Check Your Mixed Air Temperature!

Commercial and industrial air conditioning systems typically draw in air from outside to provide ventilation air for the building. This air is mixed with the return air before going across other system components, such as filters and coils. In really cold weather, it is possible for the mixture of outdoor air and return air to drop below freezing. If you have chiller coils or even hot water heating coils, allowing large quantities of sub-freezing air to pass over them for an extended time can cause them to freeze and burst. These systems typically have some type of freeze protection that closes the dampers if the mixed air temperature gets clos e to freezing. In colder climates, people know to check for this – because they pay a price if they don’t. In warmer climates, we can get lazy and forget about the fresh air dampers, freeze stats, and the controls that are supposed to prevent this. This extreme cold weather has got folks in Georgia feeling the effects of single digit temperatures, including frozen water coils from drawing in too much super cold air. If you maintain a facility that has outdoor air ventilation dampers, you might check to see what the temperature of the mixed air is after the outdoor air is mixed with the return air. If the mixed air temperature is below freezing, you probably need to check your minimum damper position or the damper controls. Many dampers leak 10% when completely shut. They typically don’t need to be open more than a crack to maintain minimum ventilation.