Skip navigation

Serving the Greater New Braunfels, TX Area

Serving the Greater New Braunfels, TX Area


Why Your Furnace Keeps Shutting Down Early


Your furnace is designed to start up when the thermostat registers that your home has reached a specific lower temperature, run until it reaches a set higher temperature, and shut off. These cycles should last 15 or even 20 minutes and then repeat. Sometimes a furnace begins a heating cycle, shuts down too soon, and tries again just a few minutes later. This is called short cycling, and indicates that there’s a problem with your heater.

Short cycling has a variety of possible causes, and you should get to the bottom of it promptly, because it can cause additional problems with your heater. What could possibly be wrong? We’re so glad you asked.

Causes of Short Cycling

  • Clogged Air Filter: If the filter that’s meant to collect dust has been unchanged for too long, all that dust causes an obstruction. Insufficient air will be passing through, which means the temperature inside gets too high. For safety reasons, your heater will shut down to avoid overheating and creating a fire hazard. Cleaning the filter is something you can do yourself, and should be doing every 1-3 months to prevent this problem.
  • Miscalibrated Thermostat: If incorrect signals are being sent from the thermostat to the furnace, it may be shutting down at what it registers as the correct temperature. In this case, there’s no problem with the furnace at all! Having your thermostat recalibrated will correct the problem. 
  • Broken Limit Switch: This is the component that tells the furnace to shut down before it overheats. It’s possible for a failing limit switch to send incorrect signals and shut the furnace down when there’s no need to.
  • Cracked Heat Exchanger: This is the most alarming possibility, because the heat exchanger contains the combusted gasses and prevents them from escaping into your home. A crack in the heat exchanger can draw too much oxygen into the furnace and cause the burner to flare up much hotter than it should, creating the heat that trips the limit switch and shuts the furnace down. Because of the risk of carbon monoxide poisoning, it’s critical to address this immediately with furnace repair in San Antonio, TX.
  • Incorrectly-Sized Furnace: If you’ve had your furnace for a while and the short-cycling problem is new, it’s extremely unlikely that this is the cause. But if your furnace has been short cycling all along, it may be that it’s simply too big for the job. This is why our technicians carefully assess your home and calculate the size of heater that will do the job just right, so it’s very important to have an experienced professional help select and install your furnace.

Effects of Short Cycling

If this symptom is allowed to continue, it can become the cause of other issues, leading to a more expensive repair. The startup is the most energy-intensive part of the heating cycle. When it happens far more frequently than it should, it puts a lot of strain on the components of your heating system. This leads to wear and tear, and even burnout or failure of major components. Also, that extra energy being used is also wasting fuel, costing you a lot more on your utility bills. Don’t let one problem lead to a host of others! Take action today.

At Thayer Air Conditioning, your comfort is our #1 priority! Contact us today.

Comments are closed.