Heating | Australia
Gas Furnace Sequence of Operation and Safeties
A quick ladder for no-heat calls so you can prove exactly where the sequence breaks.
Built for Australia field work where reverse-cycle splits, ducted systems, zoning, latent-load issues, and roof-space installation realities shape the day-to-day call mix.
Ticket note prompts
- Document the last successful step in the heat sequence so a repeat callback starts from the real failure point.
- Note draft, ignition, flame-sense, and blower behavior separately instead of writing only 'no heat'.
- Capture any venting, filter, or temperature-rise condition that contributed to a limit or rollout issue.
Comeback prevention
- Confirm the furnace runs through a complete heat cycle with normal temperature rise before you leave.
- Recheck venting and condensate routing on high-efficiency furnaces when pressure-switch faults were involved.
- Verify flame signal stability after cleaning or grounding repairs.
Sequence ladder
- Start with thermostat call and board status, then prove line voltage and low voltage at the right moments.
- Listen for inducer start, then confirm pressure switch closure instead of assuming it happened.
- If flame lights but does not hold, clean and prove flame sense before selling a gas valve.
Failure points
| Step | If it fails here | Likely direction |
|---|---|---|
| Inducer never starts | No sequence initiation | Board, door switch, line voltage, or call issue |
| Pressure switch never proves | Draft path not proven | Blocked vent, cracked hose, weak inducer, switch issue |
| Ignition lights then drops | Flame not proven | Flame sensor, grounding, board, or gas delivery issue |
Safety mindset
- A tripped rollout means stop and inspect for flame carryover, blocked exchanger path, or venting trouble.
- Repeated high-limit trips point to airflow, blower, filter, or temperature-rise problems before they point to a bad limit.
- Do not leave jumpers in safety circuits after proving the fault path.