The 3L09-21 Board Mount Limit Control (White-Rodgers / Copeland Comfort Control) is an automatic reset temperature safety switch designed to protect HVAC equipment, control boards, transformers, air handlers, and electrical compartments from overheating. Featuring a 150°F fixed open temperature and an extended 3-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 operation.

Cross-Reference

• 3L09-21
• 3L0921
• L150F
• White-Rodgers 3L09-21
• Copeland Comfort Control 3L09-21
• 74T Series Board Mount Limit Control

Direct OEM

White-Rodgers 3L09-21

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 compartments
• Monitor temperatures within air handlers and heating 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
• Electrical Control Assembly

Technician Notes

The 3L09-21 is an automatic reset, open-on-rise board mount limit control utilizing a compact bimetal sensing disc and a 3-inch insertion probe. Under normal operating conditions, the contacts remain closed and open when the sensing surface reaches 150°F, interrupting the circuit to prevent overheating. Once temperatures fall below the reset point, the contacts automatically reclose and restore operation.

The extended 3-inch probe allows temperature sensing deeper inside electrical compartments, air handlers, blower cabinets, and OEM HVAC assemblies than standard board-mounted controls.

Common Symptoms of Failure

• Equipment shuts down unexpectedly
• Open safety circuit faults
• Intermittent operation
• Repeated thermal shutdowns
• Control board overheating faults
• No continuity through switch contacts
• Failure to reset properly
• Electrical compartment overheating
• Transformer overheating conditions

Installation Tips

1. Disconnect power before servicing.
2. Verify the replacement temperature rating matches the original control.
3. Confirm switch action is Open on Rise.
4. Verify the 3-inch insertion length matches the application.
5. Secure the sensing probe properly during installation.
6. Inspect terminals for corrosion or heat damage.
7. Test continuity after installation.
8. Verify operation through a complete equipment 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
• Probe insertion depth