Your home’s furnace is a hard-working machine. Whether you have a ductless system, heat pump, oil, gas, or electric setup, there’s the potential for problems.
When the heat stops blowing, the furnace makes funny noises, or the unit fails to turn on, it’s time to put on your troubleshooting hat to see if you can fix the problem yourself.
Like most home improvement tasks, some solutions are easier than others. In the end, some projects are best handled by professionals because of a safety concern or a warranty issue.
If you’re really unfamiliar with the work, a mistake can cost you more in the long run than hiring someone to do it right the first time, so evaluate your skills and knowledge.
Before you call in the pros, however, consider some common causes for a moody furnace.
Types of Furnaces
Not all furnaces are created equal. Each type of furnace comes with its own components. Some parts can be found on all types, while other parts may only be applicable to gas, oil, or electric furnaces.
A gas furnace burns propane or natural gas. The device draws in air and heats it before sending it through the home’s ductwork. Burning propane or natural gas generates heat in the furnace's burner.
The heat produced passes through a heat exchanger, making it hot.
Air from the home's ductwork is blown over the heat exchanger, warming the air.
The furnace's blower then forces the heated air into the supply ductwork, distributing it throughout the home.
An oil furnace works in a similar way, drawing oil into the furnace with a fuel pump. The oil is converted into a mist as it enters the burner. Air from the home is heated inside the chamber and released into the home.
An electric furnace is also similar, except heating takes place with the aid of electric elements inside the heat exchanger.
Each system comes with pros and cons around the type of fuel available, the associated costs, climate, life expectancy, efficiency, installation process, venting requirements, maintenance, and more.
In general, though, when discussing troubleshooting for furnaces, they are similar in many ways. Here are some common repairs and easy fixes you can try when addressing issues.
Open Heat Registers
If the furnace is blowing but warm air isn’t entering a space, you may have closed heat registers.
Each system is different, with some registers located in the ceiling and others on the floor. They are also commonly found underneath cabinetry in the kitchen, bathroom, and laundry room.
If your registers have an adjustment dial, rotate it to the open position.
Check Thermostat Settings
Your furnace works in response to the thermostat on the wall. If it doesn’t tell the furnace to go to work, it might explain why the heat isn’t kicking on.
Check the settings on your thermostat. First, make sure the heat is turned on. Someone may have turned off the heat on purpose or by accident. There may be a physical on/off switch or an option on the digital menu.
Next, check the temperature setting. It may have been turned so low the furnace doesn’t know to kick on. Adjust the setting to around 68 degrees. If the temp in your house is warmer than that, it won’t initiate the furnace so go above the room temp until it kicks on and then lower it to a comfortable level.
If you have a programmable thermostat, ensure the settings are correct there too. For example, it may have been set to a lower temperature on Wednesdays, or you may need to adjust the time of day warmer temperatures are expected in the home.
If your furnace is continuously running, it may also be caused by the thermostat. Check that the fan is set to auto. If the fan is set to on, it will continuously run.
It’s also possible your thermostat isn’t compatible with your furnace type. If you’ve recently replaced either the thermostat or the furnace, consider this dynamic.
Your thermostat can also malfunction, causing issues with the furnace. While checking the settings on your thermostat, set the fan to ‘auto.’ Then move the temperature up and down. If the fan continues to blow even at low temperature settings, it indicates a problem with your thermostat.
Verify the Furnace Has Power
If your furnace is up to code, it’s connected to your home’s power via circuit designated for its use. In other words, the circuit cannot supply power to anything else in the house. This makes it easy to check the power to the furnace.
Inside the central circuit box, located the circuit breaker dedicated to the furnace. Flip it off and then back on. Check to see if the furnace then begins to work.
A dirty furnace filter results in inefficient and potentially problematic operation for the furnace. Replacing your furnace filter every one to three months is the number one best course of preventative maintenance you can do.
Keep a few extras on hand. To replace, find the covering for the filter slot. It may be at the top, bottom, or side of the furnace. It may be labeled, and it may not. The door will slide or pop open to reveal the old filter. Remove the old filter and put the new one in place. It’s quick and easy to do.
Check for Obstructions
To start, locate your air intake and exhaust systems for your furnace. These are often on the outside of the home. Take a look to make sure they are not gunked up with any type of debris. Many furnaces will shut off if they sense a blockage.
Next, turn off the power to the furnace and also the gas flow if it is a gas model. Open the access panel. Look for the condensate tubing and make sure there are no clogs in the line. If there is, clean it out or replace the hose.
Clean and Inspect Electrical Components
A loose or void electrical connection can be enough to take the entire system down. If you don’t feel comfortable dealing with electrical components, call in a professional. To tackle it yourself, turn off the power (and gas if applicable).
Vacuum out any dust and dirt surrounding the components. Wipe things down with a moist cloth. Inspect wires and connections. Tighten bolts if needed. Use a cotton swab or toothbrush to get into small areas.
If you have an oiling port for your motor, check the oil level and add oil if necessary. Be sure to use electric motor oil only.
Check your belt too. If it’s loose, tighten it. If it’s cracked or shows heavy wear, replace it. Once the motor components are cleaned and maintained, reattach your panel cover.
Test the Fan Limit Control Switch
This switch is a safety feature that monitors the temperatures inside the furnace and causes it to shut off at the set temperature. If you find your furnace’s blower is constantly running, it may be because your fan limit control switch is malfunctioning. Try resetting the switch or replace it if necessary.
Pilot Light Is Out
Older furnaces rely on a pilot light to ignite the furnace. If the pilot light is out, the furnace won’t kick on. The pilot light might be blown out by a draft, or it may be an issue with the appliance. The pilot burner could be dirty or there could be debris inside the tube that delivers gas.
In addition, a faulty thermocouple may cause the gas to turn off, killing the pilot light. If you don’t have a pilot light, try to relight it. If it won’t light, work through each possible cause or call in a professional.
Note: Working with gas is particularly dangerous due to the potential for carbon monoxide poisoning. If you have an older gas system with a flickering, yellowing, or absent pilot light that won’t reignite, call in a professional to address the issue.
Check the Electronic Ignition
Newer furnaces rely on an electronic ignition instead of a pilot light. This is basically a spark that ignites the furnace.
Symptoms of an issue with your electronic ignition include the furnace producing little heat or cycling frequently. If the electronic ignition is acting up, it may be caused by a dirty filter or a supply line blockage so try other steps before assuming it’s faulty.
Test the Fuse
On an electric system, dirt, wiring issues, problems with circuit breakers, worn out parts, or an overworked system can cause a fuse to blow.
Your furnace may express a blown fuse by refusing to work altogether, or it may continuously run the blower without producing heat.
Use your hands or a fuse puller to remove the fuse. Look for a blown circuit inside the fuse. You may be able to see the broken filament or a black smudge inside the fuse. You may also see corrosion on the connections.
If in doubt, go ahead and replace it to ensure it’s not the problem. For a few bucks, it’s worth trying rather than moving on to more complicated measures only to find out later it was a simple fix.
The air ducts in your HVAC system perform the task of carrying heat throughout the home. The system also returns air to the furnace. Throughout the process, the ducts become dirty from greasy or dusty air.
Foreign objects also find their way into the ductwork, whether that’s from curious kids seeing how far a toy will go down the hole, or from critters finding their way into the system.
Like everything in the home, ducts require periodic maintenance. This not only provides peace of mind the air your family is breathing is clean, but will also save you money. The dust and debris that collects inside your ducts moves through the system and back into the furnace, where the build-up causes the furnace to work harder.
If the furnace is working harder, it will reflect in your utility bill. Professionals estimate a homeowner can save between 11% and 25% on the energy bill by cleaning ducts periodically and, as mentioned, changing furnace filters every one to three months.
In addition, your home will accumulate less dust when there is less dust in circulation.
From a safety standpoint, reducing debris buildup also reduces your chances of carbon monoxide poisoning or home fires caused by a dirty system.
If you’re buying a home or it’s been a few years, it’s time for a duct cleaning. To do a thorough job, you should hire a professional. While cleaning ducts isn’t necessarily difficult, it is much easier with commercial vacuums and a truck with a holding tank.
However, you can get the process underway by frequently removing and vacuuming all air registers in the home, including the intake vents.
Use the hose attachments on your vacuum to clean out as far as you can in each register opening.
If you see dust accumulating around your vent registers or the space surrounding your furnace filter, it’s time to have the entire system cleaned. Smells of dust or mold when the air kicks on is another indicator that it’s time to clean the ducts.
Also note that if you’ve recently performed home improvement tasks that created a lot of dust or if you have a fireplace, you may need to clean ductwork more frequently. This is also true if anyone smokes in the house or if you have pets. If you live in an area prone to wildfire smoke, it can also clog up the furnace system.
Note: If you smell mold or are confident there are rodents in the ductwork, avoid turning on the furnace until the system has been cleaned. Rodent feces and mold can both be toxic when airborne. Call for bids with local services who can use long snaking hoses to clean around every bend in your ductwork.
Companies can also send a camera through the system to ensure there aren’t any leaks or gaps that may be affecting the function of your furnace.
Replace the Unit
At some point, all good things come to an end. Even before your furnace gives out completely, you may have some indicators that the time is coming. The good news is that newer models are energy efficient, so even though it will cost you upfront, you can expect to see energy savings in the long run.
Look specifically at Oil Furnace Repair: Furnace Blower Troubleshooting here and take a look at How to Install an Electric Furnace for more information.