MIXED AIR Mixed air in a fan system is the body of air that is a mixture of outside air and return air from the building. Most conventional fan systems attempt to control the mixed air temperature in the range of 55°F. A certain percentage of the fresh air is required for ventilation purposes and exhaust air replacement, while fresh air beyond that percentage is required for cooling purposes only. Often the mixed air is referred to as “FREE COOLING”, but is it always? Most buildings require varying degrees of cooling, even during the winter. The mixed air in the range of 55°F is the design temperature for times when the building requires maximum cooling. What happens when that amount of cooling is forced into the building and the maximum amount of cooling is not required? The building will either become uncomfortably cold or the building system will have to add heat to compensate for the unwanted cooling. This is similar to going home and putting a manual switch on your central air conditioner to have refrigeration active all the time. When the load for cooling is great enough to match the capability of the refrigeration unit the building will be comfortable. When the cooling load drops, you will either have to find a way to have the cooling delivered match the actual requirement or turn on your furnace to remain comfortable. “FREE COOLING” is not “free” if the amount of cooling delivered is not required by any part of the receiving areas. This “FREE COOLING” cost is equal to the heating energy cost to correct for the over-cooling. RETURN FAN EXHAUST NC AIR RETURN AIR NO VENTILATION SUPPLY FAN OUTDOOR MIXED SUPPLY AIR AIR NC AIR FREE COOLING 5.37
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