If you have a high-efficiency gas furnace in your home, then the pressure switch is one potential failure point that does not exist on older, lower-efficiency models. The purpose of the pressure switch is to ensure that exhaust gases are flowing freely from the heat exchanger into the exhaust flue. A faulty switch or one that is stuck open will prevent your furnace from operating.
The pressure switch is part of a safety and efficiency system that also includes the draft inducer. This small blower motor sucks trapped exhaust gases out of the heat exchanger when the furnace turns on, creating a clean combustion chamber and preventing gas backflow. The pressure switch ensures that the inducer is working correctly before sending power to the igniter.
The Symptoms of a Faulty Pressure Switch
Your furnace's pressure switch connects to the exhaust flue with a small rubber tube. When the inducer turns on, the airflow creates a vacuum in the hose that closes the switch. Without detecting the proper vacuum pressure, the pressure switch will prevent your igniter from turning on. In other words, the key symptom of a faulty switch is a furnace that never turns on.
Of course, there are a variety of other problems that can prevent your furnace from igniting. Even if the pressure switch is the culprit, it may not be the underlying cause. The purpose of the pressure switch is to prevent the furnace from operating without negative exhaust pressure, so a switch that disables the system under these conditions is correctly performing its job.
Common Causes of Pressure Switch Woes
Diagnosing a pressure switch is a job best left to professionals, but a switch stuck in the open position has a few potential causes that may or may not trace back to the switch itself. In some cases, the switch may be faulty and require replacement. A clogged or damaged vacuum tube can also prevent the switch from accurately measuring the operation of the draft inducer.
If the switch and tube are both operating correctly, then the open state is most likely a response to a genuine lack of vacuum. Causes for this can be varied and may range from a failing draft inducer motor to a clogged condensate drain. Incorrect draft pressure negatively impacts the efficiency of your furnace and can pose a safety hazard, so the underlying cause must be found and addressed.
Resolving Pressure Switch Failures
If you suspect that your furnace has a pressure switch stuck in the open state, an HVAC technician can help you to diagnose and repair the issue. Not only will a professional be able to determine if the pressure switch is the actual cause, but they will also be able to look for other problems that may have caused the failure. Correctly dealing with the problem now ensures that you will enjoy a lasting repair.
If you feel your pressure switch is having issues, contact a furnace repair service today.