Furnace Keeps Turning On And Off? (Possible Causes & Fixes)

You’ve spent a lot of money on your home’s HVAC system and smart thermostat. That makes it even more frustrating when for some reason, the furnace keeps turning on and off. One minute hot air is blasting from the vents, and the next it feels like you’re in a cryochamber. What is the explanation for this annoying situation, and what can you do to fix it?

Your furnace probably keeps turning on and off because of a dirty air filter or flame sensor. You may also have an electrical issue or a faulty thermostat. Check the air filter and clean or replace it as needed. Check and replace any blown fuses in the electrical panel. And if these don’t work, you may need to replace your thermostat.

When a heating system is working properly, it turns on and off around 3-6 times an hour. If it’s more rapid than that, your furnace is short-cycling.

There are a few different reasons your furnace might keep turning on and off. It is most likely related to poor airflow. But it may also be a thermostat or electrical issue.

Whatever the reason, you’ll want to troubleshoot it as soon as you can. That way your electricity bill doesn’t skyrocket, and your home climate stays controlled. Here are common reasons your furnace may be turning on and off, and simple fixes.

Note: Before doing any work, cleaning, or troubleshooting, shut off gas and electricity to the HVAC.

Air filters don’t have an infinite lifespan. Depending on the type, the average air filter lasts between 30 days to 6 months. If you’re not changing it out regularly enough, you may have a clogged air filter.

When the air filter is blocked, the furnace doesn’t have proper airflow and can get overheated. It cannot effectively detect the temperature in your home, so it short-cycles. If your furnace won’t turn on automatically or is frequently turning off and on, check the air filter for dirt and grime.

If you haven’t replaced it in 30 days to 6 months, it’s time to clean or replace it. Depending on the type, quality, and size, air filters will cost anywhere from $5 to $100 to replace. It’s an affordable alternative to the more serious damage a long-term clogged air filter causes to your HVAC system.

When your furnace shuts on and off repeatedly — something called short-cycling — it’s likely related to airflow. Like a dirty air filter, blocked air vents affect the proper circulation of airflow through the furnace. Low airflow to the furnace confuses temperature readings.

Keep the air vents in your home opened so air can circulate. Closed vents can cause warm air to build up in the furnace. It is also a good idea to check for any foreign or large objects that may be lodged in the air vents. Major blocks in the vents will affect the climate control accuracy for your HVAC.

Another cause of a furnace shutting on and off is a dirty or corroded flame sensor. A flame sensor is a thin metal rod on the burner assembly of your furnace. It is an important safety feature for your HVAC system. It detects the flames from when the pilot light ignites the gas chamber.

If it does not detect the flame, it powers everything down to prevent dangerous gas from leaking into your home. When the flame sensor collects too much soot, it may not be able to detect the presence of a flame. This shuts down the heating system, preventing warm air from cycling through the furnace.

Your flame sensor will need to be cleaned or replaced if it’s corroded. You can complete this process yourself, but it isn’t the most simple. If you’d like to do it yourself, Here are the basic, simplified steps to cleaning the flame sensor:

Another culprit of a haywire furnace is a faulty or damaged thermostat. The primary function of a thermostat is to send signals to the furnace about what temperature to set. The thermostat lets it know to release warm air, or to shut off.

If the thermostat has faulty wiring or a dying battery, it cannot properly communicate with the furnace. It will either not be able to send any signals to the furnace, or it will send incorrect temperature readings.

So how do you know if you have a faulty thermostat? The main tell-tale sign is a short-cycling furnace, or a furnace that does not respond when you make temperature setting changes. You can also take these troubleshooting measures:

You can do this yourself, just make sure you buy the exact brand your furnace requires. Ensure it’s compatible before installing.

Always proceed with caution when performing electrical installations. If you’re not familiar with the process, you should call in an electrician to do the fuse replacement.

Related Questions

Why does the furnace fuse keep blowing? 

Loose wires in the furnace can cause an electrical short and a blown fuse. Worn furnace components may also contribute to a blown fuse. In more rare instances, if a thermostat wire touches the metal frame of a furnace, it can deliver a higher voltage than can be handled. Also, incorrectly installed fuses can blow.

What furnace filter do I need? 

There are important factors to consider when purchasing a furnace air filter. The size of the filter cabinet, the efficiency level, and pleated or non-pleated style are all important to consider. Filters with the highest MERV (efficiency) rating will cost the most, but will also purify the air the best. Pleated filters last longer. And check the required filter size for your specific furnace. The door of your filter cabinet will indicate the correct measurements to purchase.

Why is the thermostat blank? 

If your thermostat is blank, it’s likely due to a dead battery. When the thermostat battery dies, the display lights will go dark and the batteries will need to be replaced. In certain cases, a blank thermostat may also indicate a tripped circuit breaker. When this happens, reset the circuit switch.