Why Does The Furnace Fan Motor Keep Running
There are only a few things that can cause this and this is usually a safety mechanism built into modern furnaces. To better answer this question we need to narrow this down into 2 categories.
We will start with the easy stuff first. The fan motor will run continuously when the thermostat is set in the fan on position. We receive service calls every year when a homeowner misunderstands the switch marked fan (off-auto-on). This only controls the fan and the fan will come on automatically when a call for heating or cooling. This does not turn the heat on. Leave the switch in the auto position.
On very old furnaces, pre 1980, there can be a summer switch in the furnace. It is usually located where the power comes into the furnace and is a simple rocker switch. If your fan motor is running continuously the switch could be in the on position.
Above the burner chamber there is a fan switch that can turn the fan on in the event that the heat exchanger warms up past a set temperature, typically 120-140 degrees. If this switch sticks in the on position the fan will not turn off.
Most older furnaces do not have a problem operating if the neutral and power wires leading into the furnace are backwards. Backwards means that the neutral line is energized with 120v incoming instead of the power line. Bear mind that once the motor is operating the neutral has 120v so this situation is not dangerous by itself. This can pose a problem in the event that the motor has a broken winding. If one of the windings on the motor shorts out to ground it completes the circuit. The neutral line goes directly to the fan motor and only the power line is switched in the furnace. When the fan switch engages the circuit completes its path and the motor turns on. If the neutral is powered and the motor is shorted this completes a path and the motor turns on and will not turn off until the furnace is unplugged. The motor will usually run at a much slower than normal speed. The motor must be replaced.
The fan motor will run continuously when the thermostat is set in the fan on position. We receive service calls every year when a homeowner misunderstands the switch marked fan (off-auto-on). This only controls the fan and the fan will come on automatically when a call for heating or cooling. This does not turn the heat on. Leave the switch in the auto position.
There is a relay in the furnace, sometimes on the circuit board, that turns the fan motor on when a call for heat is made based on time usually 60-90 seconds after the gas valve energizes. If this relay sticks in the on position the fan motor will run continuously.
The limit controls, flame roll out switch, vent switch, high limit switch are all safety controls. If these are activated the furnaces circuit board thinks the furnace is burning up. To combat this, the circuit board will turn the blower on until the system cools off. If the limit switches do not reset the motor will run all the time.
If there is any break in the 24 volt safety circuit in the furnace the motor will run all the time. This includes the fuse, the transformer, and disconnected wire and the circuit board itself.
The main difference between hot surface ignition systems and electronic spark systems is the method in which they ignite the flame. A spark ignition system uses a pilot assembly and ignites the pilot. A hot surface ignition system uses a glow plug that operates on 87 to 120 volts. It glows white hot and the gas valve release gas to either ignite the main burners or a burner spread tube.
If your furnace doesn’t come on, no sparking or no glowing, the problem is electrical in nature. It has to be a problem with the circuit board, the wiring or the thermostat. Any other break in the system is going cause the blower motor to operate 24/7.