Short cycling is a general-purpose term that describes HVAC equipment that doesn't run for long enough. How long is enough? That depends on your particular needs. As a general rule, your furnace may be short cycling if it routinely shuts down before reaching your thermostat's set point. This behavior usually indicates a problem somewhere in your HVAC system.

However, rapid short-cycling is typically a much more obvious problem. Rapid short-cycling occurs when your furnace briefly turns on and then off again. Depending on the underlying cause, your furnace may shut down before producing any heat or before the house blower turns on. These symptoms typically indicate a more severe problem than "normal" short cycling.

Common Causes for Rapid Short Cycling

Your furnace needs to go through a sequence of checks before heating your home. These checks ensure that the furnace is working correctly, and they're crucial to protect your HVAC equipment, property, and health. Modern furnaces use various sensors to confirm that everything's working before they ignite the burners and turn on the central house blower.

If your furnace quickly turns on and then shuts down again, there's a good chance that it's tripping one or more safety switches. The three switches you'll commonly find on a modern furnace are the draft sensor, flame roll-out sensors, and limit sensors. Each switch checks for a specific condition, and either prevents the furnace from running or shuts it down when it detects anything abnormal.

A furnace that shuts down immediately may have a problem with its exhaust flue or draft inducer motor. These issues will cause the draft switch to remain open, stopping the furnace from turning on. You may hear the draft inducer motor turn on briefly, but the furnace won't ignite. If the furnace turns on for a short while, it may be shutting down to an over-limit condition or a flame roll-out.

Dealing With a Rapid Short Cycling Furnace

If your furnace is briefly turning on and then off again, it's crucial not to attempt to bypass any of the safety switches. At best, these switches are preventing further damage to your furnace. At worst, you may be putting your safety in jeopardy by overriding them. While the problem may be a faulty switch or sensor, you should never assume that this is the case.

Avoid using your furnace at all while it's quickly short cycling. You can turn your heating off at the thermostat or use the shutdown switch near your furnace. A professional technician can examine your furnace and test its safety switches. If the safety switches are working as they should, they can then diagnose the real issue preventing your furnace from providing heat and perform a furnace repair.