Short cycling is when a furnace cycles on and off too frequently. It means there is a problem in your heating system’s functioning and efficiency where it’s not completing the heating processes. Not only an inefficient way to regulate the temperature inside your home, but short cycling can also be a sign of major problems in your heating system.

When a furnace is short cycling, it will typically turn on and off more frequently and only run for a short time. It typically takes five minutes for a furnace to complete a full cycle. If you notice that your furnace shuts off in under five minutes, it could mean that it’s short cycling. The same is true if the furnace starts running again almost immediately after shutting off as this should only happen in extremely cold, below-zero weather. If you notice uneven temperatures throughout your home and that your energy bills have increased, your furnace could be short cycling.

Several signs can indicate furnace short cycling:

• Unusual noises, such as rapid cycling or clicking sounds
• Inconsistent heating or if certain rooms feel significantly colder than others
• High heating bills or a constantly running thermostat

Furnace short cycling can significantly reduce your energy efficiency by causing the furnace to repeatedly turn on and off quickly, which wastes energy due to the extra power needed to start up each cycle, leading to higher energy bills and less consistent heating in your home.

Causes and Solutions of Short Cycling
Short cycling can be caused by a range of issues, which vary in severity and necessary repair. The following are some of the most common problems associated with short cycling and what they mean.

• Improper Furnace Size: If your furnace is too large for your home, it may cycle on and off frequently. While this is not a dangerous condition, it could lead to increased energy usage and higher utility costs.
  
  Solution: You should consider having it replaced with one that’s properly matched to your home’s specific heating needs. For efficient and consistent airflow, it’s vital to select a unit that matches the size of your space accurately.
  
  
• Malfunctioning Thermostat: If the thermostat is not accurately sensing the temperature in your home, it may cause the furnace to turn on and off frequently. This can be due to faulty wiring, outdated technology or placement. Placing it near heat sources, direct sunlight or drafts can cause it to inaccurately control the HVAC system, leading to short cycling.
  
  Solution: Make sure your thermostat is working properly and set correctly. You can try setting it to “auto” instead of “on.” A professional HVAC technician can inspect and repair your thermostat to resolve any issues.
  
  
• Dirty/Frozen Coil: Dust or dirt can collect on the evaporator coil, which can make it difficult for it to absorb heat from the air. Condensation can freeze on the coil. Depending on the severity, it may short cycle or stop working altogether.
  
  Solution: Evaporator coils can be an easy fix and are usually the result of poor maintenance. Schedule service with a professional if the coils appear to be frozen or dirty.
  
  
• Blocked Airflow/Dirty Air Filter: Blocked or damaged ductwork can prevent warm air from reaching rooms, causing the furnace to overheat and cycle on and off. If the airflow through a heater’s filter is blocked by too much debris, the system won’t be able to produce heat effectively. It can even put the system at risk of overheating. Regularly replacing your air filters can prevent this issue. 
  
  Solution: Make sure all vents and registers are open and free of debris; repair them as needed or have an HVAC professional inspect your ducts for blockages and leaks. Check and replace your air filter every one to three months.

• Dirty Flame Sensor: The flame sensor senses the presence of flames at the burner and allows natural gas to flow. A dirty flame sensor will shut off the gas valve and the furnace, which prevents natural gas accumulations that could cause a fire.
  
  Solution: Schedule service with a professional if the flame sensor appears to be dirty.
  
  
• Clogged Flue: There’s a chance that the flue can become clogged with debris, foreign objects or an animal’s nest. If the flue detects a blockage, the component will tell the furnace to shut down completely.
  
  Solution: Inspect the flue and if possible, remove any blockages. A professional HVAC technician can inspect the flue to resolve any issues.

Short cycling can have several negative effects, including reducing the lifespan of your furnace, higher energy bills due to decreased energy efficiency, and can lead to more severe repairs in the future. If you’ve checked these common issues and the furnace continues to short cycle, it might be time to call a professional technician for an inspection.

How to Prevent Short Cycling
Scheduling regular furnace tune-ups with a certified HVAC professional is key to identifying and addressing potential issues before they escalate. During these inspections, technicians will thoroughly examine your furnace, replace dirty filters, check electrical connections and ensure proper airflow. By detecting and resolving any underlying problems, you can significantly reduce the risk of short cycling.

When it comes to furnace issues, it's important to know when to seek professional help. While minor problems can often be resolved with simple troubleshooting, certain indications warrant the expertise of a certified HVAC technician, such as overheating, improper sizing, a clogged flue and refrigerant leaks.

When searching for a reliable furnace repair service near you, it's important to look for a company that is licensed and insured, as this indicates their commitment to quality and professionalism. Reading customer reviews can also give you insights into their reputation and reliability. You should also consider their response time and availability since furnace issues often require attention quickly to minimize discomfort and prevent further damage.

If you experience persistent short cycling issues, you should contact your local Lennox Dealer for further assistance.

This article has been reviewed by Lee Butler, Senior Product Marketing Manager, for accuracy. Lee brings 12 years of expertise in the HVAC space.