A furnace is a complex system with many interconnected parts that all work together to keep your home warm. To properly take care of your heating system and be able to recognize when something’s gone wrong, it’s helpful to know the basics behind how your furnace operates and the roles that some of those vital parts play in the heating process. Today we’re going to help familiarize you with the work that your furnace does by giving you a glimpse behind the typical heating cycle!

Heating cycles work in phases

If you pay attention to your furnace from the moment it turns on to the moment it shuts off, you’ll notice that it goes through multiple phases. Even if you don’t know exactly what’s going on, you can hear the sound of the system turning on, followed by the whirring of a small fan, followed by the sound of the burners igniting, and finally the noise of air blowing through your ducts. So what exactly is going on in each of these steps?

The draft inducer phase of a heating cycle

The first major phase of the heating cycle begins when the thermostat detects the need for heat in your home and sends a signal to your furnace’s control board. From there, the control board turns on the draft inducer fan. This is a small fan that pulls air through your furnace’s combustion chamber. It is responsible for supplying oxygen to the burners and for ensuring that combustion gases are directed into your system’s flue pipes so that they can be safely carried out of your home.

When the draft inducer fan reaches its full speed, the control board then begins the ignition process.

The ignition phase of a heating cycle

The ignition phase begins by starting the furnace’s igniter. In older systems, the igniter is a pilot light that will already be lit. In newer systems, the igniter is an electronic ignition that sparks up on demand.

Once the igniter is ready, the gas valve to your furnace is opened and gas flows over the igniter. This lights up the burners inside of the combustion chamber. Right near the burners, there is a device called a limit switch that reads the temperature inside of the combustion chamber. When that temperature gets hot enough, the control board sends a signal to your furnace’s blower motor to start it up.

The air distribution phase of a heating cycle

With the blower motor on, your air handler begins to blow air over the walls of the hot combustion chamber. This air is directed to your home’s supply ducts and is distributed to the rooms in your home. As the heating cycle continues, the air is drawn from the rooms in your home through the return vents and back to your furnace to be heated again. This cyclical process continues until your thermostat is satisfied and your system shuts down until the next heating cycle.

If you have any questions about a furnace’s heating cycle, or if you’d like a heating system serviced or installed in your home, contact Jerry Kelly Heating & Air Conditioning, your St. Charles, MO, furnace installation and repair contractor. We provide service all over the St. Louis area, including towns like Ladue, Lake St. Louis, and Manchester, MO.

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