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Troubleshooting Your Furnace When It Stops Working

Your furnace keeps your home warm and comfortable, so maintaining it is crucial. Furnaces can stop working for various reasons. Insufficient gas supply or a tripped circuit breaker might prevent the furnace from getting the energy it needs to operate. Pipe leaks or blocked vents could disrupt the system’s smooth operation, and thermostat malfunctions might interfere with the distribution of warm air.


If your furnace stops working, some troubleshooting tips can help you identify and fix issues. Read on to learn more about furnace troubleshooting and maintenance to keep your equipment running.


How Does a Furnace Work?

A furnace burns fuel to produce hot air and evenly distribute it throughout the building. It can use electricity, oil, coal, wood, or natural gas, with gas furnaces being the most common. A furnace comprises the following components:

  • Thermostat
  • Gas valve
  • Igniter and flame sensor
  • Combustion air intake
  • Burner
  • Transformer
  • Blower fan
  • Air filter
  • Supply air plenum
  • Return air plenum
  • Heat exchanger


Exhaust pipe

The gas supply for your home is outdoors, and pipes connect the tank or local gas supply network to the indoor furnace. When gas enters the furnace, the burner ignites it, heating the cold air that enters the heat exchanger. The furnace’s gas valve regulates the gas pressure entering, and exhaust from the combustion pipe is expelled from the home through vents and exhaust pipes.


The thermostat detects when different rooms need more heat, and the blower fan directs the hot air throughout the home. As the supply air plenum sends heated air through ducts and the blower fan disperses heat, the internal room temperature rises, and the return duct redirects cool air back to the furnace.


The furnace also features a heat exchanger that separates the combustion process from the air expelled by the blower fan. Cold air passes over the exchanger, heating it before reentering the home. When the thermostat detects the set temperature is reached, it shuts off the gas valve to stop warm air distribution.

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Troubleshooting Your Furnace When It Stops Working

What Can Cause a Furnace Not to Start?

Furnaces are generally reliable heat sources, but yours might fail to start for various reasons. Whether your furnace stops overnight or you begin feeling cold air from specific areas of the house, understanding how to troubleshoot furnace issues can help maintain a warm and comfortable home. Furnaces can stop working due to:

  • Clogged air filter
  • Insufficient gas supply
  • Indicator light off
  • Closed or blocked vent
  • Pipe leaks
  • Power failure
  • Thermostat malfunction
  • Incorrect thermostat reading
  • Damaged or faulty parts


Check the Air Filter

If your system is not working, the air filter is the first furnace component you should inspect. Over time, furnace filters accumulate dust and dirt, obstructing the filter and leading to complications. A clogged filter obstructs airflow and reduces indoor air quality. In some cases, a clogged filter can cause the furnace to shut down.


Excessive dirt and dust in the filter can also lead to premature failure or fire hazards, so cleaning and replacing filters as needed is crucial. You should perform the following steps to check the furnace filter every month or every 30 days:


Turn off the thermostat and furnace switch: Always turn off the power to the entire system before performing any operations.

Hold the filter up to a light source: Holding the filter up to a light source is a quick way to check for blockages. If the light doesn’t pass through, the filter needs cleaning or replacement.

Install a new filter: To install a new filter, make sure its arrow points in the direction of airflow.

Return the access panel: After replacing or cleaning the filter, secure the access panel back in place.

Check Gas Supply and Indicator Light

If the indicator light is off or there’s no gas supply, your furnace might not work. Check the gas valve outside your home and ensure the switch is parallel to the gas pipe.


You can also check the gas supply by turning on other gas appliances in your home. If these are still working, it indicates the furnace is the issue. If not, ensure your gas supply is connected correctly or contact your utility company for assistance.


If the pilot light goes out, turn off the gas supply and reignite the pilot light. It’s crucial to consult your furnace’s user manual before reigniting the pilot light so you follow the correct steps.


Before reigniting the pilot light, turn off the gas supply and wait for 10 to 15 minutes. Smell the air for any gas odor. If you still smell gas, evacuate your home immediately, move to a safe distance away from the house, call the fire department, and contact your gas company.


A persistent gas odor could indicate a leak, which is dangerous and can lead to explosions. If you don’t smell gas after 10 to 15 minutes with the gas supply off, you can carefully relight the pilot light following the instructions in the user manual.

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Check Vents and Ducts

If warm air is exiting the vents, but you still feel cold spots, your furnace’s ductwork might need attention. Air ducts contain protruding handles called dampers to control the system’s airflow.


Dampers should be fully open for proper airflow, so if you find cold spots in the house, it might be necessary to open the dampers. Move any furniture, curtains, wall art, or plants that might be obstructing vents for maximum airflow.


If you notice cold spots in entire rooms or multiple rooms, the ducts of the system might be leaking, blocked, or improperly designed, hindering airflow. You can locate leaky or blocked ducts by checking accessible sections of the system. Inspect gaps between sections and branches and seal any open gaps with duct tape. You should also contact professionals to clean and repair your ductwork.


Duct leaks or blockages are serious issues that can significantly increase your energy costs, so addressing them promptly is crucial.


Check Electrical Panel

The power supply to your furnace can also cause it to stop working. Before checking the furnace’s power supply, make sure your feet and hands are completely dry. For safety reasons, if you notice any signs of moisture or electrical damage in the system, contact a qualified electrician.


Check the furnace switch near the furnace to ensure it’s turned on. If someone accidentally bumped into it and turned it off, simply turn it back on. If you’ve turned on the switch but the furnace is still not working, you might need to check the electrical panel.


Inspect the fuses or circuit breaker box to make sure the power is on. Next, perform the same operation on the HVAC circuit breaker. If your electrical panel is labeled, you can use those labels to locate the breaker. If your panel isn’t labeled, you can find the HVAC breaker by looking for a switch pointing in the opposite direction of the rest of the switches. Turn it back to the same direction as the other switches to turn it on.


You might also need to reset the circuit breaker. Turn off the HVAC switch completely and turn it on again. If you hear a click, the circuit breaker has been successfully reset. In some cases, you might need to supply power to the fuse box again. To restore power, check for discolored and melted fuses in the fuse box. Unscrew the blown fuse and replace it with an identical one. You can take the blown fuse to a store to help you find a replacement.


After restoring power, turn on the furnace switch and check if the system is working. If the circuit breaker keeps tripping or the furnace power remains off, contact professionals for assistance.


Check Your Thermostat

If your thermostat is malfunctioning or has programming errors, troubleshooting potential thermostat issues can also help restore your furnace’s operation. First, ensure the thermostat is set to “Heat” mode and adjust the temperature to be five degrees higher than the current room temperature.


If necessary, replace the batteries in your thermostat; if you have a mechanical thermostat, dust it off. You can gently brush off dust and dirt using a soft brush or similar object.


The location of your thermostat matters too. If your furnace isn’t kicking on with the thermostat, it might be reading too much heat. Heat sources like space heaters, lamps, sunlight, and ovens can lead to incorrect readings for the thermostat. Ensure your thermostat receives accurate input so it can produce the correct output.


Turn Off and On the Furnace

Sometimes, a simple reset can fix your furnace. Try turning off the furnace and then turning it back on to see if the heat is restored. Since each furnace is different, make sure to follow the user manual for proper resetting procedures.


Call in the Professionals

Some furnace issues require professional maintenance. If your system has damaged or faulty components, diagnosing them can be challenging unless you know what to look for. Professionals can identify problems and fix them promptly.


Always have a backup heat source in case your furnace needs servicing. If you need to schedule maintenance and have to wait for repairs, having an alternative heat source and ample blankets can help keep you and your family warm.


What Does It Mean When a Gas Furnace Is Running but No Heat?

If you find your furnace running but your home lacking warmth, the ignition system of your system might be at fault. A furnace can operate when there’s sufficient energy. However, if there’s no ignition, the blower fan motor will run without distributing warm air. The furnace requires ignition to heat the air it distributes, and the igniter is responsible for producing the necessary sparks.


If the igniter fails to produce sparks, the furnace will emit cold air. If you’re experiencing cold air circulation in your home, check the thermostat and gas valve to ensure they are working correctly. You might need to relight the pilot light to restore a heat source for the furnace’s distributed air. If these troubleshooting tips don’t work, contact professional technicians for help.


How Should I Maintain My Furnace and Ensure My Home’s Safety?

Maintaining your furnace and adhering to safety guidelines can prevent damage and harm. It can also help extend the lifespan of your furnace. To care for your furnace and ensure your family’s safety, consider the following tips:

  • Ensure the area around the furnace is clear of flammable materials.
  • Remember to replace the air filter regularly.
  • Schedule annual maintenance by professional technicians to ensure smooth system operation.
  • Keep at least two-thirds of your vents open to prevent excessive heat buildup.
  • Keep vents clean and avoid blocking them with furniture or objects.
  • Install at least one carbon monoxide detector in your home and test it monthly.
  • Check or replace the CO detector’s batteries at least twice a year.


Keeping flammable items away from the furnace is crucial. Since furnaces use gas, nearby flammable items can pose a potential fire hazard. Regularly replacing air filters prevents system clogs, scheduling routine maintenance allows technicians to catch potential problems before they cause damage.


You should always maintain at least two-thirds of your vents open so heat disperses throughout your home rather than accumulating within the furnace. Keeping vents unobstructed and free from dust and dirt is also important.


Your furnace exhausts gases outside to ensure indoor air remains free of carbon monoxide. However, your home should still be equipped with one or more carbon monoxide detectors to alert you in case of leaks. Carbon monoxide poisoning can lead to severe health complications and death, so evacuate your home immediately if your detector sounds and call 911.


Regular furnace maintenance and safety practices are essential to keep your home comfortable and secure.

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