HVAC Technician Academy
Learn how HVAC technicians diagnose furnace sequence of operation, inducer motors, pressure switches, igniters, gas valves, flame sensors, rollout switches, high limits, blower airflow, temperature rise, and combustion-related safety problems.
Furnace diagnosis follows the heating sequence of operation from the thermostat call through inducer operation, pressure-switch proving, ignition, gas-valve operation, flame proving, blower operation, and safety monitoring. The technician determines exactly where the sequence stops or becomes abnormal.
Gas furnaces are designed to operate in a specific sequence.
Each step depends on the previous step being completed and proven.
That makes furnace troubleshooting much easier when technicians stop guessing at parts and instead ask:
The exact point where the sequence stops provides the next diagnostic direction.
| Step | Expected Event | If It Does Not Happen |
|---|---|---|
| 1 | Thermostat calls for heat | Check thermostat, 24V circuit, board input |
| 2 | Inducer motor starts | Check board output, voltage, inducer motor |
| 3 | Pressure switch proves draft | Check venting, inducer, condensate, tubing, switch |
| 4 | Igniter energizes | Check board output, igniter resistance, wiring |
| 5 | Gas valve opens | Check control voltage and gas supply |
| 6 | Burners ignite | Check ignition, gas pressure, burner condition |
| 7 | Flame is proven | Check flame sensor, grounding, flame signal |
| 8 | Blower starts and furnace heats normally | Check blower, airflow, limits, temperature rise |
Establishes draft and helps prove safe venting before ignition.
Verifies draft conditions before the ignition sequence can continue.
Provides the ignition source for burner operation.
Controls fuel flow when the control system has satisfied required safeties.
Proves flame so the control board can continue gas flow safely.
Protect the furnace from overheating or unsafe flame conditions.
1. Confirm the call for heat. Verify thermostat output and board input.
2. Watch the sequence. Observe which steps occur and where operation stops.
3. Read fault codes. Use them as diagnostic clues, not automatic part-replacement instructions.
4. Verify inducer operation. Check voltage, current, wheel condition, venting, and condensate where applicable.
5. Verify pressure-switch conditions. Determine whether proper draft is actually being produced.
6. Verify ignition. Test the igniter and confirm required control voltage.
7. Verify gas-valve operation and flame. Confirm proper control command and safe fuel delivery.
8. Verify flame proving. Measure flame signal where appropriate.
9. Verify blower and temperature rise. Confirm airflow is sufficient for safe heat removal.
10. Verify the complete heating cycle. Confirm stable operation through shutdown and restart.
The inducer establishes airflow through the combustion system before the furnace is allowed to ignite.
If the inducer does not start, verify:
Do not assume a pressure-switch code means the pressure switch is defective if the inducer cannot create proper draft.
The pressure switch proves that the furnace is creating the draft condition required for safe operation.
A switch that will not close may be reacting correctly to:
When the inducer and pressure-switch sequence are satisfied, the control board typically energizes the igniter.
If the igniter does not operate, verify:
A gas valve should only be energized after required safeties and ignition conditions have been satisfied.
If the igniter glows but burners do not light, determine whether the gas valve is receiving the required control voltage.
If correct control voltage is present but fuel does not flow, the technician must also verify gas supply, shutoff position, pressure, and valve condition.
A classic flame-sensor complaint is:
The flame-sensing circuit confirms that burner flame is actually present.
Diagnosis may involve:
Replacing a flame sensor without testing the flame-sensing circuit can miss grounding or combustion problems.
A high-limit switch protects the furnace from excessive internal temperature.
Repeated high-limit opening often points toward a heat-removal problem.
Common diagnostic directions include:
If the high limit is opening, determine why the furnace is overheating. Replacing the safety without correcting the cause does not repair the furnace.
Rollout switches are critical safety devices intended to detect abnormal flame or heat outside the normal combustion area.
A tripped rollout requires careful investigation.
Possible causes can include:
Do not bypass, defeat, or repeatedly reset a rollout safety as a repair. Identify and correct the unsafe condition before returning the equipment to service.
Temperature rise is the difference between return-air temperature and supply-air temperature while the furnace is operating.
The measured rise should be compared with the manufacturer's rated range.
Excessive temperature rise can suggest:
Low temperature rise may suggest excessive airflow, low input, staging issues, or other operating conditions.
Airflow problems do not only affect cooling.
In heating mode, inadequate airflow can cause:
Study: HVAC Airflow Testing Guide
Diagnostic Case
Furnace ignites normally.
Flame remains stable.
Blower starts.
Several minutes later, burners shut down.
Previous diagnosis: flame sensor.
New flame sensor did not solve the problem.
Temperature rise is excessive.
Static pressure is high.
Airflow is restricted by a dirty blower wheel and indoor coil.
The high-limit switch is opening correctly because the furnace is overheating.
Lesson: Burner shutdown after several minutes is not the same failure pattern as loss of flame signal within a few seconds. Timing is diagnostic information.
| Symptom | Possible Direction | Next Test |
|---|---|---|
| Inducer never starts | Board, voltage, inducer motor | Verify call and inducer output voltage |
| Inducer runs, pressure switch will not close | Draft, vent, condensate, switch circuit | Measure draft and inspect tubing/venting |
| Igniter glows, no flame | Gas valve, gas supply, control voltage | Verify gas-valve voltage and fuel supply |
| Flame lights then shuts off in seconds | Flame proving circuit | Measure flame signal and check grounding |
| Burners shut off after several minutes | High-limit or airflow problem | Measure temperature rise and static pressure |
| Rollout switch trips | Potential combustion or venting hazard | Shut down and perform combustion/safety investigation |
Proper furnace diagnostics extend beyond whether burners light.
Depending on equipment and service requirements, qualified technicians may evaluate:
Combustion readings should be interpreted using equipment-specific procedures and manufacturer specifications.
Master furnace control circuits and sequence logic.
Airflow & Static Pressure →Diagnose airflow problems that cause high-limit trips.
Heat Pump Diagnostics →Continue into electric and refrigerant-based heating systems.
A common direction is a flame-proving problem, but technicians should verify flame signal, grounding, burner flame quality, sensor condition, and control operation before replacing parts.
No. The code may indicate that required draft was not proven. Venting, inducer performance, condensate, tubing, ports, and actual pressure should be checked.
High-limit trips frequently involve excessive furnace temperature caused by low airflow, restrictions, blower problems, incorrect setup, or excessive input.
A tripped rollout should be treated as a potential safety problem. The cause should be identified before the furnace is returned to service.
Temperature rise helps technicians evaluate whether heat input and airflow are reasonably balanced and should be compared with the furnace manufacturer's specified range.
Gas furnaces involve combustible fuel, ignition sources, high voltage, hot surfaces, combustion gases, and potentially lethal carbon monoxide. Testing, gas-pressure adjustment, combustion analysis, venting diagnosis, and safety-device evaluation should be performed only by qualified personnel following manufacturer instructions, applicable fuel-gas and mechanical codes, approved instruments, and appropriate safety procedures.
Furnace failures can involve controls, airflow, ignition, venting, flame proving, safeties, or combustion conditions. Proper diagnosis follows the entire heating sequence before parts are replaced.
Furnace Repair Technician Academy