generated moisture and removing it with the return air.

If the sensible indoor load drops below the minimum cooling capacity provided by the minimum flow of supply air, sensible heat must be added either at the terminal unit (to temper the supply air) or within the zone. Failing to temper the supply air (or to increase the sensible load) overcools the zone without dehumidifying it, making it feel clammy.

Typically, some VAV zones require tempering heat even when high sensible loads exist in the others. Always consider on-site recovered energy as the source for supply-air tempering, whether the system is VAV or constant volume.

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