A furnace that shuts down can leave you in the cold. Fortunately, most issues can be quickly repaired once the cause is discovered.
Types of Furnace Shutdowns
Furnace shutdowns typically fall into two categories — short-cycle shutdowns and total shutdowns. A short-cycle shutdown means the furnace stops running in the middle of a heating cycle before the house reaches the desired temperature. Often, the furnace will come back on a few minutes later, in an attempt to finish the heating cycle. In extreme cases, this will happen repeatedly, with the home never quite getting warm.
A total shutdown can occur mid-cycle or between cycles. The motor will simply turn off and it won't come back on again. Occasionally, the motor may come back on, but it will take much longer than it does in a short-cycle situation. With a total shutdown, the wait is more likely to be hours rather than minutes.
Common Shutdown Causes
For short-cycling, the most common reason is a lack of airflow. Air must be pulled into the system to both be heated and cycled through, and to prevent the motor from overheating. When there is a lack of airflow, the furnace will shut down mid-cycle. It starts back up in a minute or two when airflow is at sufficient levels again. Causes of poor airflow include dirty air filters, blocked heating vents and returns, and an extremely dirty or dusty blower assembly.
There are several things that can lead to a total shutdown. Lack of fuel or an electrical short is common, particularly if the furnace won't come back on. A faulty pilot light or ignition assembly could also be the culprit. Sometimes the issue is with the blower motor assembly itself. Broken parts, worn bearing, or a snapped belt can cause the furnace to grind to a stop. If the furnace starts back up after waiting a while, then extreme overheating of the motor could be the cause.
Short cycling is typically easy to repair. Your technician will inspect the motor for any issues, but usually, a deep cleaning is all that is needed. The air filter will also be swapped out and the filter housing will be thoroughly cleaned. Finally, the ducts and vent openings will be inspected and cleared to ensure airflow is sufficient.
A total shutdown will require a full inspection, beginning with the ignition and electrical systems and finishing with the blower assembly. In some cases, you may just need to have the furnace lubricated and tuned. In other instances failed parts may require replacement.
A heating contractor can inspect your heating system to find the problem.