SF 35 Furnace Manual

The SF 35 Furnace is a high-efficiency residential heating system with 35,000 BTU output capacity, featuring electronic ignition, variable-speed blower motor, multi-position installation capability, energy-saving operation modes, built-in safety controls, and compatibility with standard thermostat systems. Designed for reliable performance in moderate climates. Below are key sections for safety, features, installation, setup, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Heat exchanger, blower assembly, gas valve, electronic control board, ignition system, pressure switches, flame sensor, and venting system.

Feature	Description
Heat Output	35,000 BTU/hour capacity
Efficiency Rating	95% AFUE
Blower Motor	Variable-speed ECM motor
Ignition System	Hot surface electronic ignition
Installation Positions	Upflow, downflow, horizontal left, horizontal right
Gas Type	Natural gas (convertible to LP)
Control System	Microprocessor-based control board
Safety Features	Flame rollout switch, pressure switches, limit switches
Venting	PVC pipe compatible
Warranty	20-year heat exchanger, 5-year parts

Installation Requirements

Professional installation required. Verify local codes and obtain permits.

1. Location: Install in dry, clean area with adequate combustion air and service clearance.
2. Gas Connection: Use black iron pipe; test for leaks with soap solution.
3. Electrical: 120V, 60Hz dedicated circuit; proper grounding required.
4. Venting: Follow manufacturer specifications for PVC vent pipe sizing and routing.
5. Ductwork: Properly sized and sealed return and supply ducts.

WARNING! Gas appliances can be dangerous if improperly installed. Use qualified HVAC technician.

Initial Setup & Startup

Complete installation verification before first startup.

1. System Check: Verify gas supply, electrical power, thermostat wiring.
2. Venting Inspection: Confirm proper vent termination and clearances.
3. Startup Procedure: Follow control board sequence; observe ignition cycle.
4. Performance Testing: Check temperature rise, gas pressure, combustion analysis.
5. Customer Orientation: Explain thermostat operation and basic maintenance.

CAUTION! Never bypass safety controls. Test all safety functions during startup.

Technical Specifications

• Model: SF 35
• Heating Capacity: 35,000 BTU/h
• AFUE Rating: 95%
• Electrical: 120V, 60Hz, 5.2 amps
• Gas Pressure: Natural gas: 3.5" WC; LP gas: 10" WC
• Temperature Rise: 35-65°F
• External Static Pressure: 0.1-0.8" WC
• Blower Speeds: 4 heating speeds, 2 cooling speeds
• Dimensions: 17.5" W × 30" H × 21" D
• Shipping Weight: 98 lbs

Control Panel & Settings

Integrated control board with status LED for diagnostics. Access panel must be securely in place for operation.

Status Lights: Green - normal operation; Yellow - service required; Red - lockout condition.

DIP Switch Settings: Heating airflow, blower off delay, heating stages. Thermostat Compatibility: Single-stage conventional, programmable, and smart thermostats.

Venting & Ductwork

Direct vent system using PVC pipe for intake and exhaust.

1. Vent Pipe: Schedule 40 PVC, 2" minimum diameter.
2. Termination: Maintain proper clearances from windows, doors, and air intakes.
3. Slope: Horizontal runs slope 1/4" per foot toward furnace.
4. Duct Sizing: Follow Manual J calculation for proper airflow.
5. Sealing: Use mastic or foil tape on all duct connections.

Tip: Never use single-wall metal vent pipe. PVC must be supported every 4 feet.

Operating Instructions

Normal operation controlled by thermostat settings.

1. Thermostat Setting: Set to HEAT mode and desired temperature.
2. Startup Sequence: Inducer motor starts, pressure switch proves, igniter heats, gas valve opens.
3. Flame Establishment: Flame sensor confirms ignition, main blower starts after delay.
4. Shutdown: Gas valve closes, inducer continues purge, blower runs during off delay.
5. Seasonal Operation: Follow recommended temperature settings for energy efficiency.

WARNING! Do not operate with access panel removed. Keep combustible materials away from furnace.

Maintenance Schedule

Annual professional maintenance recommended.

• Monthly: Check air filter; replace when dirty
• Annual (Professional): Clean heat exchanger, check burners, test safety controls
• Seasonal: Inspect vent system, check drain lines, verify thermostat operation
• Every 5 Years: Inspect inducer motor, check heat exchanger for cracks

Safety Features

Multiple safety systems protect against hazardous conditions.

Safety Device	Function
Flame Rollout Switch	Detects flame outside burners
Pressure Switches	Verify proper venting airflow
High Limit Switch	Prevents overheating
Flame Sensor	Confirms burner ignition
Gas Valve	Automatic shutoff if flame lost

Regulatory Compliance

ANSI Z21.47: Certified for gas-fired central furnaces. UL 1995: Standard for heating and cooling equipment. Meets DOE energy efficiency standards. CSA International certified. For technical support, contact 1-800-555-HEAT or visit www.sffurnace.com.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No heat	Thermostat setting, power, gas supply	Check thermostat, circuit breaker, gas valve; verify pilot/ignition
Furnace short cycles	Dirty filter, blocked vent, limit switch	Replace filter, clear vent obstructions, check airflow
Blower doesn't run	Motor, capacitor, control board	Check power to blower, test capacitor, verify control signals
Unusual noises	Loose parts, airflow restriction	Tighten access panel, check for duct restrictions
Lockout condition	Ignition failure, safety trip	Reset power, check flame sensor, verify gas pressure

Reset Procedure: Turn thermostat off, wait 5 minutes, restore power, turn thermostat on.

Service: 1-800-555-HEAT; www.sffurnace.com