Water Blowoff

Water blowoff is caused by unconditioned air entering the return air stream from leaks in the return duct, wall penetrations or anywhere unconditioned air can infiltrate the return air supply. Normal return air is at about 50 to 55% relative humidity with the system operating normally. Infiltration in the return adds water vapor and increase the RH to 60% or more in the return air.

This added humidity load increases the water load on the surface of the coil and decreases the airflow through the coil, causing more draw from the blower section. This increase in draw also increases velocity of air moving through the coil fins pulling water with it.

This problem is more apparent with variable speed air handlers because variable speed motors will not slip as static across the coil increases with the water load. Non variable speed motors will slip or pull less air as static increases.

High velocity air also increases the chances for water blow off. Duct systems that has air exceeding manual “D” recommended air velocities will have water blow off problems and sweating ducts.