Everything seems to work fine until the blower comes on. The heater comes on normally, the gas lights, and the furnace warms up just like you want it.
However, as soon as the blower starts, the gas cuts off, and you’re left battling the biting cold. What could be the problem?
The worst part about this issue is that the heater will happily relight with the blower off. However, the constant restarts can be annoying. Combine that with the whining noises during restart, and the level of frustration goes through the roof.
Several things can cause the furnace to go off when the blower comes on. It could be the rollout switch, control board, or pressure circuit. It could also be an airflow problem. Read on to find out how to diagnose the problem and learn potential fixes.
How the Blower System Works
Perhaps it’s best to begin by understanding how the blower system works in a furnace. The blower system is just that – a blower system. It blows hot air into the duct system and creates the motion that helps circulate warm air throughout your home.
The blower system consists of two main fans – the blower motor and the blower fan. These two work together harmoniously to bring warm air from the furnace to your home to keep you and your family warm.
The way the whole process works is straightforward. After warm air is created in the furnace from the combustion of propane or natural gas (or whichever fuel you use), the blower motor turns the fan, sending the warm air through the ducts and into the various rooms within your home. Blower motors can be single-speed, dual-stage, or variable-speed.
However, the blower system and how it engages the rest of the furnace can be compromised. For example, the blower motor can become bent or broken. The lubrication may also dry up. And sometimes, it could be the blower assembly.
Wiring issues are also possible. For instance, the wiring between the blower and the rest of the furnace can become loose. Or, it could be the wiring between the fan and the thermostat or circuit board. All these can cause blower system malfunction and ultimately affect the heating process.
4 Reasons Your Furnace May Go Off When the Blower Comes On
However, before you worry about the blower, the two main reasons furnaces go off when the blower comes on are as follows;
The Flame Rollout Switch
The flame rollout switch is a unique device that monitors the level of the heat surrounding the burners. It automatically turns off the furnace if it’s tripped due to excessive heat.
Typically, the rollout switch will be tripped when the furnace exceeds 350˚F. This helps to prevent potential fire and also protects furnace damage.
However, the rollout switch doesn’t only trip when the furnace is boiling. It may also trip if it detects conditions that may cause overheating.
For instance, the flame rollout switch will trip if it detects clogged furnace vents, a recent fluctuation in gas pressure, a damaged heat exchanger, or a clogged heat exchanger.
If the rollout switch detects that the burners aren’t being drawn into the heat exchanger and the exhaust ends up expelling the flue, it will open and stop the furnace from heating at all. Even if the heater starts, the flame will go out within several seconds.
Never bypass the flame rollout switch. You need it for your safety and the safety of your family. It also protects your furnace. Instead, when it trips, treat it as a sign of a small problem. For example, maybe the furnace vents are clogged, or there’s been a gas fluctuation recently.
You can reset the breaker even on your own. But make sure to fix the underlying issue first. Switch off the heater and turn off the gas supply. Then fix the issue before resetting the switch.
The Flame Sensor
If it’s not the flame rollout switch, it’s very likely to be the flame sensor. The flame sensor is another safety feature integrated into gas heating appliances to prevent fires and furnace damage. It undergoes a phase where either a hot surface igniter or a spark ignites the gas during the ignition cycle.
As the gas ignites, the sensor generates a small electric current (a few microamperes) to inform the circuit board that the gas is being burned and not escaping into the vent system. It does so by sensing heat from the flame inside the burner assembly.
If the furnace is ON, but the flame sensor cannot detect a flame, the sensor (a small metal rod with a porcelain base that conducts an electric current) cuts off the gas supply, automatically shutting off the furnace.
Most furnaces can be reset after a flame sensor has “locked out” the unit. Begin by shutting off the furnace and turning off the gas supply. Then wait a few seconds before turning the furnace back on.
Additionally, the flame sensor may malfunction if it’s damaged or dirty. To clean it, turn off the furnace entirely and shut off the gas supply. Then open the combustion chamber to access the sensor. Remove the sensor and clean it using steel wool, then put it back.
Aside from the flame rollout switch and flame sensor issues, the furnace can also go off immediately after the blower comes on for the following reasons.
The furnace needs an ample supply of fresh air to run effectively. Therefore, if it detects a low oxygen supply, it will go off. This may happen even during startup.
If, after lighting the unit, the sensors detect a low oxygen supply, they will instruct the unit to shut down to prevent inefficient combustion and carbon monoxide poisoning.
Low oxygen levels often result from poor airflow in the house. It may also be a sign of a damaged or blocked blower system.
Solution: Make sure there’s sufficient airflow in your home. You can improve airflow by opening the windows and doors for a few seconds. Also, check the heating system to ensure the air inlet vents are not blocked and the blower system is working correctly.
Bad Draft Inducer Motor
The draft inducer motor is a unique blower system located inside the furnace near the heat exchanger. It’s one of the first things that come on when the furnace begins a heating cycle. Typically, it comes on 30-60 seconds before the burners are lit.
The primary purpose of the inducer motors is to clear out any combustion gases. It does so by pushing all combustion gases into the flue pipes and out of the home. The draft inducer motor also provides the furnace with a constant source of oxygen.
Owing to the importance of the inducer motor, it’s constantly monitored by a relay known as a pressure switch. If the draft inducer motor is defective or not working correctly, the pressure switch won’t close. If the pressure switch doesn’t close, the ignition process will stop until the problem is resolved.
Solution: The only solution here is to make sure the draft inducer motor is functioning optimally. It’s best to involve a professional here.
What Else Could It Be?
If even after diagnosing and fixing the four issues above, the flame continues to go out when the blower comes on, it may be time to call an HVAC professional.
Perhaps you have a malfunctioned control board, or the pressure switch has stopped working. Shut off the heating system, turn off the gas supply, and call your preferred HVAC technician.