If your furnace turns on and blows warm air but it doesn't run very long, the problem might be restricted airflow, a dirty furnace, or a tripped safety mechanism. You don't want to run your furnace when it shuts down too soon, especially if it can't run long enough to warm up your house, or the furnace might be damaged. Here are some possible causes of this problem.

1. The Blower Motor Is Bad

The blower that moves air through your furnace is controlled by a motor. If the motor or parts that help it work go bad, the blower won't blow as much air as it should, or the blower may not work at all, and that can cause the furnace to overheat. Some parts of the motor, such as the capacitor, can be replaced, but if the motor is bad, the motor might have to be replaced too.

2. The Thermostat Is To Blame

Don't forget to check your thermostat before calling a furnace repair service. The batteries might be bad or a setting might be wrong. If you can't find a problem, the repair technician might check the thermostat more thoroughly by opening it and looking for bad connections. If the thermostat is bad, you might need to have it replaced.

3. An Exhaust Vent Is Clogged

A clogged exhaust vent can make your furnace overheat and shut down. A clogged vent might even make your carbon monoxide alarm go off since the gas normally flows out the exhaust vent. The vent can be clogged by snow, tree debris, or even a bird's nest.

The furnace repair technician has to track down the reason your furnace shuts off too soon, and when the clog is found, they can clear the vent and return your furnace to normal.

4. The Furnace Is Dirty

A dirty furnace restricts airflow and prevents proper functioning of the parts. The problem might be a dirty filter, and if that's the only problem, your furnace may start working again after you've put in a new filter. The problem might also be a dirty blower fan that can't produce enough airflow. This situation can cause your furnace to overheat.

Another area of your furnace that can get dirty is the combustion area where the flame sensor is located. If the sensor is coated in soot, it can't detect flames and your furnace will shut down right after it starts.

The solution for a dirty furnace is to have the parts cleaned off by the furnace repair technician. If any parts were damaged because of the furnace overheating, the technician might have to repair some parts too, but cleaning them may be all that's needed.