Picture this: Your furnace seems to be running normally. But suddenly it shuts down, after barely heating your home!
So what just happened?
If your furnace keeps turning off, there is most likely an issue with the thermostat settings or airflow in your furnace. Check the thermostat to ensure that your furnace is in the correct mode, and change the air filter so your furnace gets adequate airflow.
In this article, I’ll go over a few different causes of a furnace that keeps turning off. I’ll also go over what you should do if your furnace keeps turning off.
What you should do when your furnace keeps turning off
If your furnace keeps turning off or short cycling, it’s almost always due to a safety or control issue.
I’ve broken down the possible causes into a list of things that you should check if your furnace keeps turning off.
Here is a list of what you should do if your furnace keeps turning off:
- Change the thermostat setting
- Check the control board for errors
- Change the air filter
- Open your home’s air vents
- Clean the flame sensor
- Clean the condensate drain pipe
- Clear the exhaust vent
Change the thermostat setting
The thermostat is the first thing that you should check if your furnace keeps turning off.
If your thermostat’s fan mode is set to “Auto”, then the blower will cycle on and off as your home reaches its temperature setpoint. This is completely normal behavior.
If this is what’s happening, you can switch the thermostat’s fan mode to “On” to keep your blower running all the time. However, your furnace will consume more energy since the blower runs continuously—even when your home doesn’t need any more heat.
Check the control board for errors
The next thing to check is your control board. Your furnace’s control board should have a diagnostic indicator light. The light will blink in a certain pattern depending on the status of your furnace.
For example, a single blink of the light will indicate normal operation.
If your control board’s diagnostic light is blinking in an erroneous pattern, check the error code chart to see what the issue is.
Once you know the description of the error, you’ll be able to diagnose the problem with your furnace.
It’s worth mentioning that the control board itself could be the issue. The control board’s relays turn on parts such as the blower, inducer, and gas valve.
If any relays go bad, the control board will need to be replaced.
For more information about troubleshooting your furnace’s control board, check out my article below:
Change the air filter
If your control board isn’t showing any errors, the next thing that you should do is change the air filter.
Airflow restrictions from a dirty air filter are one of the most common causes of short cycling in a furnace.
If your air filter is dirty, your furnace will have less airflow. If there is an excessive amount of airflow restriction in your furnace, it could keep turning off. Why does that happen?
In the furnace, there is a device called the high limit switch. The high limit switch’s job is to shut off the furnace’s burners if the temperature inside the furnace gets too high.
When there is excessive airflow restriction, the temperature inside the furnace increases since there isn’t enough airflow to blow the heat into your home. If the temperature in the furnace gets too hot, the high limit switch will trip and turn off your furnace’s burners.
A tripping high limit switch is a common cause of a furnace that keeps turning off.
One of the telltale signs of a tripping high limit switch is a furnace that runs for a few minutes and then stops heating. If your furnace is exhibiting this behavior, there’s a good chance that the high limit switch is tripping due to airflow issues.
Open your home’s air vents
Closed air vents in your home are another cause of airflow restriction. Similar to a dirty air filter, closed air vents will starve your furnace of airflow and cause its high limit switch to trip.
If you’ve closed off some air vents in your home and the furnace keeps turning off, open those vents back up. Your furnace needs sufficient airflow to work properly and you’re only going to cause damage to your furnace in the long run if you choke off airflow in the system.
Clean the flame sensor
When the burners light up in your furnace, the flame sensor detects the flame to ensure that the burners are successful in firing up.
If the flame sensor doesn’t detect any flames, your furnace will turn off to ensure that unburned gas doesn’t leak into your home.
But what happens if the flame sensor gets dirty?
When your furnace’s flame sensor gets dirty, it won’t be able to sense the flames from the burners. The result is a furnace that keeps turning off—even when everything is ok.
The easiest way to tell if you have a dirty flame sensor is to watch your furnace turn on. If your furnace starts up normally but turns off quickly after the burners light up, there’s a good chance that the flame sensor is dirty.
Sometimes the flame sensor may be just dirty enough where it works—but only for a little while. This will cause your furnace to short cycle.
Check out my article below for more information on how to find and clean your furnace’s flame sensor:
Clean the condensate drain pipe
If you have a high-efficiency furnace, it produces liquid condensate. The condensate drain pipe is responsible for draining the condensate from your furnace to the outside of your home.
If the condensate drain pipe gets clogged, then your furnace will turn off. This is a protective measure to ensure that condensate doesn’t leak out all over your home.
If your furnace’s condensate drain is clogged, you’ll need to clear out the drain line so your furnace can drain properly.
Check out my article below for information on how to unclog your furnace’s condensate drain:
Clear the exhaust vent
Your furnace’s exhaust vent directs harmful exhaust fumes out of your home.
How does that work?
The inducer motor sucks in air from the furnace’s combustion chamber, through the heat exchanger, and blows it out of the exhaust piping.
If the exhaust vent isn’t clear, your furnace will turn off. But how does your furnace know to turn off if the exhaust vent is blocked?
The furnace has a pressure switch that’s connected to the inducer. The pressure switch sends a signal to the furnace’s control board when air flows through the furnace’s exhaust system.
If the exhaust vent is blocked, the pressure switch won’t send a signal to your control board. So your furnace shuts down.
Here are a few common things that can cause blockages in your furnace’s exhaust vent:
- Birds nests
- Soot buildup
For more information about what you should do when your furnace’s exhaust vent is blocked, check out my article below: