How to Fix a Gas Furnace That Short Cycles

Service technician testing a furnace.

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Identifying and addressing an overactive, or short cycling, the HVAC system will help you avoid increased energy costs and long-term damage to your home and furnace. Here’s a quick guide to help you diagnose and fix your overactive furnace.

Identifying Short Cycling

Short cycling is a malfunction that causes your furnace to turn on for short periods of time, normally between four and seven seconds. If you notice constant, short bursts of activity, you’re likely experiencing short cycling. The causes of short cycling vary from minor malfunctions to unit-replacing problems. Here’s a look at common short cycling causes:

Thermostat Malfunctions

Broken or poorly placed thermostats can cause short cycling. Thermostats near heat sources or in direct sunlight can warm too quickly, signaling your furnace to shut off. You should also test your thermostat to ensure it’s working correctly. Testing your thermostat can involve handling live wires. Call a pro if you’re unsure about performing the tests yourself. Many homeowners forget to switch their thermostats over to heat mode. This can cause short cycling depending on your temperature setting. Make sure your thermostat is in the "heat" position before making any changes.

Rusted Flame Sensors

Burning fuels produce trace amounts of water, which can cause certain metals to rust. A buildup of corrosion can cause irregular furnace cycles and pose a threat to your home. Cleaning your sensor will require working with electricity and flammable gasses. Call a professional to inspect and clean your flame sensor.

Dirty Air Filters

Dirty air filters restrict airflow and cause backups. The blocked warm air that remains in your furnace raises the internal temperature, causing it to shut off. The lack of warm air entering your home lowers the temperature and leads to extra furnace cycles. Replace your air filter at the beginning of each season to avoid blockage-related short cycling.

Always consult your owner’s manual before changing your air filter. If you have any questions, don’t be afraid to call a furnace professional. Mistakes in installing your new filter can lead to additional short cycling.


Modern furnaces have built-in safety devices that monitor temperatures. Faulty temperature-monitoring features can cause your furnace to overheat and turn off. Constant overheating is a common source of short cycling.

A damaged heat exchanger can also cause short cycling. Your heat exchanger transfers heat from one medium to another. In some cases, a cracked heat exchanger releases heat into other parts of your furnace, causing overheating and frequent on-off cycling.

Oversized Furnaces

Overpowered furnaces heat smaller homes quickly but unequally. This results in short cycling in order to maintain consistent temperatures throughout your home. Poor insulation will also cause short cycling with an oversized furnace. Furnaces designed for larger homes produce ample heat but will lose most of their energy through cracks, holes, or faulty seals. The massive amount of energy produced and lost via sub-par insulation causes your furnace to short cycle to maintain a constant temperature.

What Is a Faulty Seal?

If a seal runs dry or gets installed improperly, it may be faulty, which can be quite dangerous. A faulty seal may lead to heat accumulation, causing fires or thermal shock.

Addressing Short Cycling

It’s important to pinpoint the exact cause of your short cycling furnace before making any changes. In most cases, it’s best to hire a pro to examine your furnace. A professional HVAC technician will identify your problem and offer the appropriate solution. Fixing a short cycling furnace requires working with electrical and gas systems, which can result in serious injury to yourself, your furnace, and your home if handled incorrectly.

Why It’s Important to Fix Short Cycling

The advantages of eliminating short cycling fall into three categories:

  • Energy: Short cycling results in your furnace being in almost constant use. Non-stop running means reduced energy efficiency and increased utility bills. Additionally, if your home suffers from poor insulation, the hike in energy costs could be severe.
  • Safety: Short cycling can be the sign of a larger, more serious problem. Some persisting furnace issues can threaten the safety of your family and home. It’s important to immediately address any short cycling to avoid problems in the future.
  • Convenience: A short cycling furnace will result in unsteady temperatures in your home—too hot or too cold temperatures are uncomfortable for anyone. A malfunctioning furnace will also cause additional financial burdens with extra energy costs and repairs.