Furnace Won't Turn On? Furnace Troubleshooting Tips

Furnace Won't Turn On? Try These Furnace Troubleshooting Tips

Types Of Furnaces

There are different types of furnaces and many different vendors. It helps to know what kind of furnace you have and a basic understanding of how they work when trying to diagnose problems. You can read more about the types of furnaces and how they work here.

Each type of furnace and different manufacturers may have unique attributes that require different fixes when there’s a problem. An oil furnace repair may be different from a gas furnace or heat pump repair.

but here are the most common ones that prevent furnaces from working properly.

Furnace Not Working? Furnace Not Igniting? Furnace Won’t Turn On?

Before you make the call for service, you should do a little furnace troubleshooting to see if there’s something simple you can fix yourself.

Check The Thermostat

The first thing to check is your thermostat. A lot of problems originate there and are easy to fix without having to call for service.

• Make sure the thermostat is set to Heat or Auto
• Turn the temp up to at least five degrees above room temperature to see if the heat kicks on
• Make sure the date and time are correct. On programmable thermostat models, the system might be using a timer.
• Check for dirt clogging up the mechanism. Open the thermostat up and gently blow on dirt, dust, or debris.
• If you’re not seeing anything on the electronic panel, check to make sure the power is working. Some models contain batteries, which may need to be replaced.
  

Check The Power

Another common cause of your heating system not working is a power problem. Check the circuit breaker panel to make sure the breaker is on. If it in the off position, flip it to on, and see what happens. If the heater trips the circuit again, you’ll want to get a technician to take a look.



Most furnaces also have a fuse. Check to make sure the fuse is still good. If you need to replace it, turn the power off first.

Check The Air Filters

Dirty filters not only reduce the airflow (and heat) that travels through the system, but it also makes your furnace work much harder.

This can run up your energy bill and reduce your efficiency. It can also cause your heat exchanger to overheat and shut down.

Check The Furnace Door And The Safety Switch

A furnace door safety switch will prevent the burner and fan from operating when the access panel is open. If the access panel isn’t positioned properly, that may lead to the furnace not turning on.

Furnace Not Igniting? Check The Pilot Light / Electronic Ignition

If you are using a gas furnace, check the pilot light to make sure it’s still lit. If it’s not lit, it won’t heat. Most units shut down completely when the pilot light is out to prevent gas from seeping out. If your pilot light goes out regularly, there may be a clog in the system or a draft that needs to be addressed.

Not all gas furnaces have pilot lights anymore. Many models have electronic controls that fire on demand rather than producing a constant flame.

Thermocouples in furnaces can also go bad. This can affect the ignition system and prevent the furnace from working or reduce the amount of heat it puts out.

Check The Gas Line

Another thing to check before calling for service is the gas line. It should be in the fully On or Open position leading to a gas furnace. If it’s closed, or only partially open, the unit won’t get enough gas to operate.

If you have any reason to believe there’s a gas leak or your smell a strong gas odor, the best advice is to call the utility company after you exit your house. Gas leaks are nothing to mess around with. Not only can it hurt the operation of your furnace, but gas leaks and carbon monoxide leaks can be very dangerous.

Check The Heat Exchanger

A heat exchanger creates combustion to generate heat from a fuel source. It’s contained in a combustion chamber that separates it from the blower )which distributes the heat throughout your home). As the combustion chamber heats, it blows air across the surface on to rapidly heat the air. If there’s a crack in the heat exchanger, it can cause your system to fail. It can also produce dangerous situations due to gas leaks.

An erratic flame could be a sign that oxygen is mixing in with the gas. That usually happens when there’s a leak or crack in the heat exchanger.

Furnace High Limit Switch Keeps Tripping

One safety featured required by law is a high limit switch. This turns off the burners and turns on the blower fan if the furnace is operating at a “too high” or unsafe temperature. If the furnace high limits switch keeps tripping, it may indicate damage to the heat exchanger. If the furnace high limit switch trips several times in a short period, the furnace may enter lockout mode and need servicing before it can be reset as a safety precaution.



If you have an old furnace, the heat exchanger can crack over time as it constantly contracts and expands. It may be time to replace the entire heating unit.

Furnace Pressure Switch Stuck Open

The furnace pressure switch prevents back-drafting. The draft inducer motor pulls gases out of the heat exchanger and vents it so it doesn’t enter your home. An issue with the draft inducer motor can cause your pressure switch to be stuck in the open position.

Common Heat Pump Problems

If you have an electric heat pump, you’ll want to check most of the things above, including thermostat, power, and air filters. If your heat pump isn’t working, the next most common causes are a broken starter capacitor, broken reversing valve, or problem with the defroster.



• Broken Starter Capacitor
  If you hear a faint click noise when the heat pump is supposed to be turning on, but it doesn’t, it may be the capacitor. The starter capacitor is what transmits the electrical charge to turn on the motor.
• Broken Reversing Valve
  As heat pumps transition from heating and cooling, the reversing valve works to change the direction of the refrigerant in the system. When it goes bad, the valve stops working. The heat pump may turn on to cool, but not for heat.
• Freezing Up
  It’s not unusual to see some ice on a heat pump in cold weather. An automatic defrost mode should clear the icing periodically. If the condenser unit freezes up and the defrost mode doesn’t melt the ice, your unit may work inefficiently or shut down completely until it thaws.

Each of these issues will require a furnace repair by technicians.

If Your Furnace Is Running But Not Producing Enough Heat

If your furnace appears to be running but you’re not getting enough heat, a simple fix may be to replace the air filter.

Check The Air Filter

Depending on the size of your home, you may have multiple air filters. These will typically be in the ceiling, floors, or walls. Some furnaces also contain a filter.

Check all of the filters. If they look dirty, toss them out and get new ones. One easy way to check is to hold the filter next to a light source. If you can’t see through it, it’s time for a new filter. When you’re checking your filter, make note of the direction your filter points. There should be an arrow on the filter indicating the direction of the airflow. If you put it in upside down, it won’t work very well.



Dirty filters not only reduce the airflow (and heat) that travels through the system, but it also makes your furnace work much harder. This can run up your energy bill and reduce your efficiency. It can also cause your heat exchanger to overheat and your furnace won’t kick on.

Furnace Not Blowing Hot Air? Check The Burner

The burners on your furnace need to stay free from dirt, dust, and debris that can build up. Buildup is common during the off-season when you’re not using heat, so you should check the burners before firing it up for the first time when temperatures cool.



One easy way to tell if there’s an issue is to look at the flames. If they are blue and appear to be burning evenly, the burner is clean and operating properly. If they are yellow, they’re likely dirty and need cleaning. You can generally clean the burner with a vacuum cleaner. You’ll want to turn off the power and gas before doing so.

Check The Furnace Flame Sensor

If the flame sensor on your furnace is dirty or has malfunctioned, it can prevent your furnace from operating. The furnace flame sensor’s job is to protect your furnace from burning fuels that can damage it. You can typically find the sensor in the back of the furnace leading to the burner.



Flame sensors can have carbon buildup or oxidation which can cause things not to work. To determine whether there’s a problem with the sensor, technicians will do a micro amp draw reading as part of their furnace troubleshooting, clean the sensor, and then see if they get higher amps. If not, it may need to be replaced as part of your furnace service.

If Your Furnace Keeps Shutting Off And Needs To Be Reset

If your furnace fires up, runs for a short time, and then shuts down, it may not be getting the amount of airflow it needs to keep the heat exchanger from overheating. The most common cause of this is a dirty air filter.

Swapping old air filters for new ones may solve the problem. If it doesn’t, you may be looking at belts or blower motor problems which would require a furnace repair service call.

If Your Furnace Isn’t Blowing Out Air

Most furnaces will have a small plastic window on the front. You should be able to see a flashing light. Green is good. Red is bad. It the light’s red, call for service. If you don’t see any light, there can be a variety of problems, including the transformer, run capacitor, furnace control board, thermostat, or blower motor. All of these likely require a contractor.

If Your Furnace Blower Runs Constantly

Between the air return duct and the furnace is the blower. It sends cool air into the heat exchanger to warm it up before distributing it into your home. If your furnace blower runs constantly and doesn’t shut off, you should check the thermostat first. Make sure it’s not set to Fan On or Continuous Fan mode. You can also try turning down the temperature to see if the blower shuts off.



A gas furnace blows that doesn’t shut off at times can be the result of a faulty limit control switch, which tells the blower when to turn off and on

If You’re Furnace Is Exceptionally Noisy

If you’re hearing a lot of noise coming from your furnace, there might be a mechanical issue. It might be an improperly adjusted pilot light, issues with belts or burners, or lack of lubrication with the blower motor.

Some noise from the furnace is normal. For example, some pinging or popping happens when air ducts contract and expand as it heats and cools. If you’re hearing other noise, there may be issues.

• Rumbling
  Constant rumbling may be the sign of a problem with the burner
• Rattling 
  There may be loose panels that should be tightened
• Grinding
  This could indicate a problem with the motor bearings
• Squealing
  Belts might need to be tightened or replaced, or parts need to be oiled
• Scraping
  If you hear scraping or a metal on metal sound, it can be a broken motor mount, loose blower wheel, or other components that need repair
• Thumping
  Thumping sounds usually indicate an unbalanced blower wheel or motor
• Humming
  Humming may indicate an issue with a transformer, bad interior fan, or capacitor

Since these noises may help to diagnose the specific problem with your furnace, it’s a good idea to let a technician know what you’re hearing when you schedule a service call.

Routine Furnace Maintenance & Furnace Tune Up

The average furnace can last 18-20 years. If they’re properly maintained, they can last 20-25 years or more. Yet, many people don’t do the routine maintenance to protect their investment and make sure they will have heat when it’s needed.

You should have your system inspected and services at least annually during a furnace tune up. That helps find furnace maintenance problems before you need to turn on the heat. It’s better to find out now rather than in the middle of the night, which may require an expensive service call at premium rates. Routine maintenance can save you from costly repairs.

Another reason to consider an annual furnace service is to keep the warranty on the unit intact. Most manufacturers provide a 5 to 10-year warranty but will void the warranty if work is done on the furnace by anyone but a qualified technician.