Limit switch check at startup

What is going on

Verify the high limit shuts off burners when the furnace overheats. Installer check at startup. Watch it the whole time. Stay at the furnace and shut it off by hand if it does not trip.

Likely causes

• Limit switch failed to open
• Limit not seeing outlet air temperature properly
• Wiring problem in the limit circuit
• Inducer limit tripping from blower disconnection instead

Check in this order

1. Set thermostat above room temperature to call for heat.
2. Restrict airflow by blocking the return air. Disconnecting the indoor blower may trip the inducer limit.
3. Wait for outlet air to reach the maximum temperature on the rating plate.
4. Confirm the burners shut off.
5. If the burners do not shut off in a reasonable time, shut the furnace off manually and replace the limit switch.
6. Test the limit with a meter for continuity once it cools to verify it recloses.
7. Remove the blockage from the return air inlet after the test.

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)