HVAC Technician Academy
Learn furnace sequence of operation, inducer and pressure-switch diagnostics, igniter and gas-valve testing, flame proving, temperature rise, limits, airflow, combustion safety, and control-board logic.
Follow the sequence of operation. Find the last step that completed successfully, then identify the first step that did not. Verify safeties, inducer operation, pressure-switch proof, ignition, gas-valve operation, flame proving, blower operation, temperature rise, and airflow before replacing components.
Exact timing and control logic vary by manufacturer, but the sequence gives the technician a disciplined way to find where operation stops.
Step 1: Learn the furnace sequence of operation
Step 2: Understand inducer and venting operation
Step 3: Diagnose pressure-switch proof
Step 4: Verify igniter and gas-valve sequence
Step 5: Learn flame rectification and microamp testing
Step 6: Understand limits and rollout safeties
Step 7: Verify blower airflow and temperature rise
Step 8: Evaluate combustion and venting safely
Learn the complete heating sequence and how to identify the first missing step.
Furnace Pressure Switch Diagnostics →Diagnose venting, inducer, condensate, tubing, pressure ports, and proof-of-draft problems.
Flame Sensor Microamp Diagnostics →Measure flame-rectification current and diagnose sensor, grounding, burner, and board problems.
24-Volt Control Circuits →Follow thermostat calls, safeties, relays, and control-board logic through the heating circuit.
On many modern gas furnaces, the inducer starts before ignition.
Its job is to establish the required draft and venting conditions before the furnace allows gas ignition.
If the inducer does not start, investigate:
A pressure switch is a proving device.
If it does not close after the inducer starts, possible causes include:
When pressure-switch operation is questionable, measure actual inducer draft and compare it with the furnace and pressure-switch specifications.
Continuity through the switch only tells you whether its contacts are open or closed. It does not prove the draft system is healthy.
If the sequence reaches ignition but the igniter does not operate:
Do not condemn the igniter before proving it is receiving the expected electrical command.
If ignition is established but burners do not light, verify whether the gas valve is being commanded open.
Diagnosis may involve:
Gas-pressure and combustion work must follow manufacturer procedures and applicable codes.
After ignition, many furnaces use flame rectification to prove burner flame.
A common symptom of weak flame proving is:
Possible causes include:
Flame current is commonly measured in microamps DC with the meter connected in series according to the furnace manufacturer's procedure.
There is no single universal microamp number that applies to every furnace.
Compare the reading with the exact manufacturer's specification.
A furnace may shut the burners down on high limit because heat is not being removed from the heat exchanger fast enough.
Possible causes include:
Repeated limit trips are a symptom requiring diagnosis, not a reason to bypass the limit.
A rollout switch may indicate abnormal flame or combustion conditions.
Potential causes can include:
Furnace temperature rise compares supply-air temperature with return-air temperature during heating operation.
Compare the measured rise with the range listed on the furnace nameplate or manufacturer data.
An excessively high temperature rise often points toward inadequate airflow or excessive heat input.
Heating performance depends heavily on airflow.
Verify:
Continue: Airflow & Static Pressure Training
Before replacing a furnace control board, verify:
The board should come later in the diagnostic process, not first.
Proper furnace commissioning and advanced troubleshooting may require combustion measurements.
Depending on equipment and local requirements, trained personnel may evaluate:
Compare measurements with manufacturer guidance and applicable safety standards.
Technician Case File
Burners: ignite normally.
Flame sensor: proves flame correctly.
Several minutes later: burners shut off.
Limit circuit: opens.
Filter: clean.
Static pressure: excessive.
Evaporator coil: heavily restricted.
Lesson: The limit switch was doing its job. The actual failure was inadequate airflow.
1. Verify thermostat heat call.
2. Observe the complete sequence of operation.
3. Identify the last successful step.
4. Test the first missing step.
5. Verify inducer and venting.
6. Verify pressure-switch proof.
7. Verify ignition command and igniter operation.
8. Verify gas-valve command and burner ignition.
9. Measure flame signal when applicable.
10. Verify blower operation and airflow.
11. Measure temperature rise.
12. Evaluate safeties and combustion as required.
13. Retest the entire heating sequence after repair.
Connect technical furnace diagnostics with local heating service.
Heating Not Working →Homeowner-facing guide to common heating failures.
Heating Tune-Up →Connect maintenance and safety checks with heating performance.
Gas-furnace diagnostics can involve combustible fuel, ignition sources, carbon monoxide, hot surfaces, energized electrical circuits, rotating blowers, and pressurized gas. Qualified personnel should follow manufacturer procedures, applicable codes, combustion-safety practices, and proper gas-leak and CO testing. Never bypass pressure switches, limits, rollout switches, or other safety devices as a repair.
Pressure switches, venting, flame sensors, ignition controls, airflow, limits, gas-valve operation, and combustion conditions can create overlapping symptoms. Follow the heating sequence and prove the failed step.
Furnace Repair Spring TX HVAC Technician Academy