Furnace Sequence of Operations

Most furnaces follow a similar sequence of operations when there is a call for heat from the thermostat. Understanding the sequence of operations is essential to diagnose a furnace issue and identify the point of failure.

Finding the sequence of operations for your furnace

You may find the sequence of operations in the guide book left by the furnace installer (on or near the furnace), or on the lower furnace cabinet cover (next to the error code guide).

If you don’t have a hard copy, it’s likely you can find the furnace installation and service guide for free online by searching for the model number.

Refer to the furnace glossary if you need help identifying furnace parts or terms.

Typical sequence of operations

1. Receive a call for heat from the thermostat to start the ignition sequence.
2. Check if there is a lock out due to multiple failed ignition attempts. (If locked out, power must be reset before retrying. Usually an error code is shown.)
3. Confirm all high-temperature limit switches are closed.
4. Confirm all roll out switches are closed.
5. Confirm all high-pressure switches are open.
6. Energize the inducer motor and run it for 30-60 seconds.
7. Confirm that pressure switch(es) have closed with inducer running.
8. Energize the igniter and hold for 10-30 seconds.
9. Open the gas valve to ignite burners.
10. Confirm ignition is successful by measuring current from the flame sensor.
    • Flames present: keep the gas valve open and continue.
    • No flames present after two to three seconds: close the gas valve and retry ignition.
11. Within 30-90 seconds, energize the blower motor to circulate warm air through the home.
12. Continue to run until the thermostat no longer calls for heat.