Furnace Short Cycling: Here’s What to Do

Your furnace is one of those appliances you never think about until something goes wrong. Then, when you’re sitting on your couch shivering, it’s a top priority to get it fixed as soon as possible. One especially frustrating problem is when your furnace is short cycling! In this article, we'll cover the common causes and how to troubleshoot them, so you can get your furnace running again.

Our goal is to help you identify and solve the cause of your problem, but at the end of the day, you may need to call a professional to help you. When a furnace short cycles it's an indication that something is not working as it should, so let's jump right in and look at the most common causes to this problem.

Reasons a Furnace Could Short Cycle and Fail to Heat your Home

A furnace deals with hot temperatures and explosive fuels like natural gas. For this reason, the engineers who designed them included plenty of safety mechanisms that quickly shut down the entire system if something goes wrong. 

When your furnace isn’t working properly, it usually has something to do with one of these safety mechanisms. When working properly, these switches and triggers go completely unnoticed. Until, one day they stop your furnace and prevent an emergency from happening.  

What follows is a list of reasons why your furnace might not be working, an explanation of what’s going on, and some advice about troubleshooting the problem.

Your Furnace is Too Large for Your Home

People have a tendency to believe bigger is better, but that’s not always the case, especially with your furnace. A properly sized furnace will blow hot air for extended periods of time. This allows the air to travel and disperse around your home.

If the furnace is too large, it'll heat up the areas near the vents too quickly. The hot air will reach the thermostat and trigger the off switch before the home completely heats up.

If you’re experiencing drastic temperature changes or find that the area near the vents is hot while the rest of the house is cold, then this could be the problem.

Besides causing an uncomfortable environment, an oversized furnace is very inefficient and can create utility bills that are much more expensive than they should be.

If you're experiencing these issues, you’ll need to replace the furnace with a smaller one. This job should be done by a professional. Contact an HVAC technician.

Thermostat is in the Wrong Place

A very common issue, especially when the furnace was installed by an untrained professional, is that the thermostat is positioned too close to a vent. Think about it. If a thermostat is too close to a vent, then the warm air will reach the thermostat and turn off the furnace before the warm air can reach the rest of the house.

If the rooms near the thermostat are warming up but other ones aren’t, this could be the reason why. 

The solution is to simply move the thermostat to a better location. It should be positioned in an area that will experience the warm air after the rest of the house has been heated.

Dirty or Corroded Flame Sensor

The job of the flame sensor is exactly like it sounds, it detects a flame. Without the flame sensor, your furnace could very easily expel natural gas into your home, so it’s a very important system.

If your flame sensor is malfunctioning, your furnace will short cycle, and the heating cycle will end as soon as you hear the whoosh of the flame igniting. 

Even if it works normally sometimes, and then other times the furnace short cycles, you still have an issue with the flame sensor.  A sensor that is slowly going out will trigger inconsistently, which gives you a clue to what’s going on.

A common mistake is to confuse the flame ignition for the draft inducer motor which will start blowing air 30-60 seconds before the ignition.

To troubleshoot this you can remove the furnace cover and wait for the ignition to happen, you'll hear a whooshing sounds as the gas ignites. If at this point the furnace shuts off, the sensor may be out or going out.

The first thing to do is to identify the sensor. You might have to consult your owners manual if you can’t find it on your own. Is it dirty or corroded? If so, try cleaning it, and then test it again. If it still doesn’t work properly, you’ll need to replace the sensor.

This video will help you determine if your flame sensor is working.

Watch the Video

Clogged or Dirty Air Filter

A lot of air passes through a furnace as it heats your home, and it's the job of the filter to make sure that the air is nice and clean. As the filter does its job, it will slowly fill up with tiny particles of dirt and debris. If the furnace is short cycling between 2 to 5 minutes, this is a good place to start your investigation.

The reason a dirty air filter causes short cycling is because it prevents the easy flow of air through the furnace. The build-up of hot air will cause the furnace to overheat and trigger one of the overheating-detecting safety sensors, thus turning your furnace off before it's done what it needs to do.

To troubleshoot this problem, you'll need to determine the location of the air filter and remove it. If it's very dirty, it'll need to be replaced.

When to Change Your Air Filter 

Check your owners manual for filter type and size recommendations, and of course your geographic location will play a part as well. Here's a quick guide to give you a starting point on when to change your filters:

• If your filters are between 1 to 2-inches thick, you should change them every 3-months.
• If your furnace uses a 4-inch thick filter, it should be changed every 6-months.
• A 5-inch thick filter should be changed annually.

Clogged Flue or Exhaust Vent

If your exhaust fan is clogged, the symptoms will be very similar to a dirty air filter. The air flow dwindles and causes a build-up of hot air inside the furnace which triggers a saftey switch. If the furnace is short cycling between 2 to 5 minutes, this could be your problem.

This type of problem can be especially dangerous because when gas is burned it creates carbon monoxide which is normally released through the exhaust vent. 

If the vent is blocked, the carbon monoxide has nowhere to go but inside your home. If you have a gas furnace and don't have a carbon monoxide detector, you could be at risk. We highly recommend making this small investment which has the potential of saving your life.

A common culprit of a clogged exhaust vent is insects or small animals. Birds and bees like to use the end of the exhaust vent to build their nests. Other common blockages are snow and ice, as well as screens.

You might think a screen would be a good idea because it would prevent critters from getting into the vent, but a screen can easily clog up and become the source of the problem which it was trying to prevent.

To troubleshoot this kind of problem, you'll need to identify the exhaust vent and where it goes. Your owners manual should show you where it's located. But if you're somewhat familiar with your furnace, you'll notice that the exhaust vent exits the furnace below the intake vent. 

Some vents exit through the roof, while others exit through the wall. Once you find the exit point, make sure it's cleaned out, and then give your furnace a test run. 

Closed Heat Grates are Overheating your Furnace

If your furnace is short cycling a little longer, say between 5 to 10 minutes, but it is still turning off before it heats the entire home, the solution could be as simple as making sure the heat grates are open.

As your furnace pushes hot air around your home through the ductwork everything heats up. If the hot air can't escape because the grates are closed, the heat will build up to the point that it shuts off the furnace via a safety switch.

The solution is typically pretty simple. Open at least 75% of the heat grates and see if that solves the problem. If all the grates are open, but everything is still overheating, you might need to assess the ductwork itself and see if the installation was done properly. If you've been using this furnace for years without issue, then the ductwork most likely is not the issue.

Malfunctioning Blower Motor

Though less common, it's possible that the furnace is running without the blower motor. This would cause the furnace to overheat and shut off before any warm air can reach the vents.

To troublshoot this problem, wait for the furnace to come on and hold your hand next to a vent. If the furnace is running, but you don't feel air coming out, the blower motor might be malfunctioning.

If this is the problem with your furnace, you'll need to replace the blower motor. This is best handled by a trained professional HVAC technician.