why water is sweating from grill

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Condensing on Grilles

The key issues here are two-fold. Condensation occurs when the temperature of a surface is below the dewpoint of the surrounding air. Therefore, either the dewpoint of the space air is too high or the surface temperature of the grille(s) is/are too low.

It appears that you may have already found part of the cause in the stuck chill water valve; i.e., the fully open chill water valve is causing reduced supply air temperatures to the point that the grille surface temperature is below the dew point of the air in contact with the grille(s). IF the backs of the gilles are wet, then the attic air is a culprit.

The suggestions to investigate "leaky attic syndrome" is relevent to the extent that raw outside air that can get into the attic, then heated further so that the air is above the dewpoint of the grille temperature can cause condensation. However, insulating the back of the grilles is not the final solution to the problem. You need to cure the cause of the problem, not the symptom.

The most common mistake made by service technicians when "sweating" (actually condensation) occurs on the grilles is to close off the outside air feature of the air handler. The opposite is true.

An auditorium should have a fair amount of outside air to dilute odors and also should conform to ventilation standards per your codes. (ASHRAE recommends 15 cfm per person, as I recall). This can be a HUGE amount of outside air to deal with. If sufficient outside air is introduced to the space, thereby "pressurizing" the auditorium, you will probably solve the problem with the grilles. Even with an open attic ventilator (I recommend that you restrict the size of the ventilator or close it), the outside air that is brought through the cooling coil will be dehumidified AND pressurize the space. If you have little or no "raw" outside air in contact with the grilles, then the drier air should not cause condensation on the front OR the back of the grilles, as the air then passes up to the attic space and out the ventilator penetration in the attic. (assuming the doors are closed to the out of doors).

Auditoriums are also VERY subject to this sort of problem during startup and especially during part load performance such as a humid 80 degree day. At part load, there can be insufficient coil load to allow for dehumidifying the air effectively, causing high space humidity, thus condensing grilles.

If the space has been vacant for a few days, upon startup of the system, you probably will have condensation no-matter-what unless you bring the space temperature down slowly, while at the same time bringing in outside air to maintain pressurization of the space.

The same thing happens in restaurants where there is a LOT of outside air passed thru the space due to the kitchen hood exhaust requirements.

A worse case scenerio would be to install correctly engineered reheat coils with appropriate controls, but this should only be recommended when all other conventional solutions fail.