Air pressure switches serve many functional purposes in the home. They are found on almost all shop air compressors and some central ventilation fans, as well as a few gas-fired water heaters. When an appliance or tool utilising an air pressure switch malfunctions, testing the pressure switch for proper operation and set point will usually help determine the cause of the problem.
Disconnect the electrical power to the device This can be done by either unplugging it or cutting off the electrical breaker that feeds power to the circuit operating it.
Remove the cover from the pressure switch by taking out the centre screw with a regular screwdriver.
Disconnect the two wires running from the pressure switch to the terminal block. These wires will either both be brown or blue. The black and white wire running from the air pressure switch are for power; do not disconnect them.
Apply air to the pressure switch using an air compressor or some other air source that produces a pressure greater than that of the switch setting.
With air applied to the pressure switch, place one lead of the ohm meter to one of the loose wires, and the second lead to the second loose wire. The air pressure switch should close when air is applied. If the switch closes, the ohm meter will read zero ohms, indicating the air pressure switch is good. If the switch does not close, the ohm meter will read infinity or "OL" for open. If the latter is true, adjust the set-point switch.
Adjust the pressure switch by turning the set-point screw counterclockwise while continuing to apply air to the switch. If air pressure switch does not close after backing the set-point screw completely out, the air pressure switch is bad and should be replaced.
If available, obtain a wiring diagram of the circuit containing the air pressure switch to determine its proper setting.
Always isolate electrical power to tools and electrical equipment before performing any kind of maintenance.