When your furnace fails to start, the culprit is often a safety mechanism designed to prevent dangerous operation. The pressure switch is a critical component that ensures the furnace blower is running and the venting system is clear before gas is allowed to ignite. Understanding how this device works is the first step in diagnosing issues, and for many, the question becomes how to bypass furnace pressure switch systems temporarily to service the unit or test for specific faults.
The pressure switch operates as a safety relay, connecting the inducer motor to the control board. If the draft is insufficient, the switch opens the circuit, preventing gas flow. While bypassing this component can be useful for troubleshooting, it is essential to recognize that doing so removes a vital safety layer. This guide details the procedure, the risks involved, and the correct methods for restoring your heating system to optimal function.
Identifying Pressure Switch Issues
Before attempting any intervention, accurate diagnosis is required. A failing pressure switch will often trigger specific error codes on the control board or display a sequence of flashing lights. The most common symptom is a furnace that tries to start, the inducer motor runs, and then the system shuts down immediately after.
Listen for the distinct sound of the inducer motor attempting to start.
Check for error codes specific to your furnace model.
Inspect the plastic tubing connecting the switch to the inducer housing for blockages or disconnections.
Tools and Preparation
Safety is paramount when working on a gas furnace. Prior to touching any components, ensure the power to the unit is completely turned off at the breaker box and the gas supply is shut off. Gather the necessary tools, which typically include a screwdriver set, a multimeter for testing continuity, and a small brush for cleaning debris.
Locate the pressure switch, which is usually mounted directly to the side of the blower compartment or the heat exchanger. You will need to access the wiring harness connected to the back of the switch to perform tests or initiate a bypass.
Temporary Bypass Procedure
A temporary bypass should only be performed to verify that the switch itself is faulty or to clear a blockage in the venting system. To bypass the switch, carefully disconnect the wiring harness. Use a jumper wire to connect the two terminals on the back of the switch that the wires were originally attached to.
This action simulates the switch being closed, allowing the furnace to proceed with its ignition sequence. Remember to remove the jumper wire immediately after testing, as leaving the bypass engaged during normal operation is extremely dangerous.
Common Causes of Switch Failure
Understanding why a pressure switch fails can help prevent future issues. Often, the problem is not the switch itself but a restriction or leak in the system.
Clogged exhaust vents due to bird nests or ice buildup.
Cracked or disconnected PVC piping leading to the outdoors.
Blower motor issues generating insufficient airflow.
Physical damage to the switch due to vibration or moisture.
Risks and Safety Considerations
Bypassing a pressure switch eliminates the protection against carbon monoxide leaks and potential explosions. If the burner ignites without proper venting, toxic gases can accumulate in the home. This procedure is strictly for diagnostic purposes to confirm a faulty switch.
Never operate a furnace with a permanent bypass. If the issue is a clogged vent or a failing inducer motor, simply resetting the switch will not resolve the underlying hazard. Always prioritize correcting the root cause over taking shortcuts.
Restoring the System
Once testing is complete, the jumper wire must be removed, and the wiring harness reconnected to the pressure switch. If the switch is confirmed to be defective, replacement is the only safe solution. Ensure the new switch is calibrated to the correct pressure rating specified by the manufacturer.
