Why Does My Furnace Keep Turning On and Off?

Over the lifespan of your furnace, you’re likely to experience issues from time to time. If you’re asking yourself questions like, “Why does my furnace keep turning on and off?” it’s a key indicator of an underlying problem. Understanding the potential reasons this may be happening will allow you to act swiftly to restore functionality to your furnace.

What Is Short Cycling?

When your furnace is running properly, it will turn on and run through an entire heating cycle. Most furnaces run about two to three cycles per hour. If you notice that your furnace is turning on and shutting off a few minutes later, it’s referred to as short cycling. While this may seem like a minor problem on the surface, it creates numerous issues. It will increase your energy bills, reduce your comfort, and likely lead to more expensive repairs.

Dirty Air Filter

One of the most common reasons your furnace may experience short cycling is that your air filter is clogged. The air filter is responsible for trapping unwanted airborne debris from reaching the internal components of your system. Unfortunately, when your air filter gets packed full of debris, it restricts the air through your ductwork. When air is unable to freely move throughout your ducts, heat will back up inside your furnace.

Your furnace has a built-in safety switch called a limit switch. It’s responsible for turning your furnace off whenever it detects that it’s overheating. This is necessary to prevent further system damage and even an electrical fire. It’s always a good idea when you experience short cycling to remove your furnace filter and see if it needs to be replaced.

You can check your filter by holding it up to a light. If you can’t see through the filter, you need a new one. Realize that if you have a high-efficiency filter, you’ll need to replace it more frequently than lower-quality filters. As soon as you put in a new air filter, your furnace should be able to run through an entire heating cycle without a problem.

Malfunctioning Limit Switch

Sometimes, the reason your furnace may be short cycling is that your limit switch is malfunctioning. Normally, its job is to shut your furnace off when there’s an overheating problem. Unfortunately, if it’s shutting your furnace off even when there’s no overheating problem, it can cause short cycling. It’s necessary to call for professional furnace repair services if you suspect that you have a faulty limit switch.

Closed or Blocked Air Vents

It’s important to note that your furnace is set up to operate with all of the air vents completely open. If you close multiple air vents, it can mess with the air pressure inside your ductwork. This can lead to overheating of your furnace due to improper airflow. Whenever you experience frequent furnace cycling, it’s best to evaluate your air vents. Ensure that every vent is open and unobstructed. Having items like plants or furniture right in front of your air vents can restrict airflow.

Flame Sensor Problem

All furnaces that burn propane, natural gas, and even heating oil have a flame sensor. This is a safety device. It’s responsible for detecting when there’s a flame on the burner. Whenever your unit is running, the flame sensor should always be detecting that there’s a flame. If it doesn’t, it will automatically shut off the fuel supply.

This is to prevent problems with gas buildup if your burner isn’t working. Unfortunately, your flame sensor may malfunction from time to time. Sometimes the flame sensor may simply be faulty and in need of a full replacement. In other cases, the sensor eye may be covered in soot and falsely detect that there’s no flame when there is. In these latter cases, we can typically clean the sensor eye and restore functionality to your system.

Blocked Exhaust Vent

All fuel-burning furnaces require proper ventilation for their exhaust fumes. These exhaust fumes are the byproduct of the combustion process and are toxic when they build up in your home. If your vents are blocked, it can cause your system to shut off as a safety precaution. It’s common for exhaust vents to get blocked by bird nests, twigs, leaves, and other debris. It’s important to clear out the blockage before turning your furnace back on.

Reliable Furnace Repair Service

McCrea Heating & Air Conditioning offers reliable furnace repair services in Northern Virginia. We can also assist with all your furnace maintenance, installation, and replacement needs. Call McCrea today to schedule your next appointment.