COMMERCIAL & RESIDENTIAL HEATING TECHNICAL MANUAL (HVAC/R FIELD GUIDE) SECTION 1: GAS FURNACE IGNITION SEQUENCE & DIAGNOSTICS GAS FURNACE SEQUENCE OF OPERATION: 1. Thermostat calls for heat (W1 energized with 24VAC to board). 2. Control board verifies limit switches are closed and pressure switches are open. 3. Draft inducer motor energizes to purge heat exchanger (15 to 30 second pre- purge). 4. Inducer creates negative pressure, closing the pressure switch contacts (proving draft). 5. Hot Surface Igniter (HSI) or Direct Spark Igniter energizes for warm-up (15 to 45 seconds). 6. Main gas valve receives 24VAC, opens, and gas flows across burners to ignite.

7. Flame sensor detects microamp current (rectification) within 4 seconds of ignition. 8. Indoor blower motor starts after 30 to 45 second heat exchanger preheat delay.

GAS FURNACE COMMON ERROR CODES AND FAULT TREES: • FLASH CODE 1 - High Limit Switch Open: Cause: Insufficient airflow over heat exchanger. Checks: Inspect dirty air filters (MERV rating too high), blocked return grills, secondary heat exchanger clogged with lint, or blower capacitor failure. Check static pressure across furnace (should not exceed 0.50 in. w.c.). • FLASH CODE 2 - Pressure Switch Failed to Close: Cause: Venting blockage, cracked inducer housing, or condensate line restriction. Checks: Use a digital manometer. Measure inducer draft pressure. If draft is below rating on switch rating plate (typically -0.40 to -0.80 in. w.c.), clear bird nests, drain traps, or flue pipe condensation. If draft exceeds rating, replace defective pressure switch. • FLASH CODE 3 - Flame Sensor Signal Weak or Lost (Flame Failure): Cause: Sensor rod oxidation, improper burner ground, or gas pressure dropout. Checks: Connect multimeter in series set to DC Microamps (µA). Normal reading is 1.5 to 6.0 µA. Readings below 1.0 µA drop flame rectification. Clean sensor rod with fine steel wool or scotch-brite pad. Verify ground wire has zero resistance to chassis. SECTION 2: HEAT PUMP HEATING CYCLE & OPERATING PRESSURES (R-410A)

HEAT PUMP HEATING CYCLE FUNDAMENTALS: In heating mode, the reversing valve shifts position to redirect hot high-pressure discharge gas from the compressor directly into the indoor coil (acting as the condenser). The outdoor coil acts as the evaporator, absorbing latent heat from outdoor ambient air. TARGET PRESSURES AND TEMPERATURES (R-410A HEATING MODE): • Low Side (Suction / Outdoor Coil): Operating pressure: 100 to 140 PSI depending on outdoor temperature (47°F ambient corresponds to roughly 115-130 PSI). Target Superheat: 8°F to 14°F at compressor inlet. • High Side (Liquid Line / Indoor Coil Discharge): Operating pressure: 300 to 420 PSI. Target Subcooling: 8°F to 12°F measured at the liquid line service valve before the outdoor expansion device. • Compressor Discharge Line Temperature: Should measure between 160°F and 190°F. If discharge temp exceeds 225°F, compressor failure or low charge is imminent.

REVERSING VALVE DIAGNOSTIC PROCEDURE: 1. Verify 24VAC signal at the reversing valve solenoid coil (O/B terminal). Note: Rheem/Ruud energize on B (heating); Carrier, Trane, Goodman energize on O (cooling). 2. Measure temperature difference between the low-pressure suction line entering the valve from the outdoor coil and the true suction line leaving the valve to the compressor.

3. If temperature difference across internal slide exceeds 3°F, the valve body has an internal bleed-by leak and requires replacement. DEFROST CONTROL BOARD (DEMAND AND TIMED DEFROST): • Symptoms of Defrost Failure: Heavy frost accumulation thick enough to block airflow, low suction pressure dropping below 60 PSI, high liquid subcooling. • Diagnostic Steps: 1. Inspect Defrost Thermostat (Bimetal / Freeze Sensor) clipped on outdoor return bend. It must close below 30°F (-1°C) and open above 50°F (10°C). 2. Jump TEST pins on board to force defrost cycle.

3. Reversing valve must shift, outdoor fan motor must stop, and electric auxiliary heat strips must energize to prevent cold air delivery indoors. SECTION 3: ELECTRIC AUXILIARY HEAT & SEQUENCER DIAGNOSTICS

SEQUENCER TESTING: Sequencers prevent huge electrical spikes by staging electric heat elements. • Apply 24VAC across sequencer coil pins M1-M2. • Wait 30 to 90 seconds: contacts 1-2 must close (continuity / 0 Ohms). • Secondary contacts 3-4 must close 30 seconds later. • Remove 24VAC: contacts must stay closed for 30 to 60 seconds before opening (blower cool-down period). SECTION 4: TEMPERATURE RISE & SYSTEM AIRFLOW COMMISSIONING