This type of switch is used to monitor the pressure differential on both sides of the evaporator coil. The sail switch has a small plastic sail that is counter weighted with a small spring to keep the sail off center. When this unit is placed in the supply air stream, air velocity keeps the sail on center. If the velocity drops, the spring pulls the sail off center and opens the switch. This condition can happen if the filter becomes clogged, the fan isn’t moving enough air, or the coil is dirty. The control can be wired to set off an alarm system to alert the occupants, or it can be wired to turn off the system.
A pressure switch works with a diaphragm having a tube sensing pressure on each side of the coil. This switch can be wired just as the sail switch is. Some residential units are equipped with a switch like this which is wired to an indicator light on the thermostat. The device helps avoid refrigerant slugging and decreases the running time of the compressor by telling the owner there is a restriction. Figure 18-3 illustrates these components.