The 3L09-20 Board Mount Limit Control (White-Rodgers / Copeland Comfort Control) is an automatic reset temperature safety switch designed to protect HVAC equipment, control boards, transformers, electric heat assemblies, and electrical compartments from overheating. Featuring a 240°F fixed open temperature and an extended 7.5-inch insertion length, this compact board-mounted bimetal limit switch interrupts the circuit when excessive temperatures are detected and automatically restores operation after cooling, helping ensure safe and reliable equipment performance.

Cross-Reference

• 3L09-20
• 3L0920
• L240F
• White-Rodgers 3L09-20
• Copeland Comfort Control 3L09-20
• 74T Series Board Mount Limit Control

Direct OEM

White-Rodgers 3L09-20

Compatible Brands

• White-Rodgers
• Emerson
• Copeland
• York
• Coleman
• Luxaire
• Goodman
• Rheem
• Universal HVAC Applications

Application

Used in HVAC and electrical equipment to:

• Protect control boards from overheating
• Interrupt safety circuits at elevated temperatures
• Automatically restore operation after cooling
• Protect transformers and electrical enclosures
• Safeguard electric heat assemblies
• Prevent temperature-related component failures

Related Components

• Board Mount Limit Switch
• Snap Disc Thermostat
• Furnace Control Board
• Transformer
• Blower Relay
• High Limit Control
• Temperature Sensor
• Safety Cutoff Switch
• Electric Heat Control Assembly

Technician Notes

The 3L09-20 is an automatic reset, open-on-rise board mount limit control utilizing a compact bimetal sensing disc and an extended 7.5-inch insertion probe. The contacts remain closed during normal operation and open when the sensing temperature reaches 240°F, interrupting the circuit to prevent overheating. Once temperatures fall below the reset point, the contacts automatically reclose and restore operation.

The extended insertion length allows the switch to monitor temperatures deeper within electric heat sections, air handlers, control compartments, and OEM HVAC assemblies where standard probes may not reach the desired sensing location.

Common Symptoms of Failure

• Equipment shuts down unexpectedly
• Open safety circuit faults
• Electric heat lockouts
• Intermittent HVAC operation
• Repeated thermal shutdowns
• Control board overheating faults
• No continuity through switch contacts
• Failure to reset properly
• Excessive compartment temperatures

Installation Tips

1. Disconnect electrical power before servicing.
2. Verify the replacement temperature rating matches the original control.
3. Confirm switch action is Open on Rise.
4. Verify the 7.5-inch insertion length is correct for the application.
5. Secure the sensing probe fully into the original location.
6. Inspect terminals for corrosion or heat discoloration.
7. Test continuity after installation.
8. Verify operation through a complete heating cycle.

Recommended Inspection Items

• Control board condition
• Transformer temperature
• Electric heat elements
• Wiring harness connections
• Electrical terminals
• Blower motor operation
• Airflow restrictions
• Relay condition
• Ventilation around controls
• Probe placement depth