REthority is reader-supported. If you buy a product we link to, we may earn a commission.
Skip to Content

What Is a Furnace & What Does It Do?

What Is a Furnace & What Does It Do?

What is a furnace? And what exactly does it do? These are questions that we hear every day, and that’s why we’ve made the complete guide below. Read on to learn all you need to know about furnaces.

What Is a Furnace? The Short Answer

Close-up of a gas furnace and its pilot light being ignited for a piece on what is a furnace

Mike Fig Photo/Shutterstock

A furnace is a part of the heating, ventilation, and air conditioning (HVAC) system. You activate the furnace when you set the temperature in a room by using a thermostat.

When you adjust the temperature, the air inside the room might be cooler than the temperature you set on the thermostat. When this happens, you trigger the furnace to:

  • Turn on
  • Start a fan
  • Heat the air (which happens in different ways with different types of furnaces)
  • Move heated air throughout the space.

This cycle is repeated once the air returns to a temperature under the one set on the thermostat.

What Is a Furnace? The Long Answer

Two hands adjusting the temperature of a white electric thermostat

Daniel Krason/Shutterstock

A furnace does much more than warm the air by turning on a fan. It uses intricate processes to ensure that the air is warmed up to the correct temperature by the time it leaves the unit. The folks at AMRE Supply explain what a furnace is their video below.

Different types of furnaces use the same generic process but with unique specifics. To understand this, you must first understand what the different types of furnaces are.

Types of Furnaces

When it comes to furnaces, there are four main typesIn order of efficiency (from greatest to least), they are natural gas, oil, electric, and propane. In order of costliness (from greatest to least), they are natural gas, oil, propane, and electric.

1. Natural Gas Furnaces

Natural gas furnaces run off of what it sounds like: natural gas. For natural gas furnaces to warm the air, the gas is ignited by communicating with a thermostat.

Then, the burner ignites. The high-quality air gets separated from the poor-quality waste in the combustion chamber. Then, the warm air gets released into the space. When the air cools, it’s pushed into the furnace to be reheated through the same process.

2. Oil Furnaces

With an oil furnace, the starting point is also the thermostat. When the temperature is lower than the temperature you set the thermostat at, the system is triggered. To warm the air, it relies on an oil tank. The oil is filtered and converted into a spray, which is on the burner chamber. It then heats up.

The air that will be released into the home at the end of the process gets heated as it makes its way over the hot burner chamber. After the journey of passing over the chamber is complete, the air is released back into the space.

3. Electric Furnaces

An electric furnace follows the same generic steps that a gas and electric furnace follow: the system detects that the temperature has fallen under the temperature set on the thermostat and turns on.

An electric furnace is different in that it uses an electric motor to pump the air into and out of the furnace. When the air gets too cold, the coiled spools of wire receive electric currents to heat the electric heater. A blower forces the air over the heating elements, through a filter, and out into the environment.

4. Propane Furnaces

Propane furnaces works by burning propane and converting it into a gas. A blower, similar to the one used in an electric furnace, blows air across a heat exchanger after the heat exchanger reaches a defined temperature. The newly-warmed air gets pushed out into the space.

Things to Consider

Whether you are looking for a furnace to use in your space or you are simply interested in learning about furnaces, there are a few items to keep in mind:

  • Furnaces vary in size. Choosing the right-sized furnace isn’t as easy as it sounds – the size depends on how spacious the area is, how tall the ceilings are, and additional minute details.
  • The location of the space will affect the size and type of furnace you should get.
  • The outdoor climate also affects the type and sizing of a furnace.
  • How much it will cost you to repair the furnace in the future.
  • The problems that may surface from using a furnace.

Frequently Asked Questions

As an image for a piece on what is a furnace, a person vacuuming the inside of the hvac unit

Indy Edge/Shutterstock

Of course, there are many more questions surrounding furnaces than the basic questions we covered above. We’ve listed some commonly asked questions along with their answers.

What is a furnace in a house?

Within a house, a furnace heats the air and delivers it to the different rooms. It follows the generic process as any other furnace to keep the air in your home warm. You’ll notice that the most common type of furnace used in a house is a natural gas furnace.

Is a furnace the same thing as a heater?

No, a furnace and a heater are not the same things. Both systems solve the same problem and function based on thermodynamics, but furnaces and heaters are two entirely different systems.

A heater is an umbrella term for anything that heats another object. Heaters range from fire, heaters used within a contained space, and other surprisingly random things. A heater does not heat the air. It recirculates it.

Without a furnace to take in the cool air, warm it, and distribute it, the heater would only put out room-temperature air. One way a furnace can distribute warm air is through a heater.

Is a furnace used for air conditioning?

A furnace is part of the HVAC system, but a furnace does not take part in cooling air unless you add more pieces to the unit. Not all furnaces can connect to an AC unit. A furnace must first work through using ducts before you can add air conditioning capabilities to it.

However, you’ll end up needing to connect the furnace to an AC unit or condenser anyways, so there is no way that you can use a furnace on its own for air conditioning.

Is a furnace gas or electric?

A furnace can be gas or electric, depending on the size, outdoor climate, and needs of your space. It also depends on your budget. As we already covered, electric furnaces are more expensive than gas furnaces.

So, What Is a Furnace?

For a piece on what is a furnace, an hvac tech gives the thumbs up while squatting next to a gas furnace


To understand what a furnace is, let’s recap what it isn’t: it isn’t the same thing as a heater. Also, it doesn’t substitute an air conditioning unit.

In a nutshell, a furnace is a system that takes in the air that is cooler than the temperature set on the thermostat, warms it, and pushes it back out after being thoroughly filtered. They system works hard to keep you comfortable, even when the temperatures are frigid outside.

Read Next: