9 Common Reasons Your Furnace Isn't Turning On (And What To Do About It)

Cold temperatures outside are to be expected during the winter months. However, it becomes an entirely different story if your home is frigid, too. We often take our furnace for granted until something causes it to stop working. Beyond the obvious frustration of trying to stay warm in a cold house, a furnace that isn't turning on can also elevate fears of expensive repair or replacement costs, further adding to our concerns.

There are several reasons why your furnace may not be turning on. Some of them are relatively simple fixes that you may be able to address yourself, while others will likely require the help of a professional. To help us learn more about what can prevent a furnace from turning on as it should, and what should be done to remedy the problem, we reached out to three heating and cooling specialists. Continue reading to learn more from their exclusive interviews with House Digest to help you diagnose your furnace issues.

Sometimes, the reason your furnace isn't working isn't actually related to the furnace itself, but the thermostat. Your home's thermostat is connected to the furnace, and it's supposed to monitor the temperature and tell the furnace when it should kick on to begin heating the space. However, if something is interfering with that communication process, the furnace won't receive the sign that it should switch on.

"Always start with checking the thermostat settings," Glenn Gault, the CEO of Gault Heating & Cooling, shares with House Digest. If the thermostat is set to a colder temperature than the current temperature of the room, it isn't going to send the signal that the heat needs to be turned on. (We understand this is common sense, but it's still worth checking!) Beyond not being set to the desired temperature, there are other potential thermostat issues that could prevent your furnace from turning on. The thermostat will stop working if it needs new batteries or if there is another electrical issue, such as loose or damaged wiring. Ruling out any potential thermostat issues can save you a lot of frustration down the road — either you're able to find a quick fix, or you're at least able to realize that the problem you're facing isn't with the furnace itself.

Your circuit breaker — or in older houses, fuse box — provides you with access to the various power lines in the home. It has switches that make it possible to cut the power to specific appliances or sections of the house. However, sometimes these switches trip on their own, cutting off power to the furnace. For this reason, Alex Atkinson, an HVAC Specialist at Super Brothers, tells House Digest that, after verifying that the thermostat isn't the problem, "one would want to check the circuit breaker or fuse box to make sure the furnace is pulling power."

Checking the circuit breaker before trying to identify or troubleshoot any issues with the furnace itself can also save you time and frustration. If the breaker is tripped, you can easily switch it back on to resume the supply of power to the furnace. However, if the breaker trips again, it could indicate a larger electrical issue or problem with the furnace. This is a reason to hire a professional to make sure there are no safety hazards. They can also verify that your furnace has a dedicated circuit, which can help prevent it from overloading the existing circuit, which may be shared with other household devices.

A burnt out fuse is another (relatively) simple problem to address. Fuses are designed to prevent too much electric current from flowing into the unit, as this can damage a furnace. The fuse only allows a set amount of current to flow to the unit (either 3 or 5 amps, depending on the fuse). The fuse will blow if the current is greater than what is allowed, turning off the rest of the appliance and protecting it from damage. (Coincidentally, this is also one of the most common air conditioning failures.)

Gault recommends checking the fuse if your furnace isn't starting. He says, "If it's discolored, blackened, or has a broken wire inside, you need to replace it." Fortunately, you can pick up a new fuse and swap it out with the blown one in your furnace's control board. This should get the unit back up and running. However, if the new fuse also blows, it can signal that you have a more serious electrical issue and that you should hire a professional to assess the situation.

If there is buildup on your furnace's burner, it could be the reason your unit isn't turning on properly. Inspecting the burners can help you determine whether this is the cause of your problems. Open the access door and remove the cover for the burner (if your unit has a cover). If you see any residue on it, then that buildup could be to blame for your furnace troubles.

As to how to fix it, Simon Bernath, the founder of FurnacePrices.ca, explains to House Digest, "It is possible to try to clean the burner yourself for instance, but not something that most people should necessarily attempt unless they're really comfortable with it," Instead, after you determine that the buildup is to blame, Bernath recommends hiring a professional to ensure they are cleaned properly.

There are some warning signs that there may be too much buildup. Knowing what to look for may help you catch the problem before it completely prevents the furnace from turning on. These other warning signs include a yellow or orange flame (instead of the steady blue flame), strange noises or smells, reduced heating capabilities, and a higher-than-expected electrical bill.

The flame sensor is an important component of a furnace — it is designed to identify when there is a flame. However, as Bernath explains, the furnace "will quickly shut itself off if the gas is flowing but the flame sensor doesn't detect a flame." This is an important safety measure; if there is no flame, gas could leak and threaten the health of anyone in the home. The problem, however, is that sometimes the flame is actually present, but the sensor can't detect it because of buildup of soot or other debris.

Gault explains how you can identify if this is the issue you're facing. He says, "If your furnace turns on for a second but then shuts off immediately, it's likely because of the dirty flame sensor." Gault recommends cleaning the flame sensor with fine-grit sandpaper — like the 3M General Purpose Sandpaper Sheets — or a soft cloth — such as one of these HOMEXCEL Microfiber Cleaning Cloths— to remove any debris that might be interfering with its performance. Before cleaning the flame sensor, you should close the gas valve and turn off power to the unit at the breaker box. The sensor is located inside the access panel, but you'll likely need a ¼-inch hex wrench, like this one from Eklind, to remove it.

If you've already checked everything on your furnace, it is possible that a closed gas valve may be the problem. If natural gas is not flowing to your furnace, then it won't be able to produce any heat in your house. Start by confirming that the gas valve is open. If it isn't, opening it should solve your problem.

There are other potential gas valve issues that could also be preventing your furnace from working. There could be a leak or the line is clogged. These are both very serious issues. "If the valve is already open but you smell gas, call a professional right away," Gault explains. You should also leave your house immediately if you suspect a gas leak. Wait until the fire department or your gas company arrives and clears the house before returning inside.

As you can probably guess based on the name, the ignition system of a furnace has the important job of lighting the burners to supply heat to a home. If there is an issue with the ignition system, it's not surprising that the furnace isn't going to be working properly. According to Gault, "If your furnace makes an unusual noise like clicking and then it shuts off, there's probably a bigger issue like a faulty ignition system."

Gault doesn't recommend fixing this yourself. Instead, he suggests hiring a professional to fix it. Some potential problems that a technician will likely look for include damage to the electrical wiring or cables, gas supply interruptions, or if your furnace pilot light went out.

Furnaces rely on several electrical components for proper function. If something goes wrong with one or more of these components, then it can interfere with the furnace being able to turn on and heat your home. Gault identifies one thing to look out for when trying to diagnose your furnace issues. He says, "If your system makes a buzzing sound when you start it and then shuts off, it's likely due to electrical problems like the control board or bad wiring — only a professional can fix it." Before calling a professional, he recommends giving the whole system a reset. Sometimes, he notes, this can fix it.

If resetting the system doesn't help, furnace electrical issues are better left handled by a professional in most cases. The furnace technician you hire will examine the various electrical components to identify the reason your unit isn't working properly. One component that they'll check is the control board. This part of the furnace is responsible for making sure everything is working together properly — such as ensuring that the burners heat up when the thermostat indicates the need to do so. If something is malfunctioning, the control board won't be able to ensure that the various functions are properly coordinated, which can prevent the furnace from kicking on as it should.

One other electrical component they may check is the capacitor. This part of the unit is responsible for starting the furnace's motor and keeping it goingas. When it fails, the motor won't be able to start, which will mean the furnace isn't going to be able to do its job. If the capacitor needs to be replaced, the technician you hire should be able to take care of it for you.

There is often a simple explanation or a quick fix for why a furnace isn't turning on. However, as Atkinson explains, sometimes "the furnace can be too old and may need replacement parts or outright replacement." If you ruled out the potential causes shared above and your furnace still isn't turning on, prepare yourself for the possibility that it might be time to purchase a new one or to perform a more costly furnace repair.

The average lifespan of a furnace can vary, but most typically last between 15 and 20 years. The cost to purchase a new one averages out to just under $5,000, but various factors, including the size of your home, whether you need an electric, natural gas, oil, or propane furnace, and the labor costs, can impact the final price. You may also have to account for additional costs, such as having new duct work installed or securing a new permit in some cities.