How to Reset a Furnace Limit Switch Safely and Effectively

The furnace limit switch is a critical safety device that prevents overheating and protects the heating system. This article explains what a limit switch does, common reasons it trips, safe methods to reset it, diagnostic steps, troubleshooting tips, and when to call a professional. The guidance applies to most residential gas and electric furnaces used across the United States and emphasizes safety and code-aware practices.

Quick Reset Steps	When To Call A Pro
Turn Off Power → Inspect Filter → Check Vents → Reset Switch → Test	Repeated Trips, Burning Smell, No Heat, Faulty Controls

What Is A Furnace Limit Switch And Why It Matters

The furnace limit switch monitors furnace internal temperature and shuts the burner or furnace fan off if temperatures exceed safe levels. It prevents damage to the heat exchanger and reduces fire risk. If the limit switch trips, the furnace may stop producing heat or run the blower continuously until temperatures normalize.

Types Of Limit Switches And Locations

Most residential furnaces use one of two types: a mechanical (bi-metal) limit switch or an electronic temperature sensor with a circuit board input. Mechanical switches are usually mounted on the furnace cabinet adjacent to the heat exchanger or near the blower compartment.

Electronic limit sensors may be integrated into control modules and show fault codes. Identifying the type helps determine the reset method and diagnostics.

Common Causes For A Limit Switch Trip

• Restricted Airflow: Dirty air filters, closed vents, blocked returns, or dirty blower wheels reduce cooling air and cause overheating.
• Blower Motor Failure: A weak or failed blower motor can’t move enough air across the heat exchanger.
• Clogged Heat Exchanger Or Coils: Dirt or debris reduces heat transfer and increases internal temperature.
• Ductwork Issues: Collapsed ducts, sharp bends, or undersized runs restrict airflow.
• Short Cycling Or Long Run Times: Improper thermostat settings or furnace control faults lead to abnormal operation and excessive heat buildup.
• Faulty Limit Switch Or Wiring: Wear or electrical faults can cause false trips or failure to reset.

Safety Precautions Before Attempting A Reset

Working on a furnace involves electrical and gas components. Always shut off power at the furnace switch and the breaker before touching the limit switch. For gas furnaces, turn off the gas supply if required by a component replacement. If a homeowner is uncomfortable or unsure, contact a licensed HVAC technician.

Step-By-Step Guide To Reset A Furnace Limit Switch

Tools And Materials Needed

• Screwdriver set
• Multimeter (for continuity and voltage checks)
• Replacement filter
• Soft brush or vacuum for cleaning
• Owner’s manual or wiring diagram (if available)

Procedure

1. Turn Off Power: Switch off the furnace at the service switch and the circuit breaker to ensure zero voltage to components.
2. Locate The Limit Switch: Remove the furnace access panel and find the limit switch mounted near the heat exchanger or blower housing.
3. Inspect Visually: Look for obvious signs of damage, loose wires, burn marks, or corrosion. Check the filter, blower, vents, and return grille for blockages.
4. Reset Mechanism: Some limit switches have a manual reset button. If present, press the button once after power is off, then restore power and test the furnace. Electronic systems may reset automatically when temperatures return to normal.
5. Cycle Power: If no manual reset exists, wait 5–10 minutes for the furnace to cool, then turn power back on and observe the start-up sequence.
6. Test Operation: Set the thermostat to call for heat and watch for normal burner operation and blower activation. Note any unusual delays, noises, or error codes.
7. Measure With A Multimeter: If the switch keeps tripping, measure continuity across the switch when cold and measure voltage signals during operation to identify intermittent failures.

Diagnosing Persistent Trips: Troubleshooting Checklist

If the limit switch trips repeatedly after reset, follow a systematic checklist to find the root cause rather than repeatedly resetting the switch.

Problem	Diagnostic Action	Expected Result
Restricted Airflow	Replace filter, open supply/return registers, inspect duct path	Improved airflow and no further overheating
Blower Issues	Inspect blower wheel, motor capacitor, and bearings; measure motor current	Normal amp draw and consistent air movement
Thermostat Or Control Faults	Verify thermostat cycles, check control board for error codes	Correct on/off cycles; no false calls for heat
Limit Switch Failure	Test continuity at room temperature; replace if open when cold	New switch maintains continuity when cold and opens at set temperature
Heat Exchanger Or Combustion Problems	Inspect visually for cracks or leaks; schedule professional combustion testing	Safe combustion parameters and intact heat exchanger

How To Test A Limit Switch With A Multimeter

Testing confirms whether the switch is functioning or failing. With the furnace powered off, remove wires connected to the switch and set a multimeter to continuity or ohms.

At room temperature a normally closed (NC) limit switch should show near-zero resistance. A reading of infinite resistance indicates an open circuit and a defective switch. After safe reassembly, measure again during operation to observe if the switch opens when the furnace gets hot.

When Replacement Is Required And How To Choose A Replacement

Replace the limit switch if it fails continuity checks, shows visible damage, or trips repeatedly despite resolving airflow issues. Use the OEM part number when possible, or match specifications: switching type (NC or NO), actuating temperature, mounting style, and electrical ratings.

Installing the correct temperature rating is essential. Replacing a higher-temperature switch with a lower one can mask airflow problems and increase safety risks. If uncertain, consult the furnace manual or an HVAC technician.

Preventive Maintenance To Reduce Limit Switch Trips

• Replace or clean filters every 1–3 months depending on usage and air quality.
• Schedule annual HVAC tune-ups to check blower operation, motor capacitors, and heat exchanger condition.
• Keep supply and return registers unobstructed and ensure adequate return pathways in living spaces.
• Seal and insulate ducts to prevent loss of airflow and imbalances.
• Maintain proper thermostat settings to reduce short cycling and excessive run times.

Common Error Codes And What They Mean

Many modern furnaces display LED blink codes for limit trips, blower failures, or control board faults. Typical patterns include repetitive flashes that map to a blower, flame, or limit fault. Refer to the furnace’s service panel legend or owner’s manual for diagnostics. Documenting the blink pattern helps technicians diagnose issues faster.

Safety And Code Considerations

Limiting devices are safety components required by building and mechanical codes. Bypassing, modifying, or installing an incorrect switch compromises safety and may void warranties or violate local codes. Any permanent replacement or complex repair should follow manufacturer instructions and local regulations.

When To Call A Professional HVAC Technician

Call a licensed HVAC technician when the limit switch trips repeatedly after basic troubleshooting, when there is a burning odor, visual heat exchanger damage, gas smell, or electrical issues. Technicians provide in-depth diagnostics like airflow measurement, combustion analysis, and safe heat exchanger inspection.

Cost Considerations

Replacing a limit switch is typically one of the less expensive HVAC repairs, but total cost varies by furnace model and whether a technician is required. Expect parts and labor to range from modest DIY costs (parts only) to a few hundred dollars with professional labor and testing. Complex issues like heat exchanger repair or blower motor replacement increase costs significantly.

FAQ: Fast Answers To Common Questions

Can A Homeowner Reset The Limit Switch?

Yes, homeowners can reset a limit switch if it has a manual reset and basic safety steps are followed. However, repeated trips require professional assessment to address the underlying cause.

Will Resetting The Switch Fix The Problem Permanently?

No. Resetting restores operation temporarily but does not resolve root causes like airflow restriction or component failure. Persistent trips need proper diagnostics.

Is It Safe To Bypass The Limit Switch?

No. Bypassing a limit switch removes a primary safety control and creates serious fire and carbon monoxide risks. It is illegal and dangerous.

How Long Should The Furnace Run After Reset?

A furnace should start normally and maintain safe operating cycles. If it overheats or trips again within a short period, turn the system off and call a technician.

Resources And Further Reading

• ENERGY STAR – Heating Tips
• Air Conditioning Contractors Of America (ACCA)
• ASHRAE – HVAC Standards

Repair And Replacement Checklist For Technicians

Task	Description
Visual Inspection	Check switch, wiring, filter, blower, heat exchanger
Motor & Capacitor Test	Measure current draw and capacitance values
Airflow Measurement	Static pressure test across filter and evaporator coil
Limit Switch Test	Continuity at cold and operational open at temperature
Combustion Safety	Combustion analysis and heat exchanger integrity check

Following methodical diagnostics and safe reset procedures helps keep furnaces operating reliably and safely. For persistent or complex issues, consulting a licensed HVAC professional ensures code-compliant, durable repairs and protects occupants and property.