Lennox G51MP Series Gas Furnace
Model Numbers: G51MP Series
Manufacturer: Lennox Industries Inc.
This document provides essential information for the Lennox G51MP Series Gas Furnace. It includes critical safety warnings, operational guidelines, maintenance procedures, and parts identification.
Important Safety Information
WARNING: FIRE OR EXPLOSION HAZARD. Failure to follow safety warnings exactly could result in serious injury, death, or property damage.
- Do not store or use gasoline or other flammable vapors and liquids in the vicinity of this or any other appliance.
- Installation and service must be performed by a qualified installer, service agency, or the gas supplier.
WHAT TO DO IF YOU SMELL GAS:
- Do not try to light any appliance.
- Do not touch any electrical switch; do not use any phone in your building.
- Leave the building immediately.
- Immediately call your gas supplier from a neighbor's phone. Follow the gas supplier's instructions.
- If you cannot reach your gas supplier, call the fire department.
Parts Identification
Figure 1: G51MP Furnace Components
The furnace unit comprises several key components:
- Top Cap
- DURALOK™ Heat Exchanger Assembly
- Burners
- Cabinet
- Upper Access Panel
- Gas Valve and Manifold
- Flue Collar
- Combustion Air Inducer
- Combustion Air Proving (Pressure) Switch
- SureLight™ Control Board
- Blower Motor
- Blower Access Panel
- Control Box
WARNING: Product contains fiberglass wool. Disturbing insulation during installation, maintenance, or repair can expose you to fiberglass wool, which may cause lung cancer (known to the State of California to cause cancer) and respiratory, skin, and eye irritation. Consult material safety data sheets for further information.
WARNING: Do not set thermostat below 60°F (16°C) in heating mode, as this reduces heating cycles and may cause damage not covered by warranty.
WARNING: If overheating occurs or gas supply fails to shut off, shut off the manual gas valve before shutting off electrical supply.
CAUTION: Before performing any service or maintenance, turn the electrical power to the unit OFF at the disconnect switch.
General Safety and Installation Precautions
- WARNING: Do not use this furnace if any part has been underwater. Flood-damaged furnaces are dangerous and can result in fire or explosion. Call a qualified service technician immediately.
- IMPORTANT: Any additions, changes, or conversions must be made by a Lennox service technician using factory-specified parts.
- WARNING: Improper installation, adjustment, alteration, service, or maintenance can cause property damage, personal injury, or loss of life. Installation and service must be performed by a qualified installer, service agency, or the gas supplier.
Safety Instructions:
- Keep the furnace area clear of combustible material, gasoline, and other flammable vapors and liquids. Ensure insulated areas are free of insulating material, which may be combustible.
- Ensure unobstructed airflow for combustion and ventilation. Do not block or obstruct air openings on the furnace or in the surrounding area. Maintain established clearances.
- Do not store or introduce chlorine or fluorine products near the unit, as they can cause furnace corrosion.
- Do not draw return air from a room where this furnace or any other gas appliance is installed. This can create negative pressure, potentially causing flue products (including carbon monoxide) to be pulled back into the room and distributed by the furnace.
WARNING: The blower door must be securely in place when the blower and burners are operating. Gas fumes, including carbon monoxide, can be drawn into the living space, resulting in personal injury or death.
Your furnace is a gas appliance. Complete combustion of gas is critical to avoid carbon monoxide production. This requires correct gas pressure, gas flow rate, adequate combustion air, and proper venting.
WARNING: Carbon monoxide gas is invisible, odorless, and toxic. Exposure can cause personal injury and death. Lennox recommends using a carbon monoxide detector in your home. Contact your Lennox dealer for more information.
Your furnace is designed to meet national standards. Proper installation and maintenance are critical for safe operation. Lennox strongly recommends annual inspection and maintenance by a qualified service technician.
Lighting Information & Operation
WARNING: Failure to follow these instructions exactly may result in fire or explosion, causing property damage, personal injury, or death.
Before lighting, smell around the appliance area for gas, especially near the floor, as gas may settle there.
Operating the Gas Valve
The gas valve may have a knob or lever. Use only your hand to operate it. Do not use tools. If it is stiff, call a qualified service technician.
Placing the G51MP Furnace into Operation:
G51MP units use a SureLight® ignition system. Burners light automatically when the thermostat calls for heat. The ignitor does not get hot when there is no call for heat.
Steps for Operation:
- Read safety information.
- Set thermostat to the lowest setting.
- Turn off all electrical power to the unit.
- This furnace has an automatic ignition device; do not attempt to light burners by hand.
- Remove the upper access panel.
- Turn the gas valve to OFF:
- White Rodgers 36G Series: Switch gas valve lever to OFF.
- Honeywell VR8205 Series: Turn knob clockwise to OFF. Do not force.
- Wait five minutes to clear out any gas. If gas is still smelled, STOP and call your gas supplier.
- Turn the gas valve to ON:
- White Rodgers 36G Series: Switch gas valve lever to ON.
- Honeywell VR8205 Series: Turn knob counterclockwise to ON. Do not force.
- Replace the upper access panel.
- Turn on all electrical power to the unit.
- Set the thermostat to the desired setting.
NOTE: Steps 1-11 may need to be repeated to purge air from the gas line during initial startup.
If the appliance does not operate, follow the "Turning Off Gas to the Unit" section and call your service technician or gas supplier.
Turning Off Gas to the Unit:
- Set the thermostat to the lowest setting.
- Turn off all electrical power to the unit if service is to be performed.
- Remove the upper access panel.
- Turn the gas valve to OFF:
- White Rodgers 36G Series: Switch gas valve lever to OFF.
- Honeywell VR8205 Series: Turn knob clockwise to OFF. Do not force.
- Replace the upper access panel.
Filters
Filters are external to the unit. Inspect monthly and clean or replace as needed for proper operation. Replacement filters must be rated for high velocity airflow.
Table 1: Recommended Filter Sizes
| Furnace Cabinet Size | Filter Size - Side Return | Filter Size - Bottom Return |
|---|---|---|
| 17-1/2" | 16 X 25 X 1 (1) | 16 X 25 X 1 (1) |
| 21" | 16 X 25 X 1 (1) | 20 X 25 X 1 (1) |
| 24-1/2" | 16 X 25 X 2 (2) | 24 X 25 X 1 (1) |
A filter must be in place whenever the unit is operating.
Maintenance
A qualified service technician should inspect the complete system each season (heating and cooling). Do not attempt to service the unit yourself.
WARNING: ELECTRICAL SHOCK, FIRE, OR EXPLOSION HAZARD. Improper servicing can lead to dangerous operation, serious injury, death, or property damage. Disconnect all electrical power before servicing. Label wires before disconnecting and reconnect them correctly. Verify proper operation after servicing.
Venting System
WARNING: Asphyxiation Hazard. The exhaust vent must be securely connected to the furnace flue collar at all times. For safety and warranty, a qualified service technician must annually inspect the furnace and its vent system. Annually inspect the venting system, vent cap, heat exchanger, and burners for corrosion, deterioration, or debris. Remove obstructions.
Inspect the furnace for signs of deterioration. Ensure the venting system is in place, sound, and free of holes, corrosion, or blockage. The vent pipe must slope upward and be clear of obstructions. Inspect the return air duct connection for a proper seal. Ensure the furnace support is sound and maintains its seal. Inspect and clean the condensate trap and drain.
During seasonal checks, technicians will inspect the indoor blower, burner flames, and electrical connections.
Blower
Check the blower wheel for debris and clean if necessary. Blower motors are prelubricated for extended bearing life; no further lubrication is needed.
WARNING: The blower access panel must be securely in place during operation. Gas fumes, including carbon monoxide, can be drawn into the living space.
Burner Flame
CAUTION: A qualified technician should annually check the burner flame for proper operation.
Electrical Checks:
- Check all wiring for loose connections.
- Check for correct voltage at the furnace while operating.
- Check amp-draw on the blower motor.
Service Reminder
Call your Lennox service technician if the unit does not operate. Before calling, check:
- Electrical disconnect switches are ON.
- Room thermostat is set correctly.
- Blown fuses or tripped circuit breakers are replaced/reset.
- Gas valve is ON.
- Air filter is not plugged.
- Gas is turned on at the meter.
- Manual main shut-off valve is open.
Record the model and serial number from the unit's nameplate.
Safety Precautions:
If you discover any of the following, shut down the unit and contact a Lennox dealer:
- Repeated new or unfamiliar sounds during operation.
- Unusual odors indicating improper operation.
- Visible signs of malfunction (e.g., condensate, burnt components, rust).
- Symptoms like headache, nausea, fatigue, or dizziness, which could indicate carbon monoxide exposure.
Prompt detection of problems can prevent injury or damage. Contact a qualified service technician.
Planned Service
A service technician should check the following items during an annual inspection (power must be shut off):
- Fresh air grilles and louvers: Must be open and unobstructed.
- Burners: Inspect for rust, dirt, or water signs.
- Vent pipe: Inspect for water, damage, sagging, or disconnected joints.
- Unit appearance: Inspect for rust, dirt, water signs, or burnt/damaged wires/components.
- Blower access door: Must be in place and provide a seal.
- Return air duct: Must be properly attached and sealed.
- Operating performance: Observe unit and vent system performance.
- Combustion gases: Analyze flue products and compare to specifications.
Problems found may require temporary shutdown until repairs are made. Be aware of potential hazards, such as stored items obstructing combustion air, which can lead to incomplete combustion and carbon monoxide production.
Repair Parts List
Repair parts are available through Lennox dealers. When ordering, include the complete furnace model number (e.g., G51MP-24B-045-1).
Cabinet Parts
- Upper access panel
- Blower access panel
- Top Cap
Control Panel Parts
- Transformer
- SureLight® integrated control board
- Door interlock switch
- Circuit Breaker
Blower Parts
- Blower wheel
- Motor
- Motor mounting frame
- Motor capacitor
- Blower housing cutoff plate
Heating Parts
- Flame Sensor
- Heat exchanger assembly
- Gas manifold
- Combustion air inducer
- Gas valve
- Main burner cluster
- Main burner orifices
- Pressure switches
- Ignitor
- Primary limit control
- Secondary limit
- Flame rollout switches
- Combustion air inducer auxiliary limit
