What a furnace limit switch is, and what happens when it trips

What a furnace limit switch does

Carrier, a furnace maker, describes the limit switch as a safety and operating part that monitors the internal temperature of the furnace. It has two jobs. It turns the blower fan on and off at the right times, and it shuts down the burners if the system overheats.

Carrier says the switch waits until the plenum, which it calls the heated air chamber, reaches a pre-set temperature before it starts the blower. That way only warm air goes into your ductwork.

The switch senses heat with a temperature-sensitive probe, often a bimetallic strip. Carrier says the element expands or contracts, which opens or closes electrical contacts inside the switch.

A Resideo installation sheet describes a limit control that provides high limit cutout with manual reset. It says that in warm air systems, such a control may be used as a secondary high limit to shut down the burners if the fan fails, among other uses.

What it means when the limit switch trips

Carrier says that if the temperature exceeds the upper safety limit, the switch opens the circuit to the gas valve. Opening that circuit immediately halts the heating process.

Carrier lists three signs that the switch itself may be failing. The blower fan may run constantly, because a switch stuck closed acts as if the furnace is always hot. The furnace may refuse to ignite, because a switch stuck open tells the system it is overheating.

The third sign is short cycling, which means the furnace turns on and off in rapid succession. Carrier says that if the furnace heats up briefly and shuts down before warming your home, the switch may be overly sensitive or failing. If ignition problems are your symptom, see why a furnace won't ignite.

Why a limit switch trips: usually airflow

Carrier says a limit switch typically trips due to severely restricted airflow across the heat exchanger. With too little air moving, the inside of the furnace gets too hot.

Carrier names dirty air filters, blocked return vents and a malfunctioning blower motor as the most common culprits. It says a severely blocked filter starves the furnace of return air.

Carrier also says that closing too many supply registers disrupts the airflow. So can blocking return grilles with furniture. In homes with central air conditioning, the evaporator coil sits above the furnace, and a dust-caked coil blocks airflow too.

A Carrier homeowner manual says the same about filters. It warns that a dirty filter can cause the furnace to overheat and automatically shut down. For help with filters, see which way a furnace filter goes and our guide to HVAC air filters.

Why the switch must never be bypassed

Carrier says you should never bypass a furnace limit switch. Bypassing it removes the system's ability to shut down during an overheating event. Carrier says that can lead to severe fire hazards, a cracked heat exchanger, or carbon monoxide leaks.

A Trane owner's manual calls carbon monoxide, written CO, a colorless, odorless gas that is produced during incomplete combustion of any fuel. It says CO can cause death without warning. If you suspect CO poisoning, that manual says to get fresh air immediately and leave the home.

Carrier also ties the switch to the heat exchanger. It says the switch prevents the heat exchanger from cracking, and that a cracked one can leak deadly carbon monoxide into the home. Read more in our guide to furnace heat exchanger cracks.

Limit switch, rollout switch and reset buttons

The Trane owner's manual also describes a flame roll-out switch. Trane says that if the furnace runs without enough air for combustion, that switch opens and turns off the gas supply to the burners.

Trane says this indicates a serious problem with the burners that must be corrected immediately. It adds that only a licensed HVAC technician can reset this switch.

Some limit controls have a manual reset, like the Resideo control described above. Resideo's sheet says its installer must be a trained, experienced service technician. Carrier says a switch may need replacing if it repeatedly trips even after all airflow and maintenance issues are resolved. Finding why a switch trips, and any reset, is work for a licensed HVAC contractor.

What you can check, and what Florida homeowners should know

Your safe checks are about airflow and settings. ENERGY STAR, a federal efficiency program, says to inspect, clean or change air filters once a month. The Trane manual says to arrange furniture and drapes so supply registers and return grilles are not obstructed. Its troubleshooting list also asks whether the thermostat setting is correct.

Everything inside the furnace belongs to a technician. Carrier says that before testing the switch, a technician shuts off power at the breaker. ENERGY STAR says a yearly check-up should cover the system's controls to ensure proper and safe operation. See what a heating tune-up covers.

The Energy Information Administration (EIA), a federal statistics agency, says Florida residents use space heating equipment much less than people in other states. It says most Florida households still use some heating equipment in winter, and most of those use electric equipment. The Carrier and Trane sources in this guide describe gas furnaces. If your home heats with electric equipment, these sources do not describe its safety switches. Ask a licensed HVAC contractor which ones your system has.