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How Does a Furnace Work?

A uniformed George Brazil tech looking at a furnace with a client

A furnace is essentially a heating system designed to produce and distribute warm air throughout a space. While the concept of furnaces has existed for millennia, with early central heating systems tracing back centuries, their significance in contemporary homes remains undiminished. Today's furnaces, however, are a blend of age-old principles and cutting-edge technologies.

Three fuel sources primarily power modern furnaces: propane, natural gas, and electricity. And while there are several furnace designs, such as single-stage, two-stage, and variable speed, their core functionality is consistent. In this article, we'll delve into:

Remember, like all machines, furnaces require regular upkeep. Always consult a professional for maintenance and repairs.

Reach Out to the Professionals

George Brazil Heating & Cooling is Phoenix's most trusted HVAC company. In business since 1955, our furnace repair and heating tune-up services have a 100% satisfaction guarantee. We treat our customers like family and have more than 2,000 5-star reviews.

The Basics of Furnace Operation

Understanding the operation of a furnace is akin to unraveling a fascinating dance of science and engineering. At its core, a furnace is designed to convert fuel into heat and then distribute this heat throughout a designated space.

Furnace Components

There are six basic components in a furnace. They are the thermostat, gas valve, burners, heat exchanger, blower, and air ducts. Each of these components interacts to make the system work.

Combustion Process

The thermostat activates the furnace when temperatures in your home drop below a set level. This starts a burner and opens the gas valve, igniting the air via an electronic ignition switch. (Using a pilot light for this purpose is out of date today.) The gas burns in a combustion chamber and heats a heat exchanger. Any byproducts of the combustion process are vented outside through the vent pipes and flue.

Heat Transfer

The heat exchanger transfers the heat from the combustion chamber into the plenum chamber. That heat generated by the combustion process is then distributed through your home's air ducts by the blower to provide warmth. As part of this process, a separate vent draws colder air into the system to be warmed.

Temperature Regulation

The temperature of the furnace is regulated by a thermostat in the house. Heating continues until the temperature specified by the thermostat is reached. The gas valve then shuts off, activating again on a temperature drop or change on the thermostat.

It's common for a furnace to be paired with a central air conditioner. It's also increasingly common to combine them with a heat pump system to maximize comfort.

Efficiency and Safety Features

An important part of a furnace's function is its efficiency features. Condensing furnaces have a second heat exchanger to extract more heat from the exhaust gases. A furnace filter protects the blower fan from hair, dust, and other debris, allowing it to function more efficiently. It also improves your home's air quality by preventing the circulation of allergens and particulates. The flame sensor, which switches off the gas should the furnace not ignite or go out, is a critical safety feature.

Types of Furnace

Three basic types of furnaces are available for American homes.


In a single-stage furnace, the valve that supplies the burner has two possible settings: off and full blast. They're also called single-speed furnaces. They distribute heat either at the furnace's maximum capacity or not at all.


A two-stage furnace uses a more efficient three-position valve. It has closed, partially closed, and open settings. This design allows for two speeds: full blast and 65% capacity, depending on the need. More energy-efficient than single-stage designs, these are the most common furnaces in newer homes.

Variable Speed

A variable-speed furnace can automatically adjust the blower's fan speed according to the needed heat level. This provides finer control than a two-stage furnace. The design confers added efficiency and saves on energy costs.

Need a new furnace? Call George Brazil today!

The expert team at George Brazil Heating & Cooling makes us one of the most trusted HVAC companies in Phoenix, AZ. We provide fast and convenient scheduling and can complete most repair jobs in a single visit. Take advantage of our 5-star heating tune-up and furnace repair services and our reliable 100% satisfaction guarantee.