Check primary limit control

What is going on

The primary limit is a non-adjustable, automatic reset bi-metal control on the heat exchanger. It should read open only when the heat exchanger is hotter than its setting. Test it cold with the low voltage wires off.

Likely causes

• Limit reads open while cold, meaning it has failed
• Heat exchanger really above the limit setting because of poor airflow
• Wrong limit fitted, with a different temperature setting
• Loose or corroded connection at the limit terminals

Check in this order

1. Remove the burner compartment door with power off
2. Disconnect the low voltage wires at the limit control terminals
3. Use an ohmmeter across the two terminals. It should read continuous unless the heat exchanger is above the limit setting
4. If the reading does not match, replace the control
5. Use the Primary Limit Charts in the furnace technical manual to confirm part number, temperature setting and color code
6. If the limit trips in normal use, check the filter, registers and blower before replacing it
7. Reinstall the burner compartment door, restore power, and verify operation

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)