The 3L09-4 Board Mount Limit Control (White-Rodgers / Copeland Comfort Control) is an automatic reset temperature safety switch designed to protect HVAC equipment, electrical controls, and heating systems from overheating. Featuring a 150°F fixed open temperature, this compact board-mounted bimetal limit switch interrupts the circuit when excessive temperatures are detected and automatically restores operation after cooling, helping protect sensitive components from heat-related damage.

Cross-Reference

• 3L09-4
• 3L094
• L150F
• White-Rodgers 3L09-4
• Copeland Comfort Control 3L09-4
• 74T Series Board Mount Limit Control

Direct OEM

White-Rodgers 3L09-4

Compatible Brands

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

Application

Used in HVAC and heating equipment to:

• Protect control boards from overheating
• Open safety circuits at elevated temperatures
• Automatically restore operation after cooling
• Protect transformers and electrical compartments
• Prevent temperature-related component failures
• Provide automatic thermal protection

Related Components

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

Technician Notes

The 3L09-4 is an automatic reset, open-on-rise board mount limit control utilizing a compact bimetal snap disc. Under normal operating conditions, the contacts remain closed and open when the sensing surface reaches 150°F, interrupting the circuit to prevent overheating. Once the temperature falls below the reset point, the switch automatically closes and restores operation.

Board-mounted limit controls are commonly used in furnace control compartments, blower sections, transformer protection circuits, and OEM HVAC equipment where compact, dependable overtemperature protection is required.

Common Symptoms of Failure

• Equipment shuts down unexpectedly
• Open safety circuit fault
• Intermittent operation
• Repeated temperature-related lockouts
• Control board overheating faults
• No continuity through switch contacts
• Failure to reset properly
• Electrical compartment overheating

Installation Tips

1. Disconnect power before servicing.
2. Verify temperature ratings match the original component.
3. Confirm switch action is Open on Rise.
4. Mount securely in the original location.
5. Inspect terminals for corrosion or heat damage.
6. Verify airflow around electrical components.
7. Test continuity after installation.
8. Confirm proper operation through a complete heating cycle.

Recommended Inspection Items

• Control board condition
• Transformer temperature
• Wiring harness connections
• Electrical terminals
• Blower motor operation
• Airflow restrictions
• Ventilation around controls
• Relay condition
• Equipment operating temperatures