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Let's look at an empty U trap and incorrectly sized. In this simulation at a negative 1 inch or 249Pa of static pressure. Many fan coil units and air handling systems are often operating at this pressure or more negative, especially when filters are at its final pressure drop. In instances after a heating cycle where the U trap is empty, the U traps will allow air ingestion into the unit shown in the deflection of the flame. It is how contaminated air can be drawn into the unit and spread into the conditioned space.

Properly sizing the depth of the U-trap is another factor to ensure discharge of condensate water from the tray. Incorrectly sized U-trap will not discharge condensate, its water will fill the pan and subsequently overflow. The carrying over of condensate water into the air stream shown in the wetting of the paper will cause damage to insulation and other components.

The problem of incorrect U-traps are common, shown here when the supply air fan intake is at negative 60pa (about 0.2 inches); there is no water flow through the condensate drain pipe, immediately after switching off the fan water flows out. When condensate water does not continuously discharge from the condensate drain tray, the risk of overflow may occur.

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