How Does a Furnace Work?

A forced air heating and air conditioning system remains the #1 type of HVAC system in North American homes. Here is the explanation for how these systems heat and distribute warm air throughout your abode.

Note – The focus here is on a system that heats using a furnace – gas or oil. A heat pump HVAC system also uses forced air distribution, but the heating mechanism is very different.

Explaining a Forced Air Heating System

This infographic is your visual guide to the question, “How does a hot air distribution HVAC system work?” This question is often asked as, “How does a forced air system work?”

How Does Furnace Work

Along the way, we define key terms to explain a forced air HVAC system operation.

Step 1: A Thermostat Calls for Heat

The system in the picture is a zoned system with three thermostats. Zoning is explained in Step 5.

When the indoor temperature in your home or one of its zones drops to the thermostat set point, the thermostat triggers the furnace to start. For example, in the fall, daytime temperatures might be in the comfortable mid-70s. But in the evening, the temperature drops.

If the thermostat set point (or setpoint) is 70 degrees F, when the indoor temperature falls below that temperature, the thermostat will open the contact to the furnace, and the furnace will begin a heat cycle.

Here are key definitions:

  • Set Point: The temperature at which your thermostat will signal the furnace to fire up. The thermostat might be a programmable model that can change the set point up to four times a day. Or it might be a learning thermostat like nest models . Or it might be a non-programmable thermostat that you have to manually change every time you want to adjust the set point. Learn more about thermostats in our Ultimate Thermostat Buying Guide.
  • Ducts/Ductwork: The series of sheet metal or insulated flexible ducts used to distribute heated air (aka treated air) to living spaces and pull cool, untreated air back to the furnace.

Step 2: The Furnace Fires Up

It’s a several step process.

1). Ensure Fresh Air: First, the draft inducer motor and fan start. This fan pushes combustion gas out the flue/chimney. Its purpose is also to ensure that there is no blockage in the chimney, such as a bird or animal nest that would prevent proper venting.

2). Start the Igniter: Then the igniter begins to heat. Most are hot surface igniters, aka glow plugs, that use electric resistance heat to get hot enough to ignite gas or oil.

3). Fire Up: The gas valve opens to allow gas to flow into the burners and be ignited.

4). Exhaust Combustion Gases through the Heat Exchanger: The air in the combustion chamber never mixes with air in your home because it contains deadly carbon monoxide. Instead, it is vented out of your home through one or two heat exchangers. A heat exchanger is a section of back-and-forth piping inside the furnace. The design, with long runs of pipe, allows more heat to transfer through it.

heat exchanger

Furnaces with efficiency over 90% AFUE have a secondary heat exchanger to draw as much heat as possible out of the combustion gases before they exit your home.

5). Push Out Warm Air; Bring in Cold Air: This is the blower’s job. It’s powerful fan circulates air over the heat exchanger. The air gets warm, and is pushed of the furnace while it also pulls in cool air.

Definitions:

  • Draft Inducer: A fan located in or next to your furnace that pulls in fresh air and pushes out dangerous combustion gases. The inducer motor is the first thing to start in the furnace once the thermostat calls for heat. If you’re near the furnace, you might hear it come on up to 60 seconds before the burner fires up.
  • Blower: The motor-driven fan that circulates air through the ducts.
  • Gas Valve: A valve regulating the amount of gas entering the furnace. Single-stage (1-stage), 2-stage and variable capacity gas valves are used in gas furnaces and explained in our Gas Furnace Buying Guide.
  • Burners: Tubes through which the gas flows and burns.

Step 3: Distribute Warm Air

Also called treated air, warm air created by the burner and transferred through the heat exchanger is pushed through the supply ducts and enters various rooms through the heat grates.

Notice that supply grates are located in or near the floor. This is for the obvious reason that warm air rises.

What about air conditioning? The air will flow in the same direction as it does when heating. Cooled air, aka treated air, will be pushed up and through supply/floor grates. Warm, humid air will be pulled in through return grates, explained in the next step.

  • Supply ducts and grates: These are the ducts and grates through which freshly heated air flows into the rooms of your home.

Step 4: Pull Cold Air to the Furnace

Steps 3 & 4 happen in a continuous cycle of airflow. The blower fan simultaneously pushes warmed air into the supply ducts and creates negative pressure that sucks cool air into the return ducts.

  • Return ducts and grates: This is the system of grates and ducts through which cold air is pulled back to the furnace to be heated. The ducts are also referred to as cold air return ducts.
  • Note on grate location: Depending on your climate and the design of your HVAC system, return grates might be located on the floor and supply grates on the ceiling.

Step 5: Zoned Heating

Notice that there are three thermostats in the drawing? Each one controls the temperature in a separate zone. To do this effectively, each zone must have separate ductwork – meaning air flowing through one set of ducts doesn’t mix with air in the other sets.

Each zone’s main supply duct has a motor-controlled damper. If Thermostat #1 calls for heat, the damper to the zone’s ductwork opens up allowing heated air to be pushed into it to the grates in the rooms comprising the zone. When the thermostat is satisfied, meaning the thermostat set point has been reached, the furnace will shut down and the damper will close.

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