Checking a furnace pressure switch is a fundamental diagnostic task that ensures your heating system operates safely and efficiently. This small, often overlooked component acts as a guardian, verifying that the inducer fan is creating sufficient airflow to clear combustion gases. When this switch fails or malfunctions, it can prevent the furnace from igniting or cause it to shut down unexpectedly, leaving you without heat. Understanding how to test this device empowers homeowners and technicians to quickly identify whether the issue lies with the switch itself, the fan, or a simple blockage in the flue.
Understanding the Furnace Pressure Switch
The furnace pressure switch is a safety device installed between the inducer motor and the heat exchanger. Its primary role is to detect the negative pressure generated by the inducer fan. If the switch does not register the correct pressure within a few seconds of the fan starting, it signals the control board to halt the ignition process. This prevents the furnace from running with blocked vents or a failing blower, which could lead to dangerous carbon monoxide buildup. Diagnosing the switch requires verifying that it is receiving the correct physical force and that its internal contacts are functioning properly.
Safety Precautions Before Starting
Safety is paramount when working on any HVAC equipment. Before you begin, you must turn off the power to the furnace at the circuit breaker. Locate the unit's disconnect switch near the unit itself and ensure it is also turned off. Furnaces involve high voltage and sensitive components, and skipping this step poses a serious risk of electrical shock. Additionally, allow the system to cool down if it has been running recently, as you will be working near the heat exchanger and vents.
Required Tools and Materials
Multimeter (digital or analog)
Screwdriver set (flathead and Phillips)
Vacuum cleaner with a small brush attachment
Manometer (optional, for advanced diagnostics)
Replacement switch (if testing confirms failure)
Visual Inspection of the Switch and Venting
Begin by conducting a visual inspection of the pressure switch and its connection. Locate the switch, which is usually a small, rectangular device with electrical wires connected to the back and a rubber or plastic tubing connecting to the inducer motor housing. Check the tubing for cracks, holes, or disconnections, as a leak here is a common cause of false pressure switch failures. Also, examine the venting outside your home to ensure it is not clogged with snow, leaves, or bird nests, which can create excessive backpressure.
Testing the Pressure Switch with a Multimeter
The most reliable way to determine if a pressure switch is faulty is to test it with a multimeter. First, disconnect the two wires connected to the switch terminals. Set your multimeter to the continuity setting (usually symbolized by a speaker icon or the letter "Ω"). Touch the probes to the two terminals on the switch. With the furnace powered off and the inducer fan manually spun (if possible), you should hear a click or see a change in resistance. If the meter shows no continuity (infinite resistance), the switch is open and needs replacement. If continuity is present, the switch may be stuck closed or the issue might lie elsewhere in the control board.
Checking for Blockages and Airflow Issues
A pressure switch can fail not because it is broken, but because it is doing its job correctly. Blockages in the flue vent, a failing inducer motor, or a clogged air filter can prevent the switch from sensing the proper airflow. To check for blockages, inspect the vent pipe running to the exterior of your home. Use a vacuum to clear any debris you find. Next, remove the tubing from the switch and blow through it; if air does not flow freely, the system is obstructed. Addressing these airflow issues often resolves the problem without needing to replace the switch.
