Explanation of the Phenomenon: Why Does a Gas Furnace Light Then Go Out?
The phenomenon of a gas furnace lighting and then shutting off repeatedly is often related to the furnace’s safety mechanisms and the presence of a flame. When the furnace is called to heat, the igniter (usually a hot surface igniter or pilot light) heats up, and the gas valve opens, allowing gas to flow to the burners. The burners ignite, and the flame sensor detects the presence of the flame. However, if the flame sensor fails to detect a flame or if there is an issue with the gas flow, the control board will shut off the gas supply, causing the furnace to shut down. This is a safety feature designed to prevent gas leaks or other hazards.
What Causes a Gas Furnace to Light Then Go Out?
Several components and conditions can cause a gas furnace to ignite and then shut down:
1. Flame Sensor Issues
- Symptoms: The furnace lights for a few seconds and then shuts off. This can happen multiple times before the system locks out.
- Causes: A dirty or faulty flame sensor may not detect the flame properly, leading to shutdown.
2. Control Board Problems
- Symptoms: Similar to flame sensor issues, the furnace may light and then shut off repeatedly.
- Causes: The control board, which manages the furnace’s operation, can malfunction, leading to improper shutdown.
3. Gas Flow Issues
- Symptoms: The flame may appear weak or irregular, leading to repeated shutdowns.
- Causes: Clogged gas ports or insufficient gas supply can prevent the flame from being stable, triggering the flame sensor to shut off the furnace.
4. Thermocouple Issues (less common in modern furnaces)
- Symptoms: The pilot light may go out, and the furnace will not stay lit.
- Causes: A faulty thermocouple (used in older furnaces with pilot lights) can cause the furnace to shut off.
5. Other Issues
- Symptoms: The furnace may shut down shortly after ignition.
- Causes: Blocked return vents, clogged filters, or condensate drain blockages can also cause the furnace to shut down.
How to Troubleshoot a Gas Furnace That Lights Then Goes Out
To diagnose the issue, follow these steps:
- Check the Flame Sensor:
- Ensure the flame sensor is clean and free of debris. Use steel wool or a similar abrasive cloth to clean it.
-
Verify the sensor is positioned correctly in the flame path.
-
Inspect Gas Flow:
- Check for clogged gas ports or obstructions in the gas supply lines.
-
Ensure there are no leaks in the gas system.
-
If the flame sensor is clean and correctly positioned, suspect a control board issue. Check wiring and ensure all connections are secure.
-
Check for Other Obstructions:
- Ensure the return vents are not blocked and the air filter is clean.
- Verify the condensate drain is clear.
Costs and Timeframes for Repairing a Gas Furnace That Lights Then Goes Out
- Flame Sensor Replacement:
- Cost: Typically inexpensive, around $10-$50.
-
Timeframe: Can be replaced in about 30 minutes to an hour, depending on accessibility.
-
Control Board Replacement:
- Cost: More expensive, ranging from $100-$500 or more, depending on the model and brand.
-
Timeframe: Can take several hours, especially if the technician needs to diagnose the issue first.
-
Gas Valve or Port Cleaning/Replacement:
- Cost: Varies widely, but typically ranges from $50-$200 for cleaning and up to $500 for replacement.
- Timeframe: Cleaning can take about an hour, while replacement may take several hours.
