Primary (high) limit open, circulator blower runs continuously

What is going on

The primary limit circuit is open, so the furnace will not fire, and the circulator blower keeps running to cool the heat exchanger. Before you touch anything, remember the furnace is hot and the blower is live. A limit that opens is a safety doing exactly what it should, so never defeat it. The aim is to find out why the heat exchanger got too hot or why the circuit reads open. Common reasons are airflow, blower speed or a poor connection in the limit wiring.

Likely causes

• Not enough conditioned air over the heat exchanger
• Blocked filters, restrictive ductwork, improper circulator blower speed, or failed circulator blower motor
• Loose or improperly connected wiring in the high limit circuit
• Closed or blocked supply registers and returns

Check in this order

1. Turn power off before any repair or wiring check
2. Check filters and ductwork for blockage. Clean filters or remove obstructions
3. Check circulator blower speed and performance. Correct speed or replace the motor if needed
4. Tighten or correct the wiring connection at the limit
5. Check the specification sheet for the allowable temperature rise and proper circulator speed
6. Measure temperature rise after repairs to confirm it is within range
7. If the limit still opens with good airflow, have a licensed technician test it and check the heat exchanger

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)