Furnace Not Putting Out Hot Air

Quick answer

Turn off the furnace power, check the air filter, and ensure the thermostat is set correctly. If the blower runs but no heat, check the gas valve or pilot light.

Confirm the symptom

• Furnace fan is running but no hot air
• Thermostat is set to heat but no warm air
• Furnace makes noise but no heat output
• Air vents blowing cold or room-temperature air

Tools and prep

• Screwdriver set
• Thermometer (infrared preferred)
• New furnace filter (if needed)
• Flashlight
• Safety gloves
• Turn off furnace power at the circuit breaker or gas valve

Step-by-step troubleshooting

1. Check Thermostat Settings
   Do: Verify the thermostat is set to 'Heat' and the desired temperature is higher than room temperature.
   Observe: Thermostat is set correctly but furnace still not heating.
   Means: Thermostat settings are not the issue.
   Next: CheckAirFilter
2. Check Air Filter
   Do: Remove and inspect the air filter for dirt or blockage.
   Observe: Filter is dirty or clogged.
   Means: Restricted airflow can prevent proper heating.
   Next: ReplaceFilter
3. Replace Filter
   Do: Install a clean or new air filter.
   Observe: Airflow improves but still no heat.
   Means: Filter was a contributing factor but not the main issue.
   Next: CheckGasSupply
4. Check Gas Supply
   Do: Ensure the gas valve is fully open and there's no gas leak smell.
   Observe: Gas valve is closed or there's a gas leak smell.
   Means: Gas supply is interrupted or dangerous.
   Next: CallProfessional
5. Inspect Pilot Light
   Do: Look at the pilot light (if your furnace has one) to see if it's lit.
   Observe: Pilot light is out.
   Means: Pilot light failure prevents ignition.
   Next: RelightPilot
6. Relight Pilot Light
   Do: Follow manufacturer instructions to safely relight the pilot light.
   Observe: Pilot light stays lit and furnace produces heat.
   Means: Pilot light was the issue.
   Next: Done
7. Check Blower Motor
   Do: Listen for unusual noises or check if the blower motor is running.
   Observe: Blower motor is not running or making loud noises.
   Means: Blower motor failure or electrical issue.
   Next: CallProfessional
8. Inspect Limit Switch
   Do: Check if the furnace limit switch is tripped (often near the heat exchanger).
   Observe: Limit switch is in the 'off' position.
   Means: Overheating or blockage caused the switch to trip.
   Next: ResetSwitch
9. Reset Limit Switch
   Do: Push the limit switch back to the 'on' position.
   Observe: Switch resets but trips again quickly.
   Means: Persistent overheating or airflow issue.
   Next: CallProfessional
10. Check for Error Codes
    Do: Look for blinking lights or error codes on the furnace control panel.
    Observe: Error code is displayed.
    Means: Specific issue identified by the furnace's diagnostics.
    Next: ConsultManual

Common causes

Parts matching

Common parts needed for a furnace not putting out hot air:

• Air filter (replace every 1–3 months)
• Thermostat (check or replace if unresponsive)
• Gas valve (ensure it's fully open)
• Igniter or flame sensor (if pilot light or ignition issues)
• Blower motor or belt (if no airflow despite motor running)
• Limit switch (if it keeps tripping)

Always turn off the furnace power before replacing parts like filters or switches.

When to call a professional

• If you smell gas or suspect a gas leak
• If the pilot light won't stay lit after relighting
• If the furnace makes loud noises or doesn't start at all
• If error codes persist after basic troubleshooting
• If the blower motor doesn't run despite power being on
• If the limit switch keeps tripping after resetting

FAQ

Related guides

• furnace not heating
• furnace blowing cold air
• furnace not working
• furnace not blowing air
• furnace not igniting
• furnace error codes

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