Components

Draft inducer

The fan that establishes flue draught on a modern gas appliance before and during firing. It pulls combustion products through the heat exchanger and out of the building, and the pressure it creates is what a pressure switch proves before the control will allow gas to burn.

Also called inducer motor, induced draught fan

Direct answer

What does a draft inducer do?

A draft inducer is the fan that pulls combustion products through a gas appliance and out through the flue. The pressure difference it creates is what the control proves before permitting ignition.

Draft inducer: how it works

Before any gas is admitted, the inducer runs to clear the combustion chamber of anything left from a previous cycle. Its rotation creates a pressure difference between the collector box and the reference the pressure switch sees, and the switch closes once that difference is enough. Only then does the control permit the igniter and the gas valve to operate.

Where you meet the term

  • As the first sound a furnace or boiler makes after a call for heat, before any ignition noise.
  • In a pressure switch fault, where the switch is the reported device and the inducer is often the reason it could not close.
  • In a post-purge period, when the appliance keeps running the fan after the burner has stopped.

How to check it

  1. Listen for the inducer at the start of a cycle. A call for heat should produce a fan sound before anything else. Silence at that point means the sequence never began, which is a different report from a failed ignition. Owner check
  2. Note whether the sound has changed. A rumble, grinding, or an uneven note developing over weeks is a mechanical symptom worth reporting before it becomes a lockout. Owner check
  3. Measure the pressure the inducer produces. A manometer reading at the pressure switch port, compared against the value in the appliance literature, separates a weak inducer from a switch that is not responding. Technician
Read this twice

Draft inducer: what it is not

  • A draft inducer is not the circulating blower. One moves combustion products through the flue, the other moves room air across the heat exchanger, and they run at different points in the cycle.
  • An inducer that spins is not necessarily performing. Blocked passages, a fouled wheel, or a restricted flue can leave it turning while the draught it produces falls short.

Draft inducer questions

Why does the inducer run before the burner lights?

The pre-purge clears the combustion chamber and the flue of anything left from a previous cycle, and it establishes the draught the pressure switch has to prove before gas is permitted.

Is the inducer the same as the flue fan on a boiler?

They perform the same function under different names. A room-sealed boiler fan both draws combustion air in and pushes products out, while a furnace inducer chiefly pulls products through the exchanger.

Why does the fan keep running after the heat stops?

A post-purge period clears remaining combustion products from the exchanger and the flue after the burner shuts down. It is designed behaviour rather than a sign that something failed to stop.

Where this definition comes from

Written from service manual material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • Deluxe 2-Stage Gas-Fired Induced-Combustion Furnaces With PSC or ICM Blower Motor Troubleshooting Guide (58-3T)Carrier · service manualCarrier troubleshooting guide for deluxe two-stage gas-fired induced-combustion furnaces with PSC or ICM blower motors. Its status code index reads 11 no previous code, 12 blower on after power up, 13 limit or flame rollout switch lockout, 14 ignition lockout, 21 gas heating lockout, 22 abnormal flame-proving signal, 23 low-heat pressure switch did not open, 24 secondary voltage fuse is open, 31 high-heat pressure switch or relay did not close or reopened, 32 low-heat pressure, draft safeguard, or auxiliary limit switch did not close or reopened, 33 limit or flame rollout switch is open, 34 ignition-proving fault, 43 low-heat pressure, draft safeguard, or auxiliary limit switch open while the high-heat pressure switch is closed, and 45 replace control. The schematic notes state that ignition lockout occurs after four consecutive unsuccessful trials for ignition and that the control resets automatically after three hours, that fuse FU1 is a 3 amp automotive blade type to be replaced only with a 3 amp fuse, and that blower-on delay is 45 seconds for gas heating and 2 seconds for cooling or heat pump operation.
  • GMVC80 furnace installation instructionsGoodman · service manualGoodman GMVC80 furnaces: the one-flash ignition lockout sequence, the troubleshooting chart entries separating a low stage pressure switch circuit closed at the start of a heating cycle, sticking contacts and a short in the switch wiring from a circuit that is not closed, the high stage version whose symptom is a furnace that runs on low stage only, the worked total external static pressure example, and the minimum recommended circulation air filter sizes by model.