Why My Furnace Won’t Turn On? How to Fix? [With Pictures]

Have you ever walked into your house on a cold winter day, and your furnace won't turn on? It is the worst feeling in the world. It is freezing outside, you are unsure of what to do, and you are thinking about what could have possibly gone wrong now!

Well, don't worry - help is available. Several causes make your furnace malfunction. So, let's dive into why your furnace will not turn on, and more importantly, how to avoid this chilling scenario.

Only an HVAC Pro can accurately diagnose your unique furnace issue. Get your furnace up and running ASAP with a professional furnace repair.

Stay warm, my friends!

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How Your Furnace Works

First, let's talk about your HVAC system's heating cycle. Understanding how your furnace works is valuable, especially if you want to know what is preventing your furnace from delivering heat.

So let's begin!

Once the indoor temperature drops below the thermostat's set temperature, the thermostat signals the control board inside the furnace to begin the heating process. The combustion process starts as the gas valve opens and the ignition turns on. The fire ignited inside the furnace warms the room temperature air, then the warmed air is redistributed in your home through your air ducts. Finally, the flue gasses from combustion travel through the heat exchanger and exit your home through the flue pipe.


If you have a malfunctioning heating system, it could be due to many causes, and our HVAC experts have seen it all before! Now, let's get into the top reasons that cause a furnace to not startup.

Why Your Furnace Won't Turn On ?

Furnace Won't Turn On Because The Furnace Switch Is OFF

If you have a heating system that will not startup, you should check the furnace power switch. The furnace switch is the power source and is typically located next to the furnace or on its side. It closely resembles a light switch. First, confirm the switch is in the "on" position and see if your furnace starts up then. If the switch is flipped down in the "Off" position, then your furnace will not turn on.

furnace switch

Furnace Switch

If your heating system still does not start up, it might be due to a blown furnace fuse or tripped circuit breaker.

Tripped Circuit Breaker Explains Why Furnace Won't Turn On

tripped circuit breaker

Furnace tripped circuit breaker

Without electricity, your furnace is entirely unable to turn. If the circuit breaker controlling your furnace trips, it will need to be reset. Check your breaker box to see if you have a tripped HVAC circuit breaker. If you have a tripped circuit breaker, flip the tab to restore power to your furnace.

Gas Valve Is OFF

Verify that the gas valve shutoff on the furnace's gas line is turned on. Its handle will be parallel with the gas line if it's on. However, if it's perpendicular, the gas is off. If the gas is turned off, rotate the gas valve handle parallel with the gas line and see if the furnace starts working.

Bad Thermostat

Don't forget that your thermostat is critical in maintaining the temperature exactly you want. If something goes wrong with this device, things can get really chilly! You'll also need batteries for when they eventually die out. So make sure to check now and then, or risk being home all winter without heat!

Damaged or Loose Thermostat Wiring

dirty thermostat wiring explains why furnace won't turn on

Dirty thermostat wiring

When any thermostat wires are loose or broken, your thermostat is unable to communicate with your furnace. Therefore, if the connection inside your thermostat is disrupted, then chances are your furnace will not work. You can unscrew your thermostat and check that the thermostat wires are securely attached and have no visible damage.

Incorrect Thermostat Settings

When your HVAC malfunctions, then we recommend checking your thermostat's settings. First, check to make sure it's set to heat. Your heating system will not operate if your thermostat is set to "Cool." Therefore check to ensure that your thermostat is set to "Heat" and not "Cool." Another possibility is that the thermostat has been set to "fan only." Make sure your fan thermostat setting is on "Auto."


If your heating system cannot sense that the room has reached the desired temperature, it will not turn on. Check your thermostat to make sure that it's not set too low. You may also want to consider upgrading to a programmable thermostat so that you can have more control over your home comfort.

thermostat setting

Bad Capacitor Is A Reason Why Your Furnace Won't Turn On

A furnace with a bad capacitor will not operate. The capacitor is an electrical component that helps start and run your furnace motor. If this part fails, your furnace will likely not turn on at all. A furnace technician can test your capacitor to see if it needs to be replaced.

Furnace Won't Turn On Due To Faulty or Filthy Blower Motor

If your heating system is not turning on, there may be a faulty or dirty blower motor. Without a functioning blower motor, your furnace will be useless. The blower motor pulls the air from inside your home, moves the air through your furnace, and back into your duct system to heat your home. Your furnace will not operate if there is poor or no airflow; therefore, it will shut down.

Additionally, If your blower motor is dirty, it will struggle to push the amount of air over the heat exchanger necessary for optimal performance. Without proper airflow from the blower, the heat exchanger will overheat from not being exposed to enough moving air. When your furnace overheats, it will shut down. Insufficient airflow from a broken or dirty blower motor will disable your furnace.

Dirty Air Filter Is Why Your Furnace Won't Turn On

dirty clogged filter

Clogged Filter

When airflow circulation is blocked, you prevent your HVAC system from functioning correctly. Without proper airflow, your furnace struggles to keep up with the household demands and is at risk of overheating. It can be easy to forget about your air filters, but if they are clogged with debris and filth, your HVAC system won't be able to breathe! It's recommended that the average household without a pet change their filter every 90 days. When was the last time you changed yours?

If you're unsure how to replace your filter, our experts are more than happy to help show you how! We can also provide a step-by-step guide on how to change your air filters here!

Flame Sensor Is Triggered

If your furnace does not turn on, you may have a dirty flame sensor. The flame sensor is a safety device that confirms whether a flame is present to allow the gas valve to open. Over time this metal rod, known as the flame sensor, gets coated in carbon deposits, dust, and other contaminating materials. When the sensor becomes dirty from combustion soot, it stops sensing the flame correctly. Because the flame sensor is so dirty, the safety device malfunctions and does not allow the ignition process to begin. The flame sensor won't "sense" the flame, so your furnace may shut down for safety, even though everything else is working correctly.

Set OFF Door Switch

The furnace door switch is a safety feature that prevents a furnace from operating if the front panel door is unhinged or ajar. So if the front panel on your furnace is not on securely, or your door safety switch needs to be replaced, your furnace won't turn on. You should check the cabinet carefully, looking for screws, loose panels, or other indicators of tampering.

Safety Float Switch Shuts Furnace Down

HVAC systems often have a condensation drain pan underneath them, collecting excess water. In this case, a Safety Float Switch shuts down the HVAC system when water reaches a certain level in the pan. This is to help prevent water damage in your home. While draining the pan may temporarily solve the problem, you're likely dealing with a clogged condensate line or drain. A heating expert can help you diagnose the problem and address it properly.

Float Safety Switch

Float Safety Switch on a Furnace

High Limit Switch Is Set OFF

One potential reason your heating system may not turn on is a tripped high limit switch. The purpose of the high limit switch is to communicate the temperature inside the furnace to the thermostat to turn on the furnace blower motor at the proper time. This feature is essential because the high limit switch turns the furnace off if the furnace is overheating. Consequently, if your high limit switch is dirty, damaged, or needs to be replaced, your furnace won't turn on.

triggered or faulty high limit switch explains why furnace won't turn on

HVAC Pro inspecting High Limit Switch

Triggered Pressure Switch

If the pressure switch is set off or malfunctioning, your heating system will not run.


A pressure switch is a safety feature that will shut down your HVAC system if it senses that the flue gasses are not appropriately exhausted. This safety device is triggered when it detects negative static pressure in the flue pipe or the draft inducer motor inside your furnace. This prevents dangerous exhaust back drafting, which could make you sick or even kill you! The automatic shutdown also guards against gas leaks and protects our homes by not allowing air circulation when there's a mechanical failure in the furnace. This ensures no unwanted gases are pushed into our house from the furnace.


Also, consider that if your pressure switch is dirty, worn, or damaged, the safety device will wrongly trigger your furnace off.

Alerted Flame Roll Out Switch

As your burner is ignited, its flames will fill the combustion chamber in your furnace. Certain conditions, such as cracked heat exchanger, a dirty combustion chamber, or a blockage in the flue pipe, may cause these flames to "roll out" of the combustion chamber. This poses a severe fire hazard and the potential to damage internal equipment. For your protection, every furnace has a flame roll-out switch meant to shut off your heating system if it senses that there are flames rolling out of the combustion chamber. If your furnace does not turn on, you could be dealing with an overheating furnace or a faulty safety flame roll-out switch.

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How To Avoid No Heat This Season

The HVAC system is one of the most critical systems in your home, so it's worth investing some time and attention into keeping it running smoothly. However, it also pays to get ahead of your furnace problems before having no heat! Consistent maintenance is the best way to prevent issues with your heating system this season. If you have a heat pump system it can also limit how often the auxiliary heat runs, explore more here! The last thing anyone wants during a cold winter when they come inside from work is an unresponsive heater that won't turn on. This can happen if the heat pump freezes. Regular maintenance and inspections will help prevent this issue before it becomes too difficult for your HVAC Pro to fix with just one visit!

During a furnace tune-up, a certificated HVAC pro inspects the condition of every part of your furnace, makes adjustments where necessary, and cleans each of its components. That means that you won't have to worry that a dirty blower motor or clogged furnace filter will shut down your heat. As part of a furnace maintenance visit, an expert will test all safety features and check the thermostat for proper operation. Not only does maintenance prevents your furnace from breaking down, but it also offers peace of mind that everything is working how it ought to be. Learn more about the steps involved in properly maintaining your furnace here!

So Your Furnace Won't Turn On?  

We know all too well that a furnace can be a lifesaver during the winter months. But what happens when your system goes down, and you don't have any heat? Call us at (443) 219-7798 or schedule online here if this has happened to you. There are many reasons why your furnace may not be turning on. The best way to find out is to contact an HVAC Pro for a furnace repair consultation. They will listen to the issue you're having, inspect your heating system and provide recommendations for what needs to happen next. 

Also, if you have any other questions about furnaces or need help with anything else related to home comfort systems in general, please do not hesitate to ask us! We would love nothing more than to help our customers feel comfortable in their homes. An if your AC does not turn on when it gets warm, learn what to do here!


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