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Gulf Coast

Technical Service Manager

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.

Recommended velocities (F.P.M.)

Flex – 600-800 FPM Main supply

        – 400-600 FPM Branch supply

        – 500-700 FPM Return Main duct

        – 300-500 FPM Return Branch duct

Metal Ductwork – 700-900 FPM Main Supply

                             – 500-700 FPM Branch Supply

                             – 600-800 FPM Return Main duct

                             – 400-600 FPM Return Branch duct

The duct system must be sized properly not to exceed a -.2″ return static and .3″ supply static, with a .5″ total external static at less than 700 FPM with filters and grills installed.

Equipment capable of controlling moisture is readily available; the problem has been the lack of an affordable and dependable sensor feedback control for measuring latent content of air. The right thing to measure in buildings is dew point.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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