Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Furnace Sequence: From Thermostat Call to Flame

9 Sept 2026 · 1 min

Watch on YouTube

1:16

In this instructional monologue, the speaker walks newcomers through the complete operational sequence of a gas furnace, starting with the thermostat’s call for heat and ending with the circulating blower delivering warm air. The steps covered include the inducer motor starting to create draft, the pressure switch confirming draft, the hot surface igniter heating, the gas valve opening to the burners, the burners igniting, the flame sensor verifying flame, a blower delay, and finally the blower turning on. The speaker emphasizes watching the startup order to diagnose problems, noting that a stop before the igniter points to draft issues, while a glowing igniter without flame suggests a gas‑side fault. The tone is instructional and supportive, urging viewers to use the sequence as a troubleshooting roadmap rather than immediately replacing parts.

Chapters

Hey, are you new to the trades? Here's something you may not know about a gas furnace sequence of operations. When the thermostat calls for heat, it sends a signal to the furnace control board. First, the inducer motor starts. That is the small fan that pulls combustion gases toward the vent and creates draft. Next, the pressure switch has to prove that draft. If the switch does not close, the furnace should not move forward. Then the hot surface igniter begins to heat. Once it is hot enough, the gas valve opens and gas reaches the burners. The burners light, and the flame sensor proves that flame to the board. Only after that proof does the furnace wait through its blower delay, then start the circulating blower to move warm air through the home. Know what comes next. Do not jump straight to replacing parts. Watch the startup in order. If it quits before the igniter, think draft-proof. If it gets glow but no flame, look at the gas side. The sequence tells you where it stopped. Now that you know, you can help someone else know.