Why Your Furnace is Shutting On & Off Repeatedly

In the middle of a frigid winter night, your furnace begins to repeatedly turn on and off. Yet, your home seems to keep getting colder. What is it doing? Furnaces shutting off repeatedly is known as short-cycling. Short-cycling most often occurs when air filters are clogged, air vents are closed, or something else is obstructing the airflow.

This can also be caused by faulty thermostats or duct work issues. When your furnace begins to short-cycle, Damiani’s Comfort Design’s background-checked HVAC technicians will be there. We’ve helped thousands of homeowners across the San Antonio Metro area with their heating problems, including furnace short-cycling.

What Is a Normal Furnace Cycle & How Long Does It Last?

Typically, your furnace is meant to turn on three to eight times every hour on low-temperature days for around 10 minutes per cycle. During the lowest temperatures of a cold day, it can occur more often and take a bit longer. The important thing is that each time your furnace does turn on and off, it has enough time to complete its job.

The full cycle of your furnace includes your unit’s burner igniting, your heat exchanger using the heat to warm the air flowing into the furnace, and then a blower moving the warm air into your home via the ductwork. If your furnace’s cycle only lasts a few seconds before shutting off again, it doesn’t have time to complete its cycle.

What Causes Furnace Short Cycling & How to Fix It

While several things can cause your furnace to short-cycle, we’ve listed the most common explanations and their fixes for your convenience:

Clogged air filters: Filters are meant to improve the air quality of your home. They can become clogged by debris, dirt, or grease. When your furnace’s blower motor can’t draw in enough air, it may overheat and trip a safety sensor known as the high limit switch, which causes an immediate shutoff.

Closed air vents: A blockage in either the heat distribution vent or intake vents can result in your furnace overheating. This is why it’s critical to leave vents open, even in rooms you’re not using as often.

The blower motor is failing: If your furnace or HVAC unit is older, the blower motor may have reached the end of its life. Without a working blower motor, the furnace won’t operate correctly. A simple test is to put your hand up to one of the vents. When the system is running and you don’t feel any air coming out, there’s a decent chance the blower motor is the issue.

Faulty thermostat: At times, the issue doesn’t have to do with your furnace itself. A malfunctioning thermostat can send incorrect signals to the heating unit and tell it to turn off and on. Another possibility is the thermostat sensor is located too close to a heat source, such as direct sunlight. This can cause confusion within your system as to your home’s actual temperature.

Incorrect furnace size: If your HVAC system is too big for your home, this extra power isn’t useful for heating and cooling. If your furnace puts out heat too quickly, each cycle will only take a few seconds.

Dirty flame sensor: Your flame sensor is meant to prevent gas from leaking when the flame is not on. When the sensor gets dirty, it may not accurately detect the flame. This can lead to the furnace shutting down or not igniting at all because it thinks there is no flame present.

Many of these issues can be fixed by changing your air filters, opening vents, or cleaning your flame sensor. However, for issues related to your blower motor, thermostat, or the furnace as a whole, you’ll need an HVAC inspection. One of our qualified HVAC technicians will quickly diagnose and repair the issue. If necessary, we may also suggest a replacement.

Ensure Your Home Is Prepared for Winter With Preventative Maintenance

By investing in your furnace through preventative maintenance, you take the important step of ensuring no system issues come up before the temperature drops. We strive to take the hassle out of maintaining your furnace with Damiani’s Comfort Club Memberships.

During the off-season, our licensed HVAC technicians come in and perform tune-ups to ensure all systems are fully functioning and prepared for winter.

Are you looking for an efficient HVAC technician in the San Antonio area to service your short-cycling furnace? Contact Damiani’s today!