High-limit switch
A furnace high-limit switch is a temperature safety control that opens the burner circuit when the air inside the furnace or supply plenum exceeds its allowed limit. The blower can continue running to remove stored heat.
Also called furnace limit switch, main limit, fan limit switch
What happens when a furnace high-limit switch opens?
A furnace high-limit switch opens when the temperature at its sensing point exceeds the allowed limit, shutting the burners off while the blower may keep cooling the furnace.
High-limit switch: how it works
The switch keeps a normally closed safety circuit while temperature remains below its limit. If heat rises past that threshold, the contacts open and the furnace control removes the burner command. Airflow from the blower can continue so the heat exchanger and cabinet cool before the switch remakes, if the model uses an automatic-reset limit.
Where you meet the term
- In a heating complaint where the burners stop before the thermostat is satisfied but the supply blower continues moving cooler air.
- In a furnace fault table that reports an open main limit or an open limit circuit rather than naming an airflow cause.
- In an airflow diagnosis where filter condition, return blockage, blower operation, temperature rise, and duct resistance are evaluated together.
How to check it
- Check the filter against its airflow arrow. With the equipment off, inspect an accessible filter for loading, correct size, and an arrow pointing toward the furnace or air handler. Owner check
- Open blocked supply and return paths. Confirm furniture and stored items are clear of return grilles and that ordinary supply registers have not all been closed at once. Owner check
- Measure temperature rise and static pressure. A technician compares measured rise and duct pressure with the furnace data plate and installation table before condemning the switch. Technician
High-limit switch: what it is not
- An open high-limit code does not prove the switch failed. It can be the correct response to excess temperature caused by restricted airflow, blower trouble, duct design, or firing conditions.
- A high-limit switch is not a flame-rollout switch. Both can open a safety circuit, but rollout concerns burner-area heat and carries a different combustion risk.
High-limit switch questions
Why does the blower keep running after a limit trip?
The burners stop producing heat, while blower airflow removes heat already stored in the heat exchanger and cabinet. That response is consistent with the safety doing its job.
Will changing the filter clear an open-limit fault?
It can remove one visible airflow restriction, but it does not prove why the switch opened. If the fault returns with the correct clean filter and open grilles, service is needed.
Can I reset a furnace high-limit switch?
Do not reset or bypass a safety to keep the furnace operating. Some main limits reset automatically after cooling; any returning trip needs the temperature and airflow cause measured.
Where this definition comes from
Written from manufacturer support article, service manual material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- What is a furnace limit switchCarrier homeowner resource on the furnace limit switch: a bimetallic strip that opens and closes contacts controlling the blower and the gas valve, an approximate blower-on point near 130 degrees Fahrenheit, an approximate upper safety limit near 200 degrees, a fan-off point near 100 degrees, the symptoms of a constantly running blower, a furnace that will not ignite and rapid short cycling, the causes it names as dirty filters, blocked or closed vents, blower motor failure and a dirty evaporator coil, and the instruction never to bypass the switch.
- Affinity YP9C furnace installation manualYork Affinity YP9C modulating furnaces: the seven-red-flash ignition lockout sequence, and the two-red-flash entry reading pressure switch closed with inducer pressure below the switch setpoint, described as the switch being closed when it should be open.