Lennox G51 Furnace Installation Manual

The Lennox G51 Furnace is a high-efficiency gas furnace designed for residential heating applications. This multi-speed furnace features advanced combustion technology, electronic ignition, and built-in safety controls. The G51 series offers reliable performance with AFUE ratings up to 95.2%, making it an energy-efficient choice for home heating. Below are key sections covering safety precautions, installation procedures, electrical connections, gas line connections, ventilation requirements, startup procedures, and maintenance guidelines.

1. Safety Information 2. Pre-Installation Requirements 3. Installation Procedures 4. Technical Specifications 5. Electrical Connections 6. Gas Line Connections 7. Venting and Combustion Air 8. Startup and Commissioning 9. Maintenance Guidelines 10. Troubleshooting 11. Warranty Information

Safety Information

Critical safety precautions must be followed during installation and service.

Hazard Type	Precaution
Gas Leaks	Check all connections with leak detection solution
Electrical Shock	Disconnect power before servicing
Carbon Monoxide	Ensure proper venting and combustion air
Fire Hazard	Maintain clearances to combustible materials
Burn Hazard	Allow furnace to cool before handling

Pre-Installation Requirements

Verify site conditions and gather necessary tools and materials.

Inspect furnace for shipping damage before installation.
Verify adequate clearances: 1" sides, 1" rear, 6" front, 24" above.
Check local codes for gas, electrical, and venting requirements.
Ensure proper combustion air supply and venting pathways.
Verify gas type and pressure requirements match supply.

WARNING! Installation must comply with all local codes and regulations. Professional installation recommended.

Installation Procedures

Step-by-step installation guide for proper furnace setup.

Position furnace: Place on solid, level surface in approved location.
Install vent pipes: Use approved PVC materials for intake and exhaust.
Connect gas line: Use black iron pipe with proper thread sealant.
Electrical wiring: Connect 120V power and thermostat controls.
Ductwork connections: Secure supply and return air ducts properly.
Condensate drain: Install trap and route to approved drain location.

CAUTION! Furnace must be level for proper condensate drainage.

Technical Specifications

Model Series: G51MP, G51MPP, G51MPV
Input BTU/H: 40,000 to 120,000
AFUE Rating: Up to 95.2%
Gas Type: Natural Gas or LP (with conversion kit)
Electrical: 120V, 60Hz, 1.5-3.0 Amps
Motor Type: Multi-speed PSC or ECM
Venting: PVC, CPVC, or ABS (2" or 3")
Operating Temp Range: -30°F to 150°F
Dimensions: Varies by model (approx 21"W x 21"D x 40"H)

Electrical Connections

Proper electrical wiring is critical for safe operation.

Power Supply: 120V AC, 15A circuit with disconnect switch required.

Thermostat: Connect R, W, G, C terminals. Use 18-gauge thermostat wire.

Grounding: Proper equipment grounding required per NEC standards.

Control Board: Follow wiring diagram on furnace access panel.

Gas Line Connections

Gas piping must be properly sized and tested.

Shut off gas supply at main valve.
Install sediment trap before gas valve connection.
Use pipe joint compound rated for natural/LP gas.
Pressure test: 14" WC for 10 minutes (no drop).
Check manifold pressure: 3.5" WC natural gas, 10" WC LP.

Tip: Always use gas leak detection solution on all connections.

Venting and Combustion Air

Proper venting is essential for safe operation and efficiency.

Use only approved PVC, CPVC, or ABS pipe materials.
Maintain minimum 1/4" per foot slope toward furnace.
Maximum vent length: 100 feet equivalent (consult tables).
Termination: 12" minimum above grade, 3' from openings.
Combustion air: Provide adequate fresh air supply.
Seal all joints with approved PVC cement.

WARNING! Improper venting can cause carbon monoxide hazards.

Startup and Commissioning

Follow proper startup sequence for initial operation.

Verify all connections are complete and secure.
Restore electrical power and gas supply.
Set thermostat to heating mode above room temperature.
Check ignition sequence: inducer motor, hot surface igniter, gas valve.
Measure temperature rise: 35-65°F across heat exchanger.
Verify gas pressure and combustion analysis.
Check safety controls: limit switches, pressure switches.
Program thermostat for proper cycling.

CAUTION! Never bypass safety controls during testing.

Maintenance Guidelines

Regular maintenance ensures optimal performance and longevity.

Annual maintenance recommended by qualified technician.

CAUTION! Disconnect power and gas before performing any maintenance.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Furnace won't start	Power, thermostat, door switch	Check circuit breaker, thermostat settings, door switch engagement
No heat	Gas supply, igniter, pressure switch	Verify gas valve open, check igniter, test pressure switch
Short cycling	Airflow, filter, limit switch	Check filter, blower wheel, heat exchanger limits
Blower not running	Motor, capacitor, control board	Test motor windings, check capacitor, verify control signals
Error codes flashing	Sensor faults, lockout conditions	Refer to diagnostic chart in service manual

Reset: Power cycle furnace to clear temporary lockouts.

Service: Contact authorized Lennox dealer for professional service.

Warranty Information

Lennox provides limited warranties on G51 furnace components. Heat exchanger: 20-year limited warranty. Other parts: 5-year limited warranty. Labor: 1-year from installation. Registration required within 60 days of installation. For warranty claims, contact authorized Lennox dealers. Keep original sales receipt and installation date for verification.