Enter your zip to
find local pros.


How Does a Furnace Work?

On this page:
  • Types of furnaces by how they work.
  • Types of furnaces by the fuel they use.
  • How to choose the right furnace for your home.

Find the best HVAC contractors in your area.

What is a Furnace?

A furnace is a heating unit that works by generating and distributing heated air throughout your home. It is one of the oldest types of heating systems still in use today for both residential homes and commercial buildings. Older versions were powered by coal and wood versus today’s modern versions, which burn natural gas, propane, and electricity to produce heat. 

The various types of furnaces differ in how they work— single-stage, two-stage, variable speed, and the fuels they are powered by— gas, oil, and electric. While the basic operation remains the same, these have specific characteristics, costs, and efficiency ratings. 

How Does a Furnace Work?

To understand how a furnace works, we will take an example of a gas-powered furnace, also known as a forced-warm air distribution system.

A furnace consists of six main components— a thermostat, gas valve, burners, heat exchanger, blower, and air ducts.

smart home thermostat for electric furnace

A thermostat activates a furnace when the temperature settings drop below the set level on the controller. The process starts at the burner that drives gas to the heat exchanger when a gas valve is turned on. The process involves air combustion where the air is ignited by the gas fuel supplied through the valve to create heat. While older furnaces used a pilot light, newer systems consist of an ignition switch to ignite the gas.

The heat exchanger turns the intake air from ducts and gas fuel into heat which is distributed through the air ducts in your home.  

A separate vent pushes the colder air from the homes into this system which is then warmed to the desired temperature by mixing with heated air from the exchanger.

The desired heat is passed through a filter, to remove dust and debris, to the furnace plenum. A plenum is a box that acts as a junction to connect the furnace system to the ductwork. From here a blower distributes the heated clean air in the home through the air vents/ducts.

The flue and vent pipes in the system exhaust any harmful by-products of the combustion process in the exchanger to the outside environment. The process repeats itself by drawing colder air to the exchanger through return ducts until the desired temperature as set on the thermostat is achieved. The gas valve shuts off and activates again when the temperature drops or when thermostat settings are changed.

Furnaces are generally paired with a central air conditioner and in some cases, combined with a heating pump system to keep the temperature comfortable for your homes around the year.

Types of Furnaces: How They Work

Single-Stage Furnace

A single-stage furnace means that the valve supplying the burner with gas only has two settings during operation, on full blast and off. They are known as single-speed furnaces because as the name suggests, they work on a single speed. They continuously distribute heated air in the environment at the furnace’s maximum capacity. 

Two-Stage Furnace

Two-stage furnaces utilize a three-position valve that is much more efficient than its single-stage counterpart. The valve settings are on, partially closed, and closed. Therefore, they work at two speeds and operate much more efficiently. The first stage (partially closed state) runs at a 65% capacity and works comfortably in colder climates. And when the temperature drops and stage one heating is not sufficient it switches to stage two heating (on or full blast state) for additional heat. Two-stage furnaces are the most common furnace types used in homes due to their higher energy efficiency model.

Variable speed furnace

Variable speed furnace does not refer to the furnace working at different speeds to achieve desired temperatures, but it rather refers to the blower’s motor/fan speed of the furnace. As the fan is adjustable, it the needed heat to dissipate in the environment by saving your energy costs.

Types of Furnaces: Fuel Used

Gas Furnace

A fully functioning gas furnace works on natural gas. The natural gas is ignited in the furnace providing heat in the environment and this heat can be controlled by an external thermostat. Gas furnaces are the most common heating unit of an HVAC system and are usually paired with a central air conditioner unit installation. They can last between 15 to 20 years.

Electric Furnace

An electric furnace works in the same way as a gas furnace, but instead of using natural gas to produce heat, it has electrical heating elements to heat up the environment and distribute it into the surroundings through a blower. An electric furnace has a life expectancy of 20 to 30 years.

Oil Burning Furnace

An important element of an oil furnace is an oil tank. When the temperature drops and the room requires heating, oil is drawn from the oil tank reserve through a filter into a burner where heated air is produced which is then pushed through the ducts in the environment. Oil-burning furnaces can last for more than 20 years if maintained properly.

Which Furnace Should You Get?

Several factors are needed to determine which furnace system is right for your home such as average temperature, costs, the top furnace brands, square footage, and insulation values.

A two-stage furnace is well worth the additional costs over a single-stage system. You must determine, based on your needs, whether it is suited for you and your budget. A professional HVAC contractor can help you with this decision by performing a thorough inspection and evaluation of your home. They can also point out any deficiencies around your home such as inadequate insulation, potential air leaks around windows, doors, and exterior doors as well as necessary upgrades to improve your home’s heating and cooling efficiency. With this HVAC guide, you should now know how a furnace works and how to find the best heating unit for your home.

If you need to replace both your furnace and central AC unit at the same time, you can expect to pay more in total HVAC system replacement costs.