Air conditioning systems are primarily on/off devices of a fixed capacity, controlled by the limits of the thermostat. A standard thermostat, when set at 70°F (with +/- 2°F range), will turn the air conditioning on at 72°F and cool the space, at full capacity, until the temperature is 68°F and the thermostat is satisfied. This is repeated continuously.
The Problem: Oversized Systems
Standards for air conditioning design require that the system has sufficient capacity to meet several design criteria (i.e. outdoor temperature, sunshine, internal occupancy, and ventilation requirements). As most of these criteria are maximums for the summer peak, the system will be designed larger than necessary most of the time. When the system is oversized, the temperature may remain cool, but the unit does not operate long enough to sufficiently dehumidify the space. The air conditioning system is “short cycling,” meaning that for each 5 minutes the air conditioning system is actively cooling the space, there are 20 minutes when the system is merely recirculating air and not dehumidifying it. During the off cycle, the humidity level rises and the combination of low temperature and high relative humidity creates an uncomfortable space.