Furnace blower not working

Two opposite symptoms

The blower will not run. The burners light, the furnace gets hot, and then it shuts itself down on its high-limit switch because nothing is carrying the heat away. Little or no air comes from the vents. In summer the air conditioner will not move air either, because it uses the same blower.

The blower will not stop. Air keeps blowing after the burners are off, sometimes cold air all the time. Check the thermostat first: a fan setting of On, rather than Auto, runs the blower continuously by design. If the fan is on Auto and it still runs, a relay on the control board or a limit switch is the usual suspect.

What fails

• The blower motor. Motors wear, bearings dry out and windings fail. A motor that hums without turning, or turns slowly, is on its way out.
• The motor's run capacitor, on many older motors. A weak capacitor makes a good motor struggle to start or run slowly.
• The control board or the relay that switches the blower.
• A clogged blower wheel. The motor is fine but the wheel is so caked with dust it moves a fraction of the air it should. It sounds normal and performs badly.
• The belt, on some older furnaces with belt-driven blowers.

What you can safely check

The thermostat fan setting. The filter, because a clogged filter makes a good blower look weak. And whether the furnace's lower panel is fully seated: on most furnaces taking it off cuts the power, and putting it back carelessly can leave it off.

Do not reach into the blower compartment. The motor can start at any moment, and on many furnaces there are high-voltage terminals within reach.

What a technician checks

The motor's current draw against its rating, the capacitor's value, the voltage reaching the motor from the board, and the condition of the wheel. Together those separate a failed motor from a failed part that only makes the motor look failed, which is the difference between a large repair and a small one.