Furnace Not Heating? Complete Troubleshooting Guide for HVAC Pros

Safety Considerations

Gas furnace troubleshooting requires extra caution due to combustion hazards. Always follow these safety protocols:

• Turn off gas supply before working on gas valve or burners
• Use combustible gas detector to check for leaks
• Never bypass safety controls for testing
• Ensure proper ventilation before testing combustion
• Have CO detector present during operation tests

Furnace Sequence of Operation

Understanding the normal sequence helps pinpoint failures:

1. Thermostat calls for heat: Sends 24V signal to furnace
2. Inducer motor starts: Creates draft for combustion gases
3. Pressure switch closes: Confirms proper draft
4. Ignitor heats up: Hot surface ignitor glows (60-90 seconds)
5. Gas valve opens: Gas flows to burners
6. Flame established: Flame sensor confirms combustion
7. Blower motor starts: After heat exchanger warms up (30-60 seconds)
8. Heating cycle continues: Until thermostat is satisfied

Common No-Heat Problems

1. Dirty Flame Sensor

The #1 cause of "furnace lights then shuts off" complaints. The flame sensor is a simple rod that detects flame by measuring microamps of current. When dirty:

• Burners light normally
• Flame goes out after 3-10 seconds
• Furnace tries again (usually 3 attempts)
• Then locks out on safety

Solution: Clean with fine steel wool or emery cloth. Replace if pitted or cracked.

2. Failed Ignitor

Hot surface ignitors (HSI) have limited lifespan (3-7 years typically). Signs of failure:

• Inducer runs but no ignition attempt
• Ignitor doesn't glow at all
• Ignitor glows weakly (orange instead of bright white)
• Visible cracks in ceramic

3. Pressure Switch Issues

The pressure switch ensures proper draft before allowing ignition. Problems include:

• Blocked condensate drain (high-efficiency furnaces)
• Blocked vent pipe
• Failed inducer motor
• Cracked pressure switch hose
• Bad pressure switch

4. Limit Switch Tripped

The high limit switch shuts off gas if the heat exchanger overheats. Common causes:

• Dirty air filter restricting airflow
• Blocked return air
• Failed blower motor or capacitor
• Closed supply registers

Ignition System Diagnosis

Testing Hot Surface Ignitors

1. Remove ignitor from furnace
2. Check resistance: should be 40-200 ohms typically
3. Infinite resistance = open (failed)
4. Very low resistance = shorted (failed)
5. Visual check for cracks - even hairline cracks cause failure

Testing Flame Sensors

1. Connect microamp meter in series with flame sensor wire
2. Start furnace and observe reading
3. Good sensor: 2-6 microamps DC
4. Weak/dirty: less than 2 microamps
5. No reading: bad sensor or ground issue

Gas Valve Testing

Before condemning a gas valve, verify:

• 24V present at valve when calling for heat
• Gas supply is on and pressure is adequate
• All safety interlocks are closed
• Proper ground for flame sensing