This switch is seen primarily in commercial units, and in my opinion it is a valuable safety switch for both heating and cooling. It is generally located downstream of the evaporator fan. The common of the low-voltage control circuit is wired through the sail switch. The Y terminal (cooling and generally wired to the compressor) or the W terminal (heating and used with electric furnaces) is wired in series through the sail switch. In Fig. 10-5 a sail switch is shown. There has to be a certain volume of air blowing across the sail switch for it to close its circuit. Diaphragm type of pressure switches can be used also. These are found in some light commercial applications. With this control, if filters are dirty, or a fan belt is broken, or the fan motor isn’t operating, the system will not operate preventing possible damage to any of its components. In units not equipped with limit switches, this type of switch is invaluable.