If your forced air gas furnace won’t turn on after a power outage, there could be several potential causes, including issues with the thermostat, power supply, gas supply, or ignition system. In this blog post, we’ll explore the common reasons why a forced air gas furnace may not start and provide step-by-step troubleshooting tips to help you get your heating system back up and running.
What Causes a Forced Air Gas Furnace to Not Turn On?
1. Thermostat Issues
- Incorrect Settings: Ensure the thermostat is set to “HEAT” and the temperature is set above the current room temperature. Also, check that the fan switch is set to “AUTO” rather than “FAN” to ensure the furnace operates correctly.
- Battery Replacement: If the thermostat is battery-powered, check if the batteries need to be replaced. Using AA lithium batteries can extend the life and consistency of the system.
2. Power Supply Issues
- Tripped Circuit Breaker: Check the electrical panel to see if the circuit breaker controlling the furnace has tripped. If so, reset the breaker to restore power.
- Power Disconnect Switch: Verify that the power disconnect switch by the furnace is in the “ON” position. Even gas furnaces use electricity, so this switch must be on for the furnace to operate.
3. Gas Supply Issues
- Gas Valve: Ensure the gas valve is turned on. The handle should be parallel to the gas line if it is on. If it is perpendicular, the gas is off.
- Gas Line Disruption: Check if there is any disruption in the gas supply to the furnace. If other gas appliances are working, the issue may be specific to the furnace’s gas supply.
4. Ignition System Issues
- Pilot Light: For older furnaces with a pilot light, ensure it is lit. For newer furnaces with electronic ignition, check the ignition sensor for dirt or grime, which can prevent the furnace from starting.
- Flame Sensor: A dirty flame sensor can prevent the furnace from igniting. Cleaning the sensor with medium grain sandpaper may resolve the issue.
Troubleshooting Steps
Checking Power Supply
- Verify Circuit Breaker: Check the electrical panel for a tripped circuit breaker and reset it if necessary.
- Check Power Disconnect Switch: Ensure the power disconnect switch by the furnace is in the “ON” position.
Inspecting Gas Valve
- Check Gas Valve Position: Ensure the gas valve handle is parallel to the gas line, indicating it is on.
- Verify Gas Supply: Check if other gas appliances are working to ensure the gas supply is not disrupted.
Examining Ignition System
- Check Pilot Light: For older furnaces, ensure the pilot light is lit. For newer furnaces, check the ignition sensor for dirt or grime.
- Clean Flame Sensor: If the flame sensor is dirty, clean it with medium grain sandpaper and replace it as instructed in the user manual.
Other Checks
- Air Filter: Ensure the air filter is clean and not clogged, as a dirty filter can prevent the furnace from starting.
- Thermostat Settings: Check the thermostat settings to ensure it is set to “HEAT” and the temperature is above the current room temperature.
- Blower Motor: Check if the blower motor is working correctly. A faulty or dirty blower motor can prevent the furnace from operating.
What If the Furnace Won’t Turn Off?
If the furnace won’t turn off, it could be related to issues that prevent it from turning on, such as:
– Faulty Thermostat: A malfunctioning thermostat might not send the correct signals to turn the furnace off.
– Stuck or Faulty Limit Switch: The limit switch is designed to turn off the furnace if it overheats. If this switch is faulty, the furnace may not turn off.
– Control Board Issues: Problems with the control board can affect the furnace’s ability to turn on or off properly.
To resolve this issue:
1. Check Thermostat Settings: Ensure the thermostat is set correctly and not stuck in a continuous heating mode.
2. Inspect Limit Switch: Check the limit switch for any issues and replace it if necessary.
3. Control Board Diagnosis: If the control board is suspected to be faulty, it may need to be replaced by a professional.
Technical Specifications and Components
Blower Motor
- The blower motor pulls air through the furnace and into the duct system. A faulty or dirty blower motor can prevent the furnace from operating.
Limit Switch
- The limit switch turns off the furnace if it overheats. A faulty limit switch can cause the furnace to malfunction.
Control Board
- The control board manages the furnace’s operations, including ignition and blower motor control. Issues with the control board can affect the furnace’s ability to turn on or off.
Ignition System
- The ignition system includes the pilot light or electronic ignition. A dirty or faulty ignition sensor can prevent the furnace from starting.
Cost Estimate for Repairs or Replacements
The cost of repairs can vary widely depending on the specific issue:
– Thermostat Replacement: $100-$300
– Circuit Breaker Replacement: $50-$200
– Blower Motor Replacement: $200-$500
– Control Board Replacement: $200-$500
– Ignition Sensor Replacement: $50-$200
– Labor Costs: $75-$200 per hour, depending on the technician and location.
Timeline for Troubleshooting and Potential Repairs
Initial Troubleshooting
- Check Power and Gas Supply: 10-15 minutes
- Inspect Thermostat Settings: 5-10 minutes
- Check Air Filter: 5 minutes
- Inspect Ignition System: 15-30 minutes
Advanced Troubleshooting
- Check Blower Motor: 30-60 minutes
- Inspect Control Board: 30-60 minutes
- Check Limit Switch: 30-60 minutes
Repair Timeline
- Simple Repairs (e.g., Thermostat Replacement): 1-2 hours
- Complex Repairs (e.g., Control Board Replacement): 2-4 hours
Recommended Maintenance Schedule
- Monthly: Check and replace air filters.
- Annually: Inspect and clean the blower motor, limit switch, and ignition system.
- Bi-Annually: Check circuit breakers and ensure the power disconnect switch is in the “ON” position.
Reference Links
- Assured Comfort: 5 Reasons Why Your Furnace Is Not Working
- Hoffmann Brothers: Furnace Won’t Turn On? Try These Troubleshooting Tips
- SuperTech HVAC: Why My Furnace Won’t Turn On? How to Fix?
