A malfunctioning thermostat can be a reason that a furnace blower won't shut off. If your thermostat fails or becomes disconnected, it will likely cause the blower to stop running. However, if the thermostat sends a signal that the home is cooler than it actually is, the furnace and blower will both continue to run indefinitely. This can happen if a digital thermostat experiences a computer error or if any type of thermostat has its temperature gauge moved and is set too high.
Fan and Limit Controller
The fan and limit controller is an electronic furnace component that regulates the blower. Some fan and limit controllers have manual switches, which will prevent the blower from running or cause it to run continuously. If this switch becomes engaged during cleaning or repairs, the blower will continue to run until it is returned to its automatic setting. A faulty fan and limit controller, or corroded wiring leading to or from it, can also cause a blower to not shut off.
A furnace blower that won't shut off can cause several problems. A running blower is noisy and uses electricity. If the furnace continues to run, it will eventually heat the home to an uncomfortable temperature and consume an excessive amount of fuel, raising home heating bills. If the furnace shuts off and the blower continues to run, it will soon start circulating cool air throughout the house, forcing the furnace back on prematurely.
If your furnace's blower continues to run for an extended period of time, your first step should be to check your thermostat. Set it to its lowest temperature and wait for the blower to shut off. If this doesn't work, the issue is likely with the fan and limit controller. Consult the owner's manual to locate it and check for a switch that places it in automatic mode. As a last resort, you can shut off your furnace's blower by cutting off the power to the fan and limit controller.