What the High Limit Switch Does
The high limit switch is a normally-closed safety device mounted inside the heat exchanger compartment. It monitors the temperature of the air passing over the heat exchanger. When the temperature at the switch reaches its rated opening point, the switch opens and kills the gas valve circuit. The blower keeps running to cool the heat exchanger down. Once the temperature drops below the switch’s reset threshold, the switch closes and the furnace can restart.
This cycle repeats until the root cause is fixed. A limit switch that trips and resets repeatedly is called short-cycling. The furnace overheats, shuts off, cools down, fires again, and overheats in a repeating loop.
Auto-Reset vs Manual-Reset Limit Switches
Most residential furnace limit switches are auto-reset. They close themselves once the temperature drops by about 20-30 degrees below the opening temperature. You do not need to press anything. Manual-reset switches have a red or white button in the center. You must press the button firmly until it clicks. Manual-reset switches typically guard the secondary heat exchanger area or act as rollout switches. If a manual-reset switch trips, do not simply reset it and walk away. Something caused it to trip and the furnace needs diagnosis.
Top Root Causes in Order
1. Dirty or restrictive air filter. This causes more limit trips than every other cause combined. A 1-inch filter rated MERV 8 or higher that has been in service for 60 days can reduce airflow by 30% or more. The heat exchanger cannot shed enough heat without adequate airflow. Replace the filter with a MERV 8 filter. Do not use MERV 13 or higher on older furnaces. The higher restriction can trigger limit trips on marginal duct systems.
2. Blocked or closed supply registers. Walk the house and open every register. Furniture pushed over a floor register can block 80% of the airflow from that run. Return grilles blocked by furniture or drapes cause the same problem. The rule of thumb is to never close more than 20% of the supply registers in the house.
3. Dirty blower wheel. The blower wheel accumulates dust over time. A caked blower wheel can reduce airflow by 30-40%. Remove the blower access panel and inspect the wheel. Clean with a stiff brush and vacuum. This is a maintenance item that most homeowners never touch, but it causes limit trips on furnaces with otherwise clean filters.
4. Failing blower motor or capacitor. A blower motor that runs slowly because of a bad run capacitor moves less air. The heat exchanger overheats even with a clean filter and open registers. Test the run capacitor by removing it and using a capacitor meter. The microfarad reading must be within 5-10% of the value on the label. Replace if out of range.
5. Dirty A-coil. The evaporator coil sits above the furnace in a split system. If the coil is caked with dust, airflow drops. This is common in homes where the air filter is changed regularly but the evaporator coil has never been cleaned. A dirty coil requires professional cleaning with coil cleaner.
6. Undersized ductwork. Some homes have ductwork sized for a smaller furnace or for cooling-only loads. When the furnace fires on a cold day, the supply ducts cannot carry enough heated air away from the heat exchanger. The limit trips. This requires a professional duct design evaluation.
7. Faulty limit switch. The switch itself may drift out of calibration. A limit switch with a rated opening temperature of 160F may open at 140F after thousands of thermal cycles. This is the last thing to check, not the first. Test with a multimeter after ruling out airflow issues.
Temperature Rise Calculation
Temperature rise measures how much the furnace heats the air passing through it. Every furnace has a rated temperature rise range printed on the rating plate (typically 40-70F for a residential gas furnace). If the measured rise exceeds the rated maximum, the limit switch will trip.
Measure the return air temperature. Clip a thermometer into the return air plenum near the filter. Measure the supply air temperature in the plenum above the furnace, past the heat exchanger. Run the furnace for 5 minutes and record both temperatures. Subtract the return temperature from the supply temperature. The result is the temperature rise.
If the rise is 80F or higher on a furnace rated for 40-70F, you have a severe airflow restriction. Find and fix the restriction before the furnace destroys the limit switch or cracks the heat exchanger.
How to Test a Limit Switch with a Multimeter
Turn off power to the furnace at the breaker. Remove the access panels and locate the high limit switch. It is mounted on the back or side of the heat exchanger compartment, typically near the top. It has two wire terminals and a metal mounting plate that screws into the furnace.
Disconnect the two wires from the switch terminals. Set your multimeter to resistance (ohms). Touch one probe to each terminal. A cold limit switch should read near 0 ohms. Any reading above a few ohms means the switch is failing. An open reading (no continuity) on a cold switch means it has failed and needs replacement.
To test the switch hot, reconnect the wires and tape a thermometer probe to the switch body. Run the furnace and watch the temperature at which the switch opens. If it opens more than 10 degrees below the rated temperature printed on the switch, the switch has drifted out of spec and needs replacement.
Brand-Specific Limit Switch Locations
Carrier/Bryant: The primary limit switch is mounted on the back panel of the heat exchanger compartment. You access it through the blower compartment. Carrier typically uses a bi-metal disc type.
Trane/American Standard: The limit switch is mounted on the side of the heat exchanger enclosure, near the top. Trane uses a snap-disc switch design on most models.
Lennox: The limit switch is on the back wall of the heat exchanger compartment. Some Lennox models have a secondary limit switch in the supply plenum that also controls the blower off delay.
Goodman/Amana: The limit switch is mounted on the front of the heat exchanger, visible once you remove the burner access panel. Goodman uses a simple bi-metal design with two spade terminals.
Rheem/Ruud: The limit switch is on the back panel of the heat exchanger section. Rheem also uses auxiliary limit switches on the blower deck. Code 3 or code 4 flashes indicate the limit circuit is open.
Limit Switch vs Flame Rollout
A tripped limit switch means the furnace overheated but combustion stayed inside the heat exchanger. A tripped rollout switch means flames pushed out of the burner tubes and into the burner compartment. Rollout is far more dangerous. Rollout switch trips come with a manual reset button and indicate a blocked heat exchanger, cracked heat exchanger, or vent blockage that is forcing combustion gases out the front of the burner. If you reset a rollout switch and it trips again, stop and call a professional immediately.
When to Call a Professional
Call a technician if you have done all of the following and the limit switch still trips within one or two heat cycles:
- Replaced the air filter
- Opened all registers
- Cleaned or inspected the blower wheel
- Verified the temperature rise is within the rated range
- Tested and confirmed the limit switch closure is within spec
At this point the likely culprits are a cracked heat exchanger, a failing inducer motor affecting combustion airflow, or an undersized duct system. Each of these requires professional tools and expertise. A cracked heat exchanger is a carbon monoxide hazard. Do not keep running a furnace that trips its limit switch repeatedly.