When a gas furnace ignites but then shuts off immediately, only to attempt ignition multiple times, it can be due to several technical issues. This blog post will explore the common problems and their explanations, providing you with the necessary information to diagnose and resolve the issue.
What Causes a Gas Furnace to Keep Trying to Ignite?
1. Is the Flame Sensor Causing the Issue?
The flame sensor is responsible for monitoring the flame and ensuring that the gas valve remains open only when a flame is present. If the sensor is dirty or corroded, it may not detect the flame correctly, causing the gas valve to shut off and the furnace to cycle off.
Additionally, the flame sensor must be positioned correctly within the flame to function properly. If it is not in the middle of the flames, it may not detect the flame, leading to ignition failure.
2. Are There Any Issues with the Ignition System?
Problems with the ignition system, such as a faulty igniter or issues with the ignition sequence, can cause the furnace to fail to sustain ignition. The gas valve might also not be opening properly or could be faulty, preventing the furnace from maintaining a steady flame.
3. Is There an Airflow or Venting Problem?
Blockages in the exhaust vent, such as debris, nests, or snow, can prevent proper airflow and cause the furnace to shut down. This is often indicated by the pressure switch not operating correctly.
If the draft inducer motor is not functioning correctly, it may not provide sufficient airflow for ignition to continue, causing the pressure switch to trip and shut down the furnace. Improper airflow due to dirty or clogged air filters, blocked heat grates, or closed HVAC dampers can also lead to overheating and short cycling.
4. Is the Control Board or Electrical System Causing the Problem?
The control board is the central component that manages all furnace operations. A faulty control board or issues with wiring can prevent the furnace from functioning properly. Loose or frayed wires, bad solder joints, or other electrical issues can also cause intermittent problems, including repeated ignition failures.
5. Are There Any Issues with the Thermocouple or Rollout Switch?
Although less common in modern furnaces, a malfunctioning thermocouple can be a problem if it is not functioning correctly. Additionally, the flame rollout switch, designed to shut down the furnace if the flame rolls back into the burner area, can cause the furnace to shut down immediately after ignition if it is triggered.
Troubleshooting Steps
- Check the Flame Sensor: Ensure the flame sensor is clean and properly positioned. Clean the sensor if necessary and verify that the wire connections are secure.
- Inspect the Venting System: Check the exhaust vent for blockages and ensure that the draft inducer motor is working correctly.
- Verify Airflow: Check the air filter, heat grates, and HVAC dampers to ensure proper airflow. Clean or replace the air filter if necessary.
- Check Electrical Connections: Inspect the wiring and connections to the control board and other components for any signs of damage or wear.
- Consult a Professional: If the issue persists after checking these common problems, it may be necessary to consult an HVAC professional to diagnose and repair the furnace.
Reference Links
- Reddit Discussion on Furnace Ignition Issues
- Hart Home Comfort Article on Furnace Short Cycling
- HVAC-Talk Forum Discussion
These resources provide comprehensive insights into diagnosing and resolving the issue of a gas furnace that ignites but does not stay on.
