How to Locate and Understand Furnace Pressure Switches

In residential heating systems, the furnace pressure switch plays a critical role in ensuring safe and efficient operation. It monitors the airflow and combustion process, helping prevent dangerous situations such as gas leaks or incomplete combustion. Knowing where the pressure switch is located and how it functions can assist homeowners and technicians in troubleshooting furnace issues effectively.

Component	Location	Function
Furnace Pressure Switch	Near the inducer motor on the furnace	Monitors airflow and ensures the inducer motor is working before ignition
Inducer Motor	Top or side section of furnace heat exchanger	Removes exhaust gases from the furnace
Vent Pipe	Connected to inducer assembly	Allows exhaust gases to safely exit

What is a Furnace Pressure Switch?

The furnace pressure switch is a safety device integrated into a furnace’s control system. It detects whether the inducer fan is operating correctly by measuring the air pressure inside the venting system. If the pressure is not within safe parameters, the switch prevents the furnace from igniting, thereby avoiding hazards like gas buildup or carbon monoxide exposure.

The pressure switch is essential for maintaining safe furnace operation.

Typical Location of the Furnace Pressure Switch

The furnace pressure switch is usually located near the inducer motor assembly. This motor is responsible for creating airflow through the heat exchanger and venting exhaust gases out of the home. The switch connects to the furnace through small rubber or plastic tubing, which allows it to sense the internal pressure.

Most furnaces place the pressure switch inside the burner compartment or near the inducer motor housing, commonly found at the top or side of the furnace cabinet.

Common Installation Points

• Attached to the outlet or vent pipe near the inducer motor
• Connected to the combustion chamber adjacent to the heat exchanger
• In older furnace models, sometimes located closer to the blower assembly

How the Furnace Pressure Switch Works

The furnace pressure switch operates by measuring vent system pressure before the ignition sequence. When the inducer motor starts, it pulls air through the heat exchanger and out the vent pipe, creating a negative pressure. The pressure switch detects this vacuum and closes its contacts, signaling the control board that it is safe to ignite the burner.

If the negative pressure is insufficient, the switch remains open and the furnace will not ignite, preventing unsafe operation.

Pressure Switch Operation Steps

1. Furnace thermostat calls for heat.
2. Inducer motor starts running.
3. Pressure switch senses vacuum created by inducer fan.
4. Switch closes, sending signal to control board.
5. Igniter activates and burner lights.
6. Furnace runs until thermostat is satisfied.

Signs of a Faulty Pressure Switch

Since the pressure switch is integral to furnace safety, any malfunction can cause operational issues. Some common symptoms include:

• Furnace cycles on and off repeatedly or won’t start
• Error codes related to pressure switch failure on the furnace control board
• Burner flickering or shutting down shortly after turning on
• Visible cracks or damage to the pressure switch or its tubing

Regular inspection and maintenance of the pressure switch help ensure reliable furnace performance.

How to Access and Test the Furnace Pressure Switch

Accessing the pressure switch typically involves opening the furnace access panel. It is located near the inducer motor, and its identifying features include connected tubing and electrical terminals.

Procedure for Testing

• Turn off power and gas supply to the furnace.
• Inspect the tubing for blockages or damage.
• Use a multimeter to check for continuity across the switch terminals when the inducer motor is running.
• If no continuity is detected or the switch does not open and close appropriately, replacement may be required.

Maintaining Furnace Pressure Switches

Proper maintenance helps prolong the life and functionality of the pressure switch. Recommended practices include:

• Regularly inspect tubing for cracks, blockages, or deterioration
• Keep the inducer motor clean and free of debris
• Schedule annual professional furnace inspections
• Ensure vent pipes are unobstructed and sealed properly

Routine upkeep of these components helps avoid pressure switch-related furnace failures.

Differences in Pressure Switch Location by Furnace Type

The location of the pressure switch can vary depending on the type and brand of furnace.

Furnace Type	Common Pressure Switch Location	Notes
High-Efficiency Condensing Furnaces	Near inducer motor at furnace vent outlet	Often has multiple pressure switches to monitor vent and condensate drain pressures
Standard Efficiency Furnaces	Close to burner compartment near inducer fan	Simpler vent system with single pressure switch
Older Furnaces	May be located near blower or sometimes integrated in control module	Less standardized positioning, may require manual referencing

Common Troubleshooting Tips for Furnace Pressure Switch Issues

When diagnosing furnace problems related to the pressure switch, consider the following:

• Check for blocked vent pipes or dirty air filters causing poor airflow
• Verify inducer motor operation – it must run smoothly to create the required pressure
• Inspect pressure switch tubing for any obstructions or leaks
• Reset the furnace system after repairs to clear error codes

If issues persist, a qualified HVAC technician should be consulted for safety and accuracy.