Systematic troubleshooting for heating system failures
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GAS SAFETY - If you smell gas, evacuate immediately, shut off gas at meter if safe to do so, and call emergency services. Do not operate electrical switches or create sparks.
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Phase 1: Basic Power & Thermostat Checks
Verify thermostat is calling for heat
Set temp above room temp, mode on HEAT, fan to AUTO, check display is working
Check thermostat batteries (if applicable)
Replace batteries, wait for reboot, verify proper display
Verify power at equipment
Check breaker ON, disconnect switch ON, fuses intact
Measure line voltage at unit
Should read 120VAC (single phase) or 240VAC (heat pump/electric heat)
Expected: 120V ±10% or 240V ±10%
Check 24VAC transformer output
Measure at transformer secondary - should be 24-28VAC
Expected: 24-28VAC | Low voltage = failed transformer or short
Inspect for blown fuses on control board
Check 3A or 5A fuses on board - replace if blown, investigate cause
🔍 IF NO 24VAC AT TRANSFORMER
Check 120V input to transformer → Replace transformer if input OK but no output → Check for shorts on low voltage wiring if fuse keeps blowing
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Phase 2: Gas Furnace Ignition Sequence
Verify gas supply to furnace
Check main gas valve open, gas cock at furnace open, gas meter running
Check air filter condition
Dirty filter can cause limit switch lockout - replace if restricted
Observe inducer motor startup
Should start when thermostat calls for heat - listen for motor, check for 120VAC at motor
Verify pressure switch closes
Check for 24VAC across pressure switch with inducer running - 0V = closed/good
Closed = 0VAC | Open = 24VAC (check hose, inducer, or vent blockage)
Check hot surface igniter operation
Should glow bright orange - check for 120VAC at igniter, measure igniter resistance
Resistance: 40-90Ω typically | Open circuit = failed igniter
Confirm gas valve opens
Listen for click, check for 24VAC at gas valve when igniter glowing
Verify flame sensor detects flame
Measure flame sense current: 0.5-10µA DC (use µA setting on meter)
Good: 2-10µA | Weak: 0.5-2µA (clean sensor) | None: 0µA (no flame or bad sensor)
Check for limit switch lockout
Verify limit switches closed - check continuity or measure 0VAC across them
💡 COMMON FURNACE FAILURES
Dirty flame sensor: Flame lights then shuts off after 3-5 seconds Pressure switch stuck open: Inducer runs but igniter never glows Failed igniter: Igniter doesn't glow, no gas valve opening Bad gas valve: Igniter glows but no flame Cracked heat exchanger: May cause limit/rollout switch tripping
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Phase 3: Heat Pump Diagnostics
Verify outdoor unit running in heat mode
Outdoor fan and compressor should run when calling for heat
Check reversing valve operation
Verify 24VAC to reversing valve solenoid in heat mode, feel for valve shift
Measure outdoor ambient temperature
Heat pump capacity drops below 0°C - may need auxiliary/emergency heat
Below -10°C: Heat pump may be in defrost or emergency heat mode
Check for defrost cycle operation
Outdoor coil icing normal - should defrost periodically (10-90 min cycles)
Verify auxiliary heat strips operational
Check for 240VAC at heat strips, measure amp draw when energized
Amp draw per kW: ~4.2A per 1kW at 240V
Measure refrigerant pressures (if equipped)
Check suction/discharge pressures against outdoor temp chart
Check emergency heat operation
Switch to EM HEAT mode - outdoor unit should stop, aux heat should run
💡 HEAT PUMP TROUBLESHOOTING
Stuck in cooling mode: Bad reversing valve or solenoid Outdoor unit frozen solid: Defrost control failure or low airflow No aux heat: Check sequencers, contactors, limit switches, heat strips Short cycling: Low refrigerant, dirty coils, or compressor issue Customer "no heat" complaint: Often just cold air during defrost - educate customer
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Phase 4: Boiler System Diagnostics
Check boiler water pressure
Should be 12-25 PSI cold (read gauge on boiler)
Low pressure: Add water via fill valve | High: Check expansion tank/relief valve
Verify circulator pump running
Should hear/feel vibration, check for 120VAC at pump, feel for heat at motor
Check zone valves operation (if applicable)
Verify zone valves open when zone calls for heat - manual lever or end switch
Inspect for air in system
Check air vents, bleed radiators, listen for gurgling in pipes
Verify burner ignition sequence
Similar to furnace - check inducer, pressure switch, igniter, gas valve, flame sense
Check boiler temperature controls
Verify aquastat set correctly, high limit not tripped, outdoor reset working
Typical settings: 140-180°F | High limit: 200-220°F
Test expansion tank
Press Schrader valve - should have air pressure, no water discharge
Pressure: 12-15 PSI | Water at valve = waterlogged tank
💡 BOILER COMMON ISSUES
Low water pressure: Leak in system or faulty pressure relief valve Circulator failure: Boiler heats but no hot water to zones Airlock: Gurgling sounds, uneven heating, pump cavitation Zone valve stuck: One zone not heating Waterlogged expansion tank: Pressure spikes, relief valve discharges
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Phase 5: Airflow & Distribution
Verify blower motor operation
Blower should start after heat established (30-90 second delay normal)
Check air filter restriction
Replace if dirty - can cause overheating and limit switch trips
Inspect ductwork for obstructions
Check for closed dampers, collapsed ducts, blocked registers
Measure supply air temperature
Should be 40-70°F above return air temp (gas furnace) or 15-30°F (heat pump)