Your furnace has several safety features that will trigger a shutdown. These features prevent your furnace from exhausting dangerous fumes into your home and also stop the furnace from causing additional damage to itself. With many modern furnaces, you can determine what's wrong by checking the control board for error codes and consulting your unit's manual.
If you see an error relating to your pressure switch, there may be many possible underlying causes. Understanding what this error means and why it triggered is essential to restore heat to your home safely.
The Role of Your Draft Inducer and Pressure Switch
Your furnace goes through several steps each time a thermostat requests heat. Before beginning, the control board will check that its various safety switches are in their correct states. Any problems with the safety switches will prevent the furnace from starting. If your pressure switch is stuck closed, your furnace may shut down at this point.
Assuming the control board doesn't see any issues, it will signal the draft inducer motor to turn on. The draft inducer is an added efficiency and safety feature found on most modern furnaces. This blower pushes air out of the combustion chamber, ensuring that it's clear of existing exhaust gases before igniting. In doing so, it creates negative pressure along the exhaust flue.
The purpose of the pressure switch is to detect this negative pressure and ensure the draft inducer is functioning correctly. If the draft inducer isn't working, there won't be a pressure change, and the switch will remain open to prevent your furnace from lighting. The control board knows the switch should close at this point, so it will trigger an error if it remains open once some time has passed.
Diagnosing a Pressure Problem
Some furnaces have multiple possible codes relating to the pressure switch. If the code tells you that it never opened, there's a good chance you need to replace your pressure switch. On the other hand, a switch that doesn't close may be functioning correctly. Since the role of this safety feature is to detect the presence of negative pressure (suction), this error code may be alerting you to a real problem.
In this case, the problem may be a faulty inducer motor, a wiring issue, or even a blocked vent. Since there are many possible causes for an inducer failure, you shouldn't attempt to solve this problem on your own unless you're confident in your diagnostic skills. Relying on a trained HVAC professional for furnace repair is the safe bet.