Lennox Merit Series Furnace Manual

The Lennox Merit Series Furnace is a high-efficiency gas furnace designed for residential heating applications. Featuring durable construction, reliable performance, and energy-efficient operation, this furnace series includes models with AFUE ratings up to 80%. Below are key sections covering safety, installation, operation, maintenance, troubleshooting, and technical specifications.

Features at a Glance

Key components: Durable heat exchanger, electronic ignition system, multi-speed blower motor, integrated control board, pressure switches, flame sensor, and limit switches.

Installation Requirements

Professional installation required. Verify local codes and obtain necessary permits.

1. Location selection: Install in dry, clean area with adequate clearance for service access.
2. Gas connection: Use approved black iron pipe; pressure test for leaks.
3. Electrical connection: 120V AC power supply with proper grounding.
4. Venting system: Category I venting for non-condensing models; PVC for condensing models.
5. Combustion air: Ensure adequate fresh air supply per local codes.

WARNING! Gas furnaces produce carbon monoxide; improper installation can be fatal. Installation must comply with all local codes and manufacturer specifications.

First-Time Setup

Initial startup must be performed by qualified technician.

1. Verify installation: Check all connections, clearances, and venting.
2. Electrical power: Turn on furnace disconnect and verify control board power.
3. Gas supply: Open main gas valve and verify proper pressure.
4. Thermostat connection: Connect and configure compatible thermostat.
5. Test operation: Initiate heating cycle and verify proper sequence.
6. Airflow adjustment: Set blower speed for proper temperature rise.
7. Combustion analysis: Verify proper gas pressure and combustion readings.

CAUTION! Never operate furnace without proper venting and combustion air. Carbon monoxide poisoning hazard.

Technical Specifications

• Models: ML180, ML193, ML194 series
• AFUE Rating: Up to 80% efficiency
• Input BTU/H: 40,000 to 120,000 BTU
• Gas Type: Natural gas (LP conversion kits available)
• Electrical: 120V AC, 60Hz, 15 amp circuit
• Blower Motor: Multi-speed PSC or ECM
• Filter Size: Standard 1-inch media filters
• Venting: Category I (non-condensing) or PVC (condensing)
• Warranty: 20-year heat exchanger, 5-year parts
• Sound Level: Quiet operation design

Controls and Operation

Integrated control board manages all furnace operations with LED status indicators.

Thermostat Operation: Set desired temperature; furnace cycles automatically. Use fan ON for continuous circulation.

Control Board LED: Flashing patterns indicate system status and error codes.

Sequence of Operation: Thermostat call for heat → inducer motor starts → pressure switch verification → hot surface ignition → gas valve opens → flame verification → blower starts after delay.

Safety Controls: Limit switches, pressure switches, flame rollout switches, and flame sensor ensure safe operation.

Venting and Combustion Air

Proper venting is critical for safe operation and efficiency.

1. Vent pipe sizing: Follow manufacturer specifications for pipe diameter and length.
2. Vent termination: Locate away from windows, doors, and air intakes.
3. Combustion air: Provide adequate fresh air per installation requirements.
4. Condensate drainage: For high-efficiency models, ensure proper condensate trap and drainage.

Tip: Regularly inspect vent pipes for obstructions or damage.

Maintenance Procedures

Regular maintenance ensures optimal performance and longevity.

1. Monthly: Check and replace air filter as needed.
2. Annual professional service: Includes combustion analysis, heat exchanger inspection, blower cleaning, and safety check.
3. Seasonal: Inspect vent system, check drain lines, verify thermostat operation.
4. Component inspection: Check burners, flame sensor, igniter, and safety controls.

WARNING! Always turn off electrical power and gas supply before performing any maintenance.

Safety Features

Multiple safety systems protect against hazardous conditions.

Flame Rollout Switches: Detect improper burner combustion. Pressure Switches: Verify proper venting operation. Limit Switches: Prevent overheating. Flame Sensor: Confirms burner ignition. Carbon Monoxide Detection: Install CO detectors near sleeping areas.

Care & Cleaning

Keep furnace area clean and unobstructed.

Cleaning: Vacuum burner area and blower compartment annually. Clean flame sensor with emery cloth. Wipe exterior surfaces with damp cloth.

Filter Replacement: Replace 1-inch filters monthly during heating season. Use proper filter size and type.

CAUTION! Never use compressed air to clean internal components. Avoid water or chemical cleaners.

Regulatory Information

ANSI Z21.47/CSA 2.3: Certified for safety standards. UL Listed: Electrical component certification. National Fuel Gas Code: Complies with NFPA 54 requirements. Local codes: Installation must meet all applicable local regulations. For technical support, contact Lennox Industries at 1-800-9-LENNOX or visit www.lennox.com.

Troubleshooting

Reset Procedure: Turn power off for 30 seconds, then restore power.

Professional Service: 1-800-9-LENNOX; www.lennox.com