How Does a Furnace Work?

how does a furnace work
It's that time of year again — time to start thinking about our furnaces keeping our homes warm all winter. A furnace is an important part of any home, especially those located where the winters are cold. A furnace heats the air and distributes it around the house. There are several different kinds of furnaces, but their basic functions are similar. Consider this your guide to answering the question: How does a furnace work?

What Is a Furnace?

You probably already know that your furnace is what keeps your house warm throughout the colder winter months. By definition, a furnace is an enclosed unit that heats an entire building. This is different from space heaters, which may be used to heat one room or a much smaller space. Furnaces are permanently installed in homes or other buildings as part of a central heating system for the whole building. There are several different types of furnace systems, but in the most common types, the furnace heats the air in one location, and the central heating system distributes this heated air evenly around the whole home or building. Ductwork and air vents usually allow for airflow in this type of heating system, known as forced-air distribution.

Why You Should Understand How Your Furnace Works

You may think that you don't really need to understand the specifics of how your furnace works. After all, if something isn't working properly, you can just call in the experts and let them handle it. However, having a better understanding of your furnace and central heating system makes you a savvier homeowner and can help you avoid bigger disasters when the furnace isn't working properly. Having a basic understanding of how your furnace works means you are better able to try to diagnose problems and do a little general troubleshooting yourself. You may be able to fix minor issues, and even when you call in the experts, you're better able to help them know exactly what might be wrong. Understanding how your furnace works can also help you use your furnace more efficiently, possibly reducing energy usage and costs. Types of Furnaces

Types of Furnaces

Different types of furnaces are generally categorized by the type of fuel used to power the unit. The four most common types of furnaces are gas, propane, electric and oil. In an electric furnace, exposed electric heating elements heat the air. In most other types of furnaces, there is a heat exchanger or special chamber where the air is warmed before it's distributed through the ductwork and vents. The thermostat works with any of these systems to signal the furnace automatically to shut off when the home reaches the desired set temperature. Let's take a closer look at the four top types of furnaces:

Natural Gas Furnace

By far the most common type of furnace in the United States is powered by natural gas. Nationwide, 48% of homes use natural gas as the main heating source, but the percentage varies by region. Older versions of natural gas furnaces were reputed to be somewhat inefficient, but today's versions are incredibly efficient and very popular.

Propane Furnace

Less common than other types of furnaces, propane furnaces run on propane, which is a byproduct of oil and gas production and is usually stored in tanks. A small number of American households use propane furnaces. They are more common in areas where oil and gas are not as easily available.

Electric Furnace

These furnaces run on electricity and generally have the lowest initial purchase cost. They are very easy to install and last up to 10 years longer than other types of furnaces. However, electricity is more expensive than other fuels, making this type of furnace more expensive to use in the long run. Electric furnaces also go out in any power outages, while other types can keep running.

Oil Fired Furnace

This type of furnace is more common in the Northeastern area of the U.S. and uses oil as their fuel source. They are slightly less efficient than natural gas models but have a lower upfront fuel cost. The furnaces themselves have a slightly higher cost for their initial purchase.

How A Gas Furnace Works

Gas furnaces run on natural gas, which is often methane or other carbon-based gases. But just how does a natural gas furnace work? Here are the main steps for how a gas furnace works:
  1. The igniter: In this popular type of furnace, the process starts with the igniter. In older models, the igniter is likely a pilot light, while newer versions commonly have electric starters. If you have a pilot light system, the regulator continuously supplies a small stream of natural gas to keep the pilot light lit. Electric igniters use a filament where electric current causes a rise in temperature that ignites the fuel source.
  2. The combustion chamber: In this part of the gas furnace, the natural gas mixes with air, and the igniter lights the gas. The furnace's venting system allows the air mixing necessary for combustion and vents byproducts and exhaust safely out of the flue. Regular maintenance of this part of your furnace ensures efficient combustion as well as cleaner indoor air quality.
  3. The heat exchanger: Situated above the combustion chamber, this part of a gas furnace warms the air from the heat of the combustion chamber. When the temperature in the heat exchanger reaches the desired level, as indicated on the thermostat, the motor starts up and blows the heated air throughout the house's ducts, out the vents in each room. The air cycles around the house, and cooler air returns to the furnace to start the process all over again.

How A Propane Furnace Works

A propane furnace works largely the same way as a natural gas furnace. They are so similar that an experienced HVAC professional can easily convert a gas furnace to a propane system. The main difference is that the propane fuel must be kept in tanks and refilled regularly, while a gas furnace system is attached to a natural gas line with a constant stream of gas. One of the benefits of a propane furnace is that burning propane doesn't emit any greenhouse gases and that you can purchase propane before the higher-demand winter months. However, you must have tanks to store the propane and keep up with regular deliveries to fill the tanks. If your tanks run out of propane, you risk your furnace shutting down completely. You'll also need to watch for leaks in the tanks and be more careful during natural disasters, as a tornado or hurricane can cause an explosion with a propane tank. How An Electric Furnace Works

How an Electric Furnace Works

An electric furnace is usually much cheaper to purchase and install, is easy to install and maintain and has a much longer lifespan than other furnace types. However, because it runs on electricity, the cost to operate can be much higher. But just what does a furnace do when it runs on electricity? These types of furnaces run similarly to gas furnaces, with some key differences. Electric furnaces use electric heating elements to heat the air instead of burning gas or oil. The functioning of an electric furnace can actually be compared to a hair dryer. It pulls air into the heating system through the heat exchanger. Once the air is in the heat exchanger, the heating elements warm the air. These elements consist of coiled metal wires which electrically charged particles travel through, heating the wires. An electric furnace may have anywhere from three to six of these heating elements inside. Once the air is heated to the correct temperature in the heat exchanger, it is blown up into the ductwork and through the vents in each room. The heated air circulates throughout the home, and the cooled air is again heated in the furnace, according to the temperature set on the thermostat. Other important parts that make an electric furnace work include:


This important furnace part controls the voltage level for the electric heating elements in the furnace.


The sequencer turns heating elements on and off. It also ensures that not all of the elements are activated at the same time, which could result in a blown breaker.


The transformer allows for the electric power to flow appropriately to the other parts of the furnace besides the heating elements. This includes the thermostat, the contractors and the sequencers. The transformer controls the flow of electricity between these circuits and parts of the heating system, ensuring the flow is smooth.

How an Oil Furnace Works

An oil furnace also operates similarly to a gas furnace, but instead of natural gas in the combustion chamber, a mist of oil sprays into the chamber. A fuel pump draws this oil from a reserve tank and through a filter. The oil mist sprays onto the burner, and the flames heat the chamber. Air passes over the warmed chamber, becomes heated and is blown back up into the home's ducts and vents. This continues until the home reaches the desired temperature, as set on the thermostat. Oil-burning furnaces are considered the best in terms of heat generation and can warm up a building faster than other types of furnaces. The initial purchase price is lower for oil furnaces, and oil furnace components are fairly straightforward and easy to repair. However, with an oil furnace, you need to have an oil tank to store the reserves of oil, which can be costly and complicated to install. These tanks are often installed underneath the home. You'll also need regular oil deliveries, which can be expensive, and constant monitoring of the tank for leaks. Central Air & Gas Furnace Split Systems

How Central Air Conditioner & Furnace Split Systems Work

With a central air and furnace split system, a home has both heating and cooling to meet any seasonal demand. Central heat and air units share the ductwork and vents in a building, making temperature control easy. When both air conditioning and heating are installed together, it's called an HVAC system, which stands for heating, ventilation and air conditioning.

How an HVAC System Works

In an HVAC, or split system, there is typically an outdoor cabinet that houses the condenser coil and compressor of the air conditioner, and an indoor cabinet with an evaporator coil. The indoor portion of the air conditioning unit is usually installed with the furnace unit. Any of the above types of furnaces can be used with an air conditioning system. In an air conditioner, refrigerant chemicals are pumped through the unit, cooling the indoor air. Moisture is also condensed out of the air as it is cooled. The refrigerant absorbs warmth, which is then pumped to the outside unit where the heat is released outdoors. Back inside, the cooled air is blown through the ductwork of the home. The ductwork is shared with the furnace, and the thermostat settings control turning the air conditioner or furnace off or on, ensuring a comfortable indoor home temperature year-round. These split-system air conditioning systems work well with heating systems for whole-home climate control. They are the easiest type to install in homes with existing furnaces. They are more efficient than other types of air conditioners, out of the way and quiet to operate.

Shop for a New Furnace With Ingrams Water & Air

Now that you know how a furnace works, check out the wide array of options at Ingrams Water & Air. We are proud to offer many HVAC systems for you to choose from. Many options include cooling, so you can decide the furnace that is right for you and your home.
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