The inducer is the part of your furnace that houses the fan that pulls air over the furnace heat source and pushes it out through the vents. In most furnaces, the inducer assembly is made of metal, though a few models may have inducers made of plastic. Once your furnace inducer goes bad, you won't get any heat circulating through your house.
One of the first hints you'll have that your inducer may be damaged is that you don't get any heat when you turn up the thermostat. However, you may have early warnings that signal trouble before you get to that point. A loud banging noise occurs when the fan is bent out of shape or the door covering the inducer is bent inward. The inducer housing may not be set properly or your furnace may be out of balance, causing the loud noise. The banging is quieter but still loud if the inducer is made of plastic. Alternatively, if you don't hear any fan blowing or inducer motor kicking up when you turn on the heat, it may be due to problems in other related parts of the furnace.
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When starting your furnace, if the inducer does not kick in, it may be due to a problem with your thermostat that regulates the inducer motor. The wiring in the inducer motor or in the thermostat may at the root of the problem. The fan may not turn on because of a bad relay switch on the control board or damage to the transformer. All the associated parts must be operating properly before the inducer can operate.
When your inducer is operating but continues to shut off after a minute or so, there may not be enough gas getting to the flame. A hissing sound occurs when the fan shuts down, indicating the ignitor is damaged or another problem exists in the relay board. The inducer has an automatic shutoff valve to prevent gas fumes from being blown into your house and goes out to protect you and your family. You may retry starting the furnace up to three times before the inducer locks down completely.
Before replacing your inducer assembly, you should check out all the other possibilities as to why you don't have heat. Hoses attached to the inducer often become clogged and cause the inducer to shut down. When condensation builds up in the vent pipe, the inducer can go out. Feel the inducer motor when you run into difficulties with the fan to verify the motor is at fault. When an inducer goes bad, it will be hot to the touch after trying several times to start the furnace. If the motor remains cool, your problem is most likely in one of the peripherals attached to the motor or in the electrical parts that drive its operation.