You keep pressing the “up” arrow button on your wall thermostat to call for heat, but your furnace is being stubborn and won’t comply. Perhaps the furnace does start up, but then stops unexpectedly, or you’re noticing that the furnace’s blower motor appears to be running all the time. All three of these problems could be the result of a malfunctioning furnace circuit board.
What a furnace circuit board does
The furnace circuit, or control board regulates the power supply to all the furnace components. When the home’s thermostat calls for heat, the furnace goes through a timed ignition sequence that is initiated when the circuit board sends 120 volts of alternating current to the furnace’s draft inducer fan motor. The inducer fan motor draws air into the burner assembly and, when the fan motor reaches its maximum speed, a vacuum switch is closed which ensures that gas will not enter the furnace unless the toxic fumes can be exhausted properly. Once the vacuum switch closes, 24 volts will travel through the switch, and one or more limit switches, allowing the circuit board to continue the combustion sequence.
Depending on the furnace model, the next step in the sequence has the circuit board sending 120 volts of alternating current to a glow-bar style igniter or a greater amount of voltage to a spark igniter. The glow-bar style igniter will begin to heat up and will literally begin to glow orange, whereas a spark igniter will begin to pulse to create a spark. At this point, the circuit board will send 24 volts to the gas valve, opening it and allowing gas to flow over the igniter where the gas is ignited into a flame.
Once the combustion sequence is successfully initiated, the circuit board will send 120 volts of alternating current to the circulation blower fan motor. The blower fan will draw air through the return vent, blow it past the heat exchanger, and force the now-heated air through the home’s venting to keep you and your family comfortable.
What causes a furnace circuit board to fail?
As with all electronic components, a circuit board can simply burn out after years of use. This is especially true of the transistors on the board. The heat generated by the furnace can also cause the solder joints on the board to expand which can create breaks in the connection. If a condensation tube cracks, the water could drip into the furnace cabinet and onto the circuit board causing it to short. Since in most homes a furnace is only in use during the colder fall or winter months, the relay switches on the board may become stuck in the open or closed position due to the furnace’s inactivity during warmer weather.
If the furnace’s circulation blower motor appears to be running all the time, it’s quite likely that the circuit board relay switch which controls the blower motor is stuck in the closed position. If this happens, you will definitely need to replace the circuit board with a new one. However, other furnace issues, such as the failure of the furnace to heat or the furnace starting and then stopping unexpectedly, are more likely to be caused by a faulty igniter, flame sensor, or vacuum switch. It is recommended that you confirm these components, along with the gas valve, flame roll-out switches, and draft inducer and circulation blower fan motors, are all working properly before replacing the circuit board.
Where is the furnace circuit board and how can I replace it?
If you’ve determined the circuit board is the cause of your furnace problem, it’s a relatively easy part to replace. The circuit board is usually located in the furnace’s blower chamber near the bottom of the unit. This is where the circulation blower fan motor is mounted. While the procedure to access the board and replace it with a new one will vary depending on the model, here are some general steps you can follow:
- Before you remove any access panels, be sure to shut off the power supply to the furnace to prevent shock.
- Some furnaces will allow you to just remove the lower panel to access the blower chamber. You may be able to simply rotate retaining knobs to release the panel, but you may need to fully unthread mounting screws securing the panel. Other furnace models will require you to completely remove an upper panel first before you can uninstall the lower panel.
- The circuit board may be attached to mounting pins on a mounting plate or secured with one or more screws. You can often use needle-nose pliers to help release the board from the mounting pins. Be aware that you may need to detach the mounting plate in order to fully release the circuit board.
- We recommend transferring the wires from the old board directly to the new one to make sure you’re connecting them to the appropriate terminals. Keep in mind, you may need to remove some terminal covers from the new board before you can connect certain wire connecters.
- Be aware that thermostat wires may be secured to the old circuit board with screws. Unthread the screws to release the wires and then use the screws to secure the thermostat wires to the new board.
- With all of the wires connected, snap the board onto the mounting pins or secure it with the screw or screws. Reposition and secure the mounting plate if necessary.
- Replace the access panels as necessary and secure them with the screws or retaining knobs.
- Restore power to the furnace.
- Call for a higher temperature on your home’s thermostat and observe if the furnace now appears to be functioning normally.
Find the right furnace parts with Repair Clinic
Whether you need to replace the circuit board, the igniter, flame sensor, vacuum switch, or a fan motor in your furnace, your first step is to identify the specific furnace parts that match your furnace model. Repair Clinic makes this step easy. All you have to do is type the full model number of your furnace into the Repair Clinic website search bar to see a complete list of compatible parts. You can then narrow the results list by using the part category and part title filters. For example, choose “Circuit Board & Timer” using the part category filter followed by “Control Board” from the part title filter to find the exact circuit board that works with your furnace. Repair Clinic stocks furnace replacement parts for all the leading heating and cooling brands, including Bryant, Lennox, Goodman, Rheem, York, Coleman, Ruud, Heil, and Payne, but you always want to make sure you’re purchasing the right component for your particular unit.