Your furnace is the workhorse of home comfort, but when it fails to start, a faulty pressure switch is often the culprit. This small, crucial component acts as a safety gatekeeper, ensuring the system operates only when the proper airflow is present. Learning how to check furnace pressure switch is an essential skill for any homeowner or DIY enthusiast looking to troubleshoot heating issues safely and effectively.
Understanding the Role of the Pressure Switch
The pressure switch is a mechanical device located near the blower motor or inducer fan. Its primary function is to verify that the furnace is not experiencing a blockage and that the flue pipe is clear of obstructions. When the fan creates the correct amount of negative pressure, the switch closes its internal contacts, signaling the main control board that it is safe to proceed with the ignition sequence. If the switch fails to detect this pressure, the furnace will typically shut down immediately, often displaying an error code on the control board.
Safety Precautions Before You Begin
Safety is paramount when dealing with HVAC equipment. Before attempting to check the furnace pressure switch, you must turn off the power to the unit at the thermostat. Next, locate the main power switch on the side of the furnace or in the nearby utility area and turn that off as well. This ensures that no voltage is present while you are working. Additionally, you should verify that the gas valve is turned off to prevent any accidental ignition while the system is open.
Required Tools and Materials
Multimeter (digital or analog)
Screwdriver set (flathead and Phillips)
Nut driver set
Replacement tubing (if necessary)
Flashlight
Locating the Pressure Switch
To check the component, you must first locate it on the specific model of your furnace. Generally, the pressure switch is a small, rectangular device with electrical wires connected to the back. It is usually mounted directly on the furnace housing or the combustion chamber. The device is often situated close to the inducer motor and features one or two rubber or plastic hoses that connect to the draft inducer fan or the heat exchanger.
Step-by-Step Testing Procedure
With the power and gas confirmed off, you can proceed to test the switch. Begin by disconnecting the electrical connectors from the switch terminals. Set your multimeter to the continuity setting (Ω). Touch one probe to one terminal and the other probe to the opposite terminal. If the switch is functioning correctly, you should hear a distinct "click" sound, and the multimeter should display continuity (zero ohms or a closed circuit). If there is no click and the meter shows no continuity, the switch is likely defective and needs replacement.
Inspecting the Tubing and Airflow
Often, a pressure switch malfunction is not due to the switch itself, but rather an issue with the tubing that connects it to the inducer fan. Inspect the small plastic or rubber hoses for cracks, clogs, or disconnections. Blockages in these tubes, caused by dust or debris, can prevent the switch from detecting the proper pressure. Ensure the tubing is securely attached and that the path for exhaust gas is unobstructed. This visual inspection is a critical part of understanding how to check furnace pressure switch effectively.
Replacing the Switch
If your tests confirm that the pressure switch is faulty, replacing it is generally a straightforward process. Note the specifications of the old switch, particularly the voltage rating and the port size, before purchasing a replacement. Remove the mounting screws or bolts securing the old switch, disconnect the wiring, and install the new unit in its exact location. Secure it firmly and reconnect the wiring according to your notes. After tightening the mounting hardware, restore power and test the system to ensure the new switch is operating correctly.
