Why Is My Furnace Short Cycling?

Short cycling is when your furnace turns on, only runs for a minute or two, then shuts off. After a short time, the furnace turns on again, and the fast on/off cycle continues.

Because the heating cycles do not last very long, a short-cycling furnace will struggle to heat your home to your desired temperature. Not only does this make your home feel less comfortable, but it also leads to higher energy bills and more frequent repairs.

The most common issues that cause your furnace to short cycle include:

• A dirty air filter
• An ignition problem
• A thermostat problem
• An oversized furnace

We’ll explain each of these issues in more detail to help you determine why your furnace is short cycling.

Want a Portland pro to repair your furnace quickly? Call Four Seasons today at 503-538-1950. We provide convenient scheduling times and fast, same-day service for every furnace repair.

The most common cause of a short cycling furnace is a dirty air filter.

Here’s why: A dirty air filter blocks airflow into your furnace, which can make the heat exchanger (the part that heats your home’s air) get overly hot. When the heat exchanger starts to overheat, the furnace will shut itself off to protect the heat exchanger and other components from damage.

Eventually, when internal temperatures return to safe levels, the furnace will turn back on. But if the dirty filter remains, the furnace will continue to overheat and turn on and off *frequently.

*Newer furnaces may restart 3 or 4 times and then shut off for good until a technician resolves the problem.

Solution: Check your air filter. If it is dirty, replace it with a new one and let your furnace run normally. If the short cycling issue persists, you may have one of the issues below (keep reading).

Your furnace has an ignition system that lights the furnace burners. The burners essentially create the heat that warms your home’s air.

That said, if the ignition system is faulty and does not light the burners correctly, then the furnace may shut itself off to prevent gas buildup in the combustion chamber. After the excess gas dissipates, the furnace will attempt to start again. Without resolving the ignition issue, this start/stop cycle will continue.

Common ignition problems that lead to short cycling include:

• A damaged hot surface igniter
• A pilot light that won’t stay lit (older furnaces only)
• A deteriorated flame sensor

Solution: These problems will require professional expertise to repair. If your furnace is short cycling and you’ve already checked the air filter, contact a professional HVAC technician for further assistance.

The thermostat is the “brain” of your furnace. It sends electrical signals to essential components, telling them when to start and stop. But if the thermostat is malfunctioning, then it may send incorrect signals, which could explain why your system is turning off faster than it should.

For example, faulty wiring or temperature sensors in the thermostat could be causing the problem. Or, in some other cases, your thermostat might be in a location that naturally gets more heat than other home areas, skewing the temperature reading on the thermostat and causing your furnace to turn on and off more frequently than it should.

Solution: Contact a furnace professional to inspect the thermostat. The technician can replace faulty wiring or temperature sensors. To get more accurate temperature readings, they can also ensure the thermostat is away from heat sources such as windows and heat registers.

Have you noticed the short cycling problem ever since you’ve had your furnace?

If so, the explanation could be that your furnace is too big for your home. Oversized furnaces typically heat the home to the desired temperature rapidly and then quickly shut off. This frequent on/off pattern (short-cycling) overworks important motors and components in the furnace, which shortens the lifespan of the equipment and increases the system’s energy consumption.

Short cycling also prevents your furnace from delivering warm air to the far corners of a room because the heating cycle does not last very long, leading to uneven temperatures throughout the home.

Solution: Unfortunately, if your furnace is oversized, it may need to be replaced with a smaller unit that more accurately matches your home. A heating professional can perform a heat load calculation, considering factors specific to your home to determine the right-size furnace for you.

You can learn more about how furnaces are sized by reading our blog, “What Size Furnace Do I Need for My Home? A Portland Tech Explains.”

One of our trusted furnace technicians can quickly find what is causing your system to short cycle and recommend the needed repair. We offer honest, upfront pricing as well as flexible financing options for furnace repairs.

Visit our furnace repair page to learn more about what to expect when you work with us.