A warmup cycle is an option for many new packaged HVAC systems. At startup in the morning, this controls option keeps the outside air damper closed until the room temperature reaches a target value. The same volume of room air is steadily recirculated through the unit without the added burden of conditioning a portion of outside air. Setpoint for the room is achieved quickly enough so that the fresh air content of the room air does not drop much. Once the room is warm enough, the outside air damper opens, and the unit begins normal operation.
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Excerpt from Energy Conservation Projects.