Limit switch check during startup

What is going on

The high limit has to open and shut the burners off at the rated outlet temperature. This is an installer's check at startup, and I always do it, because a limit that does not trip is the difference between a nuisance call and a fire. What I usually find on a bad one is that it simply never opens, and the furnace keeps firing while the plenum gets hot. Stay with the furnace the whole test and be ready to shut it down by hand.

Likely causes

• Faulty limit switch if burners keep running and overheating is evident
• Limit sensor not seeing heat because of mounting or position
• Blower or airflow problem causing the limit to trip early instead
• Wiring fault in the limit circuit

Check in this order

1. Set the thermostat above the setpoint to bring on the gas valve.
2. Restrict airflow by blocking the return air. Disconnecting the indoor blower can trip the inducer limit.
3. Watch for the burners to shut off when outlet temperature reaches the rating plate maximum.
4. If the burners do not shut off and overheating occurs, shut the furnace off manually at once and replace the limit switch.
5. Check continuity across the limit with a meter once it cools to confirm it recloses.
6. Remove the paper or cardboard from the return air inlet after the test.
7. Never leave a limit bypassed or a faulty limit in service.

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)