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A faulty pressure switch is a common problem reported with most furnaces. When stuck open, there will be no completion of electric circuit for the furnace to fire up the burner and blower for production and circulation of heat. Let’s look at what causes this and how to troubleshoot and fix a pressure switch that is stuck open.
A furnace pressure switch is a safety mechanism located near the draft inducer motor. The use of the motor is to move combustion gasses out of the furnace and up through the vents. In pre-ignition stage, the motor runs before the flame is established to create a negative pressure ready for suction of gases out of the unit.
When the pressure switch senses the attainment of negative pressure in the system, it closes automatically completing an electrical circuit so that a furnace can fire up the burner and heat blower. If that drop in pressure doesn’t occur, the switch will remain open and the furnace won’t start or will shut automatically if it was already burning.
If your single stage or two stage furnace isn’t getting vented, it means the combustion gases are not released outside and they will be back into your unit and home’s air ventilation system. This not only affect how the furnace works but also endangers your household members when they inhale the toxic fumes.
Reasons Why a Pressure Switch is Stuck Open
Apart from acting to protect your unit or indoor air from a back drift of bad fumes, a pressure switch may stuck open as a result of the following:
- Blockage in inducer fan – This happens when an obstacle is stuck in the fan blades. Things like birds or small rodents like mice might crawl into the vent pipe and all the way into a section of inducer fan.
- Blockage in venting pipe – Debris like leaves, twigs, seeds and small branches from a nearby trees or creosote buildup may block a chimney or vent pipes to prevent proper movement of gases from a furnace.
- Faulty suction tube – A suction tube is where a negative pressure is detected by the pressure switch. If loosely fitted or cracked, this pressure won’t be realized and the switch will remain open
- Loose wire connection – A bad wire connection or a disconnection anywhere in the unit will definitely means there that no power is flowing for a proper functioning.
- Water collection in the fan housing – Excess moisture in the exhaust pipes is allowed to drain onto a pan through a small drain tube situated above the inducer motor. Any blockage in the drain line will result into water buildup in the inducer fan housing.
- Damaged switch – Wear and tear, electric failure or a clog around a pressure switch can prevent it from opening and closing.
- Failure of the draft inducer motor – A damaged inducer motor will not run to create a negative pressure required for closing of a pressure switch.
- Furnace lockout – The Integrated Furnace Control (IFC) can lockout the unit as a safety mechanism due to prolonged or repeated break in the ignition sequence mostly caused by overheating, a faulty thermostat or short cycling.
How to Fix a Pressure Switch that is Stuck Open
You can perform a quick troubleshoot of your stuck pressure switch before referring to a nearby HVAC technician. Before you do anything, turn off the main power switch on the unit or at the breaker box. You risk an electrical shock working on a powered furnace.
1. Inspect the pressure switch
Access the control board and inspect if the switch is stuck due to any blockage, a wire disconnection or any kind of mechanical jam. You may also need a multimeter to test the resistance of the switch. Unclog any blockage you discover, reattached any disconnected cable or replace the faulty switch. This can be done by the help of a technician.
2. Reset the Integrated Furnace Control unit (IFC)
When a furnace high limit switch keeps tripping, the Integrated Furnace Control may automatically shut down the furnace as a safety measure. This will be signaled by blinking lights on some models. How to reset the IFC unit is usually documented in the product manual. Carefully go through it and perform a reset.
3. Check for blockage in venting
You will need to check from the chimney down to your unit if there is any blockage in the venting. For most condensing furnace, a vent pipe extends out through the wall or roof of a house. Remove all the debris in the venting system and try to run your unit.
4. Inspect the hose/suction tube
Carefully locate where the pressure hose is connected and use a flashlight to look at it. Make sure it is tightly connected on the port, there are no debris causing blockage and it’s not broken anywhere. Clean it or replace it where applicable.
5. Check the drain line
Ensure there is no blockage in the drain line by removing any debris. This is where excess moisture from the vents is captured and directed into a drain pan. Remove any excess water from the drain pan and any that has collected in the inducer fan housing.
6. Replace the inducer motor
A failing inducer motor produces alarming noises and reduces the overall efficiency of a furnace. A motor breakdown can result from wear and tear or clogging with dirt. This part records the highest rate of failing than other components of the furnace. Replacing is the only option for a completely failed motor.
7. Call a HVAC expert
If you have tried everything above and nothing is happening, its time you call a HVAC professional. He or she will come with special tools for thorough inspection and fixing of any issue on your furnace that is leading to a stuck pressure switch.
The Bottom Line
Furnaces are very important at creating a comfortable warm indoor climate during cold seasons. Like any other appliances, these systems can experience a software, mechanical or electric problems that affect how they work. Usually a stuck open or closed pressure switch will tell you that all is not well.
A pressure switch will only resume its functioning when there is proper venting of gases out of the furnace, an inducer motor or fan is working properly and there is no serious electrical or mechanical problem in the unit. Troubleshoot any fault as guided or call a professional HVAC technician.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.