The control circuit of a furnace manages a variety of data produced by the thermostat, the sensors near the burners, the ignition system and the blower mechanism. It is in charge of starting up components in the correct order and then shutting them down properly when the furnace finishes its job. A control circuit can fail in several key ways, requiring require a replacement of the board or nearby components to keep the furnace operating correctly.
One sign that the control circuit board has failed is a lack of any type of response. In this case, the thermostat is sending the right signals to the circuit board, but wires have lost the connection or the panel itself had burned out. If you have an ohmmeter, you can test the wiring running to and from the control board to see if the wiring is at fault or the circuitry itself. Replacing a few wires can be less expensive than installing an entirely new panel.
Your control board also includes a timer that controls the furnace blower after the burner or element has been lit. The timer itself can fail and lead to a properly operating burner but a blower that never turns on. The relay switches and pressure switches that the furnace uses to open and close separate circuits can also fail, causing the timer to work but never send signals anywhere else. When the internal timer fails, you may need to replace the entire control board to fix the problem.
Thermistors and thermocouples send basic signals to the control board indicating the heat levels of the pilot light and burners. This helps the control board switch on its timer, shut down the furnace if temperatures rise dangerously high and start the primary burner. These sensors can become loose or may burn out entirely. When this occurs, you can often save money by replacing the sensors instead of assuming the problem lies directly with the control circuit.
Calling out a professional to take a look at your furnace may save you work in the long run. Modern furnaces have diagnostic indicators that show a series of lights. The pattern indicates particular problems. Charts are built into control panels or are available in manuals for more detailed looks. You may try to reset your furnace entirely to see if a diagnostic error is preventing the system from working.
- Photo Credit Comstock/Comstock/Getty Images