The furnace limit switch is a safety device that monitors the temperature inside the heat exchanger plenum and shuts off the burners if it gets too hot—typically above 150–200°F, depending on the furnace model. When it trips during a normal heating cycle (not an obvious emergency), restricted airflow is almost always the cause.
The limit switch has two jobs:
- Safety shutoff — opens the burner circuit when the heat exchanger overheats, preventing cracks, CO leaks, and fire
- Fan control (on older combo units) — delays the blower at startup until the heat exchanger is warm enough to push comfortable air, then keeps it running after burner shutoff to extract residual heat
Types of limit switches
High-limit switch — the main safety switch mounted on the supply air plenum above the heat exchanger. Auto-resets after the plenum cools below a reset temperature (usually ~30°F below the trip point).
Roll-out switch — mounts near the burner opening. Trips if flames roll out beyond the burner box, signaling a draft problem or blocked heat exchanger. Manual-reset only—you have to push a button to reset it. Do not reset a roll-out switch without finding and fixing the cause first.
Fan-limit combo control — older furnaces use a single bimetal disk that controls both the fan relay and high-limit function. Replaced today by separate electronic controls.
Why the limit switch trips during normal operation
An occasional trip on an exceptionally cold day may be normal if the furnace is slightly undersized. Repeated tripping is a problem. Causes in rough order of frequency:
- Dirty or blocked filter — by far the most common. A clogged filter starves the heat exchanger of airflow; heat builds up faster than the blower can remove it. Replace the filter and test.
- Blocked return or supply registers — furniture, drapes, or closed vents restrict airflow even with a clean filter.
- Failed blower motor — if the blower doesn’t start or runs slowly, heat accumulates. Listen for the fan; if it doesn’t come on within 90 seconds of burner ignition, the motor or capacitor may be failed.
- Limit switch itself failing — a weak or fatigued switch trips at lower-than-spec temperatures. Verify with a multimeter: the switch should read continuity (closed) at room temperature and open when the plenum is hot. If it opens at room temperature, replace it.
- Cracked heat exchanger — a crack allows flue gases to enter the supply airstream, changing combustion dynamics and sometimes causing flames to contact the limit switch. A confirmed crack means the furnace is off until repaired.
Safety: A limit switch that trips repeatedly is telling you something is wrong. Do not tape over it, bypass it, or hold it closed. Operate the furnace without the safety functioning and you risk heat exchanger failure, CO leak, or fire.
Diagnosing a limit switch trip
- Check the error code on the furnace control board (most modern furnaces flash a sequence). Limit trips usually display a specific code (e.g., 3 flashes on Carrier). Refer to the label inside the furnace door.
- Replace the air filter regardless—it’s a 30-second check that clears the most common cause.
- Confirm all supply and return registers are open and unobstructed.
- With furnace running, listen for the blower starting within ~90 seconds of the burner.
- If the blower doesn’t start: check the blower capacitor (common failure) and motor.
- If blower runs but limit still trips: measure supply plenum temperature with a probe thermometer through the draft diverter or a duct access. A reading over 160°F with a clean filter points to a failed limit switch or duct restriction.
How it connects to your thermostat
The thermostat calls for heat (W terminal energizes). The furnace control board checks the limit switch—if the switch is open (overtemp condition), it won’t allow the gas valve to open or will cut it mid-cycle. From the thermostat’s perspective, the call for heat is satisfied without actually heating—which can appear as the thermostat not responding, or the house not getting warm.
If a W-terminal call produces a heating cycle that runs for only 1–2 minutes and cuts out, check for a limit switch trip before assuming a thermostat problem.
Related glossary terms
- Short cycling — repeated limit trips cause short run cycles
- Furnace flame sensor — another burner safety that causes shutdowns
- Heat exchanger — what the limit switch is protecting
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FAQ
Q: Can I reset a limit switch that keeps tripping? You can reset it—it will auto-reset once the plenum cools. But if it trips again in the next heating cycle, the reset accomplished nothing except delaying diagnosis. Fix the airflow cause first.
Q: My furnace runs for 5 minutes, shuts off, then restarts 10 minutes later. Is that a limit switch? That’s the classic limit-trip cycle. The furnace runs, overheats, the limit opens, the system cools, the limit resets, then it tries again. Replace the filter first; if that doesn’t solve it, proceed through the diagnostic steps above.
Q: The roll-out switch tripped. Can I just reset it and go? Reset it to verify the furnace runs, but do not leave it running until you understand why the roll-out tripped. Roll-out means flames went where they shouldn’t—blocked flue, cracked heat exchanger, or failed draft inducer. Have a technician verify combustion safety before continued use.
Q: Where is the limit switch located on my furnace? On most upflow furnaces, it’s mounted on the supply air plenum above the heat exchanger—accessible from the blower compartment door. On horizontal or downflow units, the location varies; look for a bimetal disk or small bracket-mounted device with two wire leads, near the heat exchanger outlet.
