Check primary limit control (90% furnaces)

What is going on

The primary limit on these 90% furnaces is the enclosed disk with the colored tab, mounted to see heat exchanger temperature. Most times I get called for a limit, the limit itself is fine and it is doing its job because airflow is bad. Test it with the low voltage wires off. It should read continuous unless the heat exchanger is hotter than the limit setting. A control that is open when stone cold is dead, and a control that opens hot is telling you to go look at the filter and the blower first.

Likely causes

• Heat exchanger temperature above the limit setting (normal open condition when hot)
• Failed limit control reading open when cold
• Dirty filter, closed registers or a weak blower pushing the heat exchanger past the setting
• Loose or corroded connections on the limit terminals that mimic an open

Check in this order

1. Shut off power and remove the burner compartment door
2. Remove the low voltage wires from the limit control terminals so you are testing the switch alone
3. Test across the two terminals with an ohmmeter. Let the furnace cool fully first
4. If the reading is not continuous with the unit cold, replace the control
5. Use the Primary Limit Charts in the technical manual to confirm part number, setting and color code before ordering
6. Before blaming the limit on a repeat trip, check filter, ductwork and blower speed
7. Reinstall the burner door, turn power on and verify operation through a full cycle

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)