1. Introduction
This manual provides essential information for the proper installation, operation, and maintenance of the Lennox 80M51 Limit Switch. Please read this manual thoroughly before attempting any procedures to ensure safe and efficient use of the product. The Lennox 80M51 is a critical component in HVAC systems, designed to monitor and control temperature limits for safe operation.
2. Safety Information
WARNING: Installation and servicing of HVAC components should only be performed by qualified, experienced technicians. Improper installation, adjustment, alteration, service, or maintenance can cause property damage, personal injury, or loss of life.
- Always disconnect power to the unit before installing or servicing the limit switch.
- Wear appropriate personal protective equipment (PPE), including safety glasses and gloves.
- Ensure all wiring connections are secure and conform to local electrical codes.
- Do not bypass or tamper with safety devices.
3. Product Description
The Lennox 80M51 is a replacement limit switch designed for use in various Lennox HVAC furnace models. This part is a direct replacement for R20294302 and 80M5101. It is compatible with furnace series including, but not limited to: CMPB050C3B, CMPB075C3B, CMPB100C4B, CMPE050U2B, CMPE050U3B, CMPE075U2B, CMPE075U3B, CMPE100U3B, CMPE100U4B, CMPE125U5B, CMPEV050U3, CMPEV075U3, CMPEV100U4, CMPEV125U5.
A limit switch is a safety device that monitors the temperature within the furnace's heat exchanger. If the temperature exceeds a safe operating limit, the switch will open, interrupting the power to the burner and preventing overheating. This helps protect the furnace from damage and ensures the safety of the occupants.
Image 1: The Lennox 80M51 Limit Switch. This image shows the metallic body of the switch with electrical terminals, designed for integration into HVAC systems.
4. Setup and Installation
Installation of the Lennox 80M51 Limit Switch requires careful attention to safety and proper wiring. Consult your furnace's specific service manual for detailed instructions relevant to your model.
- Power Disconnection: Turn off all electrical power to the furnace at the main service panel before beginning any work. Verify power is off using a voltage meter.
- Access: Locate the existing limit switch within the furnace. This typically involves removing an access panel.
- Wiring Disconnection: Carefully disconnect the wires from the old limit switch, noting their positions for correct re-connection. Take a photograph if necessary.
- Removal: Remove the old limit switch from its mounting bracket.
- Installation of New Switch: Mount the new Lennox 80M51 Limit Switch in the same location and orientation as the old one.
- Wiring Connection: Reconnect the wires to the new limit switch according to your notes or photographs. Ensure connections are tight and secure.
- Reassembly: Replace any access panels removed during the process.
- Power Restoration and Testing: Restore power to the furnace. Initiate a heating cycle and monitor the furnace's operation to ensure the new limit switch functions correctly.
5. Operation
The Lennox 80M51 Limit Switch operates automatically as a safety control. When the furnace is operating, the temperature within the heat exchanger rises. If this temperature exceeds the factory-set limit of the 80M51 switch, the switch will open its electrical contacts, interrupting the circuit to the furnace's burner. This action prevents the furnace from overheating. Once the temperature drops back to a safe level, the switch will automatically reset, allowing the burner to reignite and resume normal operation.
The switch is designed for "Auto" operation, meaning it will automatically reset once conditions return to normal. No manual intervention is typically required for its normal function.
6. Maintenance
The Lennox 80M51 Limit Switch is a sealed component and does not require routine maintenance itself. However, regular maintenance of the overall HVAC system is crucial to ensure the proper functioning of all safety devices, including the limit switch.
- Annual Furnace Inspection: Have a qualified technician inspect your furnace annually. They will check for proper airflow, clean components, and verify the operation of safety controls like the limit switch.
- Air Filter Replacement: Regularly replace or clean your furnace's air filter as recommended by the furnace manufacturer. A clogged filter can restrict airflow, leading to overheating and frequent tripping of the limit switch.
- Clear Vents: Ensure all supply and return air vents in your home are unobstructed to maintain proper airflow.
7. Troubleshooting
If your furnace is frequently cycling off and on, or not heating properly, the limit switch might be involved. Always consult a qualified HVAC technician for diagnosis and repair.
- Frequent Tripping: If the limit switch is frequently tripping, it often indicates an underlying issue causing the furnace to overheat, rather than a faulty switch itself. Common causes include:
- Clogged air filter.
- Blocked return or supply air vents.
- Malfunctioning blower motor or fan.
- Dirty evaporator coil (if applicable).
- Oversized furnace for the ductwork.
- Furnace Not Starting: If the furnace does not start at all, and the limit switch is suspected, a technician can test the continuity of the switch. A continuously open switch (even when cool) indicates a fault.
- No Reset: While the 80M51 is an auto-reset switch, if it fails to reset after cooling, it may be faulty and require replacement.
Note: Do not attempt to bypass the limit switch. This is a critical safety device, and bypassing it can lead to dangerous operating conditions, including fire hazards.
8. Specifications
| Brand | Lennox |
| Model | 80M51 |
| Replaces Part Numbers | R20294302, 80M5101 |
| Operation Mode | Auto |
| Item Weight | 4.8 ounces |
| Manufacturer | Allied |
| Connectivity Protocol | X-10 (Note: This might refer to a general control system compatibility, not direct X-10 home automation. Consult a technician for specific application.) |
| Unit Count | 1 Count |
| ASIN | B009HEIVBU |
9. Warranty and Support
For warranty information and technical support regarding the Lennox 80M51 Limit Switch, please refer to the official Lennox website or contact your authorized Lennox dealer or service provider. Keep your purchase receipt for warranty claims.
Manufacturer: Allied
Date First Available: July 27, 2012
