Many HVAC units operate with an electric fan motor to blow hot or cool air through the ducts into the various rooms in the building. The motor typically connects to a device known as a pressure switch, which senses negative pressure from the draft inducer to start the HVAC unit and detects when air flow is interrupted to shut down the fan. Sometimes the pressure switch malfunctions or sticks in the open position, which prevents the fan motor from switching on. Your unit may also have a control panel with a flashing indicator light to alert you of the problem. Troubleshooting the part can provide the solution.
Opening the HVAC Unit
Shut off the HVAC unit at the fuse box or circuit breaker and wait for the equipment to cool to room temperature if the heat has been running. If the AC component has been in use, there's no need to wait since the internal temperature of the components will not pose a danger. If you're working on a gas furnace, close the gas valve connected to the unit, twisting the lever to the left. The panel on the top side of the unit contains screws or slotted bolts that remove with a screwdriver or nut driver to provide access to the interior.
Locating the Pressure Switch
On most condensate HVAC equipment, the pressure switch will be located near the top interior. One fast way to locate the switch is to follow the route of the flexible plastic hose from the blower motor at the bottom. The hose attaches at the other end to the pressure switch.
Disconnecting and Testing the Pressure Switch
The air hose on the pressure switch pulls off with gentle force. Removing the hose from the switch and the motor lets you check the hose for any obstructions. Blow through one end of the hose to test. The pressure switch works with a small plunger that pushes in and out. Pushing in the plunger closes the circuit to activate the blower motor, so if the switch is stuck in the open position, that means the motor will not run. Spray silicon lubricant on the plunger and work it back and forth to loosen the part. If it will not move, the spring may be broken and the switch is defective. The only solution is to replace this inexpensive part.
Removing a Pressure Swich
The pressure switch disconnects from the HVAC unit when you remove the screws in the mounting holes. The screws holding the two wires to the switch must also be loosened to disconnect the wiring. If you're unsure about the correct replacement part, take the defective switch to an appliance parts store for assistance.
Replacing the Switch and Closing the Unit
The new pressure switch fits over the mounting holes in the HVAC unit in the same orientation as the old part. Secure it with the two screws in the mounting holes and connect the two wires to the switch's two screw terminals. The air hose slides on the nozzle of the new switch and to the blower motor at the other end. With the connections complete, fasten the cover plate with screws to the exterior of the HVAC unit and turn on the power at the breaker or fuse box.