What Is Furnace Short Cycling: How to Fix

As Angelinos, we may not be used to snowfall during winter, but that doesn’t mean we are not susceptible to the cold. Furnaces assist with keeping our homes warm and cozy, but sometimes it can experience short cycling. When that happens, homes can’t get warm enough, and you’re left wondering what is furnace short cycling and how can you fix it?

At Monkey Wrench, we’ve repaired and replaced hundreds of furnace systems dealing with short cycling issues. Throughout this experience, our technicians have noticed which problems are the most common causes of furnace short-cycling and have applied different solutions to these issues. 

Here’s a quick look at the top 5 causes of a furnace short-cycling.

• Bad thermostat
• Fan limiter failure 
• Poor igniter 
• Dirty flame sensor 
• Clogged air filter or furnace flue

With this article, you’ll know more about what causes a furnace to short cycle and possible fixes that a technician may perform.

Why Is My Furnace Short Cycling?

Furnace short cycling is when your furnace turns on and off without reaching the set temperature on the thermostat. For example, if you set your thermostat to 73 degrees Fahrenheit, your furnace may turn off a few degrees below the desired temperature. It may continue to cycle on and off but won’t reach the set temperature on the thermostat. 

This occurrence can be inconvenient to your comfort, but does it put your health at risk?

Is Furnace Short Cycling Dangerous?

A short cycling furnace can potentially be dangerous if left as is. The most considerable risk that may occur from a furnace short cycling is a carbon monoxide leak. Carbon monoxide (CO) is a colorless, tasteless, and odorless gas that can be lethal if inhaled.

Carbon monoxide gas is a byproduct of natural gas combustion. Furnaces run on natural gas, and when they start short cycling, their heat exchangers can experience excess wear and tear or blocked vents, leading to a CO gas leak.

Besides a CO leak, other possible effects from short cycling furnaces include.

• Loss in comfort 
• Uneven temperatures 
• Damaged internal parts
• Shorter furnace lifespan 
• Mechanical failure
• Increased energy bills

All of these issues aren’t ideal and can be a huge inconvenience. In some cases, some of these issues are signs that you may need to replace your furnace. But what could have caused your furnace to short cycle in the first place?

Bad Thermostat

A malfunctioning thermostat may lead to your furnace short cycling. Several issues can lead to a bad thermostat, such as loose connections, faulty wiring, or old age. 

Since the thermostat acts as a command center for the furnace, these issues can cause it to misread the temperature. When this happens, the furnace can shut on and off intermittently while failing to reach the desired temperature.

Even placing the thermostat in the wrong place can lead to a short cycling furnace. If it is directly hit by sunlight or a heat source like a radiator, it can trick the thermostat into thinking that the entire home is already at a desired when it’s not. Direct sunlight can cause the thermostat to cycle on and off, causing the furnace to short cycle. 

Now we take an inside look at furnaces and what internal parts can cause them to short cycle. 

Fan Limit Switch Failure

A bad or failing fan limit switch can cause the furnace to short cycle if it cannot function as designed. The primary function of a fan limiter is to determine when to turn on the blower fan that circulates warm air and, as a safety precaution, shut down the furnace if the internal temperature becomes too hot. 

So, if your fan limit switch starts failing, one of two things can happen that can lead to short cycling. The first is that the switch keeps the blower fan on, cooling down a home when the furnace isn’t producing heat. This causes it to power on and heat a home repeatedly. 

The second issue that can occur with the limit switch is it stays in the off position. This means that it isn’t turning on the fan motor to circulate warm air, and instead, it can cause the furnace to overheat and short cycle. 

Besides the fan limiter, a bad igniter may also be responsible for a short cycling furnace. 

Poor Igniter

The igniter is part of what kickstarts a furnace’s heating and combustion process. If the igniter starts to fail because of wear, damage, or defectiveness, it can cause the furnace to short cycle. 

Since there is no combustion, the furnace shuts down immediately and attempts to restart the ignition process, leading to short cycling. 

Another critical part of the ignition process is the flame sensor, which may sometimes be responsible for a short cycling furnace. 

Dirty or Bad Flame Sensor

Your furnace can start short cycling if the flame sensor fails to detect a flame present. Flame detection failure may cause the furnace to shut down and restart combustion. The short cycling can last a few seconds to a couple of minutes. 

Eventually, your furnace may not even start at all since the flame sensor acts as a safety component that signals the thermostat to shut off the gas supply to avoid a leak or a fire hazard. The flame sensor can be damaged or defective, but it can get dirty with soot in most cases.

Soot and other debris can accumulate on different parts of your furnace, which may lead to short cycling. 

Clogged Air Filter or Furnace Flue

A dirty air filter and furnace flue may get clogged and lead to short cycling. Neglecting air filter replacements can cause airflow restrictions that can increase the chances of overheating. Once it overheats, the furnace shuts down and will attempt to restart once it cools off, leading to short cycling since the restricted airflow wasn’t addressed. 

Meanwhile, you’re left without any warmth in your home. Your home will also face problems trying to stay warm if your furnace starts short-cycling because of a dirty furnace flue.

The furnace flue is crucial in providing a vent for exhaust gases to exit the furnace. If it becomes clogged, a sensor switch shuts the furnace off. Your furnace will start short cycling once it attempts to restart. 

A short-cycling furnace can have different causes; therefore, there are multiple possible solutions.

How Do You Fix a Short Cycling Furnace in Los Angeles?

If you notice your furnace short cycling, the best thing to do is call a professional technician. With their tools and resources, a technician can thoroughly inspect your furnace and identify the cause of its short cycling.

Here are some possible solutions to the most common causes of furnace short cycling.

• Bad thermostat: If the thermostat is the cause behind short cycling, then the technician can recalibrate the unit, tighten any loose wiring or connections, or replace the thermostat altogether.
• Fan limiter failure: The most common repair for a bad fan limit switch is to replace it entirely. 
• Poor igniter: A technician can replace the igniter if it’s found to be the cause of your furnace’s short cycling. 
• Dirty flame sensor: If the flame sensor is dirty, the technician can wipe it clean and see if it addresses the problem. Otherwise, a sensor replacement may be necessary for the furnace to stop short-cycling. 
• Clogged air filter or furnace flue: A dirty air filter can easily be replaced. A technician may also recommend a duct cleaning to clear the ductwork from debris buildup. If a technician encounters a clogged furnace flue, removing the obstruction can help stop your furnace from short cycling.

Furnace short cycling can occur randomly in any furnace, but you can take preventative measures. Annual professional furnace maintenance helps protect your furnace from any little issues that can eventually lead to short cycling. 

How to Prevent My Furnace from Short Cycling?

Annual tune-ups and maintenance are some of the best ways to catch an issue before it leads to short cycling. Now that you’ve seen what can cause a furnace to short cycle, you can stay in the loop when a technician comes to inspect your heating system.

From bad thermostats to clogged air filters, we’ve seen various ways a furnace can start short cycling. At Monkey Wrench, our technicians have seen it all. And we’ve prepared ourselves to tackle any short cycling issues by receiving technical training from the top brands in the industry.

If you’re dealing with a short cycling furnace and live in Los Angeles and the surrounding areas, contact us using one of the buttons below. You can also book an appointment directly with our HVAC online scheduler.

In case you need to replace your furnace, learn more about how much a furnace installation costs.