Armstrong Ultra SX 90 Furnace Service Manual

The Armstrong Ultra SX 90 is a high-efficiency gas furnace with AFUE rating up to 90%, featuring two-stage gas valve operation, variable-speed blower motor, electronic ignition system, sealed combustion chamber, pressure switch safety, flame rollout protection, and diagnostic LED indicator. Designed for residential forced-air heating systems with input capacities ranging from 60,000 to 120,000 BTU/h. Below are key sections for safety, technical specifications, installation, operation, maintenance, troubleshooting, and service procedures.

Features at a Glance

Key components: Two-stage gas valve, variable-speed ECM blower motor, hot surface ignitor, flame sensor, pressure switches, sealed combustion chamber, multiport burners, primary and secondary heat exchangers.

Installation Requirements

Unpack and inspect furnace for shipping damage. Verify proper location with adequate clearances.

1. Location requirements: Minimum 24" front clearance, 6" sides, 6" rear, 1" combustible materials.
2. Venting: Use only approved PVC pipe for intake and exhaust; maintain proper slope for condensation drainage.
3. Gas connection: Use black iron pipe with proper thread sealant; install sediment trap and shutoff valve.
4. Electrical: 120V AC power supply with proper grounding; install disconnect switch within sight of furnace.
5. Ductwork: Ensure proper sizing and sealing; install return air filter rack with minimum MERV 8 filter.

WARNING! Gas furnaces produce carbon monoxide; improper installation can be fatal. Follow all codes and manufacturer instructions.

Initial Startup Procedure

Complete installation verification before applying power or gas.

1. Verify gas type: Ensure furnace is configured for natural gas (default) or propane with proper orifice.
2. Check electrical: Verify 120V supply, proper grounding, and thermostat wiring.
3. Inspect venting: Confirm PVC pipes are properly glued and supported; check for obstructions.
4. Gas pressure test: Measure manifold pressure (3.5" WC natural gas, 10" WC propane).
5. Startup sequence: Turn on power, set thermostat to heat mode, observe ignition sequence.
6. Performance check: Verify proper flame appearance, temperature rise within range (35-65°F), and correct airflow.

CAUTION! Only qualified technicians should perform startup procedures. Test for gas leaks with soap solution.

Technical Specifications

• Input Capacity: 60,000-120,000 BTU/h (model dependent)
• AFUE Rating: Up to 90%
• Gas Type: Natural gas or propane (field convertible)
• Manifold Pressure: 3.5" WC natural gas, 10" WC propane
• Electrical: 120V AC, 60Hz, 5-8 amps
• Blower Motor: Variable-speed ECM, 1/3 to 1/2 HP
• Temperature Rise: 35-65°F
• Venting: 2" or 3" PVC pipe (intake and exhaust)
• Dimensions: Varies by model (typically 21"W x 21"D x 46"H)
• Weight: 120-160 lbs

Control System Operation

Integrated furnace control board manages all operations with diagnostic capabilities.

Control Board: Monitors safety devices, controls ignition sequence, operates blower motor, provides diagnostic flash codes.

Ignition Sequence: Call for heat → Inducer motor on → Pressure switch proves → Hot surface ignitor heats → Gas valve opens → Flame detection → Blower motor starts. Safety Systems: Flame rollout switch, high limit switch, pressure switch, flame sensor. Diagnostics: LED flash codes indicate specific faults (1 flash = pressure switch, 2 flashes = ignition failure, etc.).

Venting and Combustion Air

Direct vent system using separate intake and exhaust PVC pipes.

1. Vent pipe sizing: Use manufacturer tables based on furnace input and pipe length.
2. Termination clearances: 12" above grade, 12" from windows/doors, 36" from gas meters.
3. Pipe support: Support every 5 feet horizontally, secure joints with proper PVC cement.
4. Combustion air: Ensure adequate fresh air supply for proper combustion and draft.
5. Condensate drainage: Install neutralizer kit if required by local codes; ensure proper slope.

Tip: Always verify vent system integrity with manometer test before operation.

Maintenance Procedures

Annual maintenance required for optimal performance and safety.

1. Filter replacement: Replace every 1-3 months depending on usage and filter type.
2. Burner inspection: Remove and clean burners; check for proper flame pattern.
3. Heat exchanger: Inspect for cracks or corrosion; perform combustion analysis.
4. Blower assembly: Clean blower wheel and housing; check motor amperage.
5. Vent system: Inspect for obstructions; check pipe connections and terminations.
6. Drain system: Clean condensate trap and drain lines; flush with clean water.
7. Electrical: Check all connections; verify proper voltage and grounding.
8. Gas system: Test for leaks; verify manifold pressure and gas input.

WARNING! Disconnect electrical power and gas supply before performing maintenance.

Safety Systems

Multiple safety devices protect against hazardous conditions. Flame Rollout Switch: Manual reset device that trips if flames escape burner area. High Limit Switch: Automatic reset that opens if heat exchanger overheats. Pressure Switches: Prove inducer operation before gas valve opening. Flame Sensor: Monitors burner flame; shuts off gas if flame not detected. Ignition Control: Limits ignition attempts to prevent gas accumulation.

Cleaning & Care

Regular cleaning maintains efficiency and extends furnace life.

Use soft brush and vacuum for heat exchanger fins. Clean burners with compressed air. Wipe blower housing with damp cloth. Keep area around furnace clean and unobstructed.

CAUTION! Never use water or liquid cleaners on electrical components. Wear safety glasses during cleaning.

Regulatory Compliance

UL certified for safety standards. ANSI Z21.47/CSA 2.3 for gas-fired central furnaces. Meets U.S. Department of Energy efficiency standards. Installation must comply with National Fuel Gas Code (NFPA 54), National Electrical Code (NFPA 70), and local building codes. For technical support, contact Armstrong Air Technical Services at 1-877-874-7378 or visit www.armstrongair.com.

Troubleshooting Guide

Reset Procedure: Turn off power for 30 seconds, then restore to reset control board.

Service: 1-877-874-7378; www.armstrongair.com