Gas Furnaces: How Do They Work? Explained!

Kenneth Wilson

One of the best inventions humankind has brought into this world are temperature control systems, namely: air conditioning, heating, and ventilation systems. In warm weather, you only need to grab a remote and push a button and a cool breeze of air will come and cool your body temperature down. If the weather is too cold to the point you get chilly, with just a click of a button, you can warm your heart down to comfortable levels. If your indoor air is smelly and is otherwise unhealthy, your ventilation systems will do the trick and will give you fresh indoor air. But one system stands out among the rest in locations where cold weather is prevalent: heating systems.

Heating systems use a heat source and are fueled by either gas or electric components. In the United States, the most commonly used heating system is gas furnaces. Gas furnaces use gas as their fuel, hence the name. It may seem like it is simple to understand the way gas furnaces work. Some may think the mechanics and physics involved in gas ranges are the same with gas furnaces, but we hate to break it to you: it is not. A gas furnace is technically more complex than gas ranges since furnaces contain more components than gas ranges. But how do gas furnaces work exactly? First, let us understand what furnaces are.

What Are Furnaces?

Furnaces are heating machines involved and oftentimes integrated into your home’s HVAC (Heating, Ventilation, Air Conditioning). These heavy and bulky machines are designed to give your home the heat and warmth you need in cold weather and distribute that heat to basically every inch and every corner of your house. Furnaces give you heat by burning fuel inside their fuel compartment to warm the air around them. Some furnaces use electricity, natural gas, or oil. But the most common furnaces used today are powered by liquefied petroleum gas, also known as LPG. Some furnaces work by boiling water inside their tanks and proceeds to distribute the heat that comes from the steam of the boiling water.

How Do Gas Furnaces Work: Heating Cycles

Understanding how gas furnaces work is no simple thing. So it is best to know about the fundamentals first before we dive deep into the more complex mechanics of gas furnaces.

In furnaces, there are heating cycles. Just like any other cycle you thought of, heating cycles are composed of stages and steps. The heating cycle of your furnace begins with an outdoor pipe leading all the way to your home. This pipe connects your furnace to the fuel source, which can be LPG or natural gas. If your furnace uses LPG, your furnace would be connected via a pipeline to the outdoor LPG storage tank. If your furnace uses natural gas, you will have underground pipelines that connect to the natural gas source. (Related: Heat Pump Or Gas Furnace: Which One Is Right For You?)

Your furnace’s heating cycle can be simplified by enumerating the stages involved:

  1. 1
    After the gas enters your home and to your furnace via the pipelines connected to your outdoor LPG tank or natural gas source, the burner in your furnace lights it, and the byproducts are flames.
  2. 2
    Then, cold air from your house enters your furnace, that is when the burning gas will make the cold air warm within the heat exchanger inside the furnace.
  3. 3
    After that, exhaust, or the byproduct of burning gas will be piped out of your furnace through a vent and will exit your home via an exhaust pipe directed outdoors.
  4. 4
    Then, the blower fan in your furnace will evenly distribute the warm air in different parts of your house and in places where the thermostat detects the need to heat up.
  5. 5
    After that, your internal room temperature will gradually rise up as the warm air is distributed evenly around the house. Cold air will eventually be redirected and go to the furnace and will be cycled again through the heat exchanger.
  6. 6
    Once your thermostat senses that the temperature you have set has been reached, it will eventually switch off the gas valve in your furnace to stop the flow of warm air.

Gas Furnaces: What are the Parts?

Gas furnaces, just as we mentioned earlier, contain a lot of complex and sensitive parts inside, and these parts make the gas furnace accurate and safe. Here are the parts of a gas furnace:

  • Thermostat: a thermostat is an electronic device that controls and measures indoor temperatures. A thermostat regulates the heat cycle of your furnace according to its set temperature point. Thermostats are also present in air-conditioning systems.
  • Transformer: this electrical component supplies electricity to the furnace and also regulates the voltage and current in the machine.
  • Burners: these are small components inside the furnace and are responsible for gas emission which is eventually ignited to get controlled flames.
  • Blower Fan: this is a small electric fan and motor that pushes the warm air from the heat exchanger unit inside the furnace to the vents that lead to various parts and rooms of your home.
  • Control Board or Circuit Board: this is a small control board or a chip that interprets the electrical signals from the thermostat and other electrical components of the furnace. It also sends a response that causes other furnace features to activate or deactivate. The control board can control the opening and closing of the gas valve, activation, and deactivation of burners, as well as the on and off operation of the blower fan among other features it can control.
  • Igniter: this component works side by side with the burners. The igniter is responsible for lighting or setting the gas on fire emitted by the burners. Igniters can work by creating sparks through friction which then lights the gas that passes through it
  • Flame Detector: a device that is programmed to prevent gas from building up to dangerous levels when the igniter fails. The flame detector shuts off the gas flow from the gas valve when no heat is detected.
  • Heat Exchanger: this component is probably one of the most important parts of a furnace. A heat exchanger contains a series of thin metal tubes that segregates the combustion process (where dangerous gases and fumes are expelled) from the inflow of air entering the home by the blower fan. Cold air passes through these metal tubes, which then warms up the air before it is redirected to the vents and to the rooms of your house.
  • Gas Valve: the gas valve regulates the amount and pressure of gas that enters your furnace from an outside source such as an outdoor LPG tank, or an underground natural gas supply.
  • Air Filter: the air filter cleans the air of debris and dust that comes along with the cool air that enters the furnace.

Gas Furnaces: How to Maintain?

Just like any machine and appliance in your home, gas furnaces need maintenance works regularly. Air-conditioning units are maintained and cleaned to make them run smoothly without any sort of problems, and it is the same for gas furnaces, especially because these furnaces handle flammable material and can be a cause of fire or explosion if any severe malfunction occurs. Here are some tips you can do to keep your gas furnace safe and working in tip-top condition:

  • Always keep all flammable and combustible materials away from the furnace. Furnaces emit heat so it would be best to keep them away as far as possible.
  • Make sure to regularly schedule maintenance and cleaning operations from a certified furnace and HVAC technician.
  • Regularly change the furnace’s air filter every 3 months or so to prevent the build-up of dust and debris from piling up.
  • Always remove any obstruction and clear a space for the furnace. Furnaces can emit substantial heat and can build up if there is no adequate space.
  • Equip your home with a carbon monoxide detector, especially in areas near the furnace and your pipelines connected to the fuel source. Make sure to test them regularly to keep track if the device is still working well.
  • Check the furnace flame regularly and if you see something odd, make sure to call your trusted technician to take a look to check if things are well.
  • If you smell something such as gas, be sure not to turn on electrical devices and be careful not to start a fire. Call your fire department and be sure to follow their instructions.


Gas furnaces are important pieces of technology that can save us from chilly weather and even hypothermia, and it is a result of beautiful engineering and innovation that can make our cold and chilly nights warm, soothing, and calming. Gas furnaces are common in the American household, and it would be nice to see furnace owners who know how the machine works so they can take care of it and make it last long. Ultimately, learning about gas furnaces is a great thing to do so you can minimize the occurrence of malfunctions through regular check-ups and maintenance works, and regular filter cleaning.

Kenneth Wilson
September 18, 2021
Contractor Tips, HVAC

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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