Main Parts of a Furnace (with Diagram)
When the temperatures start to drop in the winter months, your furnace may struggle to maintain a comfortable temperature in your home. While that may sometimes mean that you need a new furnace, it most often means that one of your furnace parts could benefit from a little maintenance. Understanding all of the various parts of a furnace can help you to make an informed decision about what is best for your home.
Your furnace will have very different parts, depending on the type of unit you have. In general, there are four different types of furnaces: gas, electric, oil, and dual fuel. However, electric and gas models tend to be the most common with the latter being considered the most efficient according to EnergyStar.
The heat exchanger, burner, flue, ductwork, and ventilation pipes are the main parts of a furnace. Electric furnaces are unique and feature a contactor, sequencer, and transformer. Gas furnaces have key parts such as gas valves and draft inducer motors that electric furnaces don’t have.
With that said, we’ve drawn up a list of all the essential parts of a furnace that you should be aware of as a homeowner.
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Parts of a Gas Furnace
There are a number of parts that make up a furnace system and they all work together to produce heat and distribute it to your indoor living spaces. A gas furnace can be broken down into 12 main parts. The following are the basic components that makeup nearly every type of gas furnace, regardless of the model or manufacturer.
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Heat Exchanger
The heat exchanger is the core of your gas furnace. Through an exchange process, the component generates heat – as the name suggests. This part is comprised of a set of metal tubes or coils that are S-shaped. These tubes are wrapped around the inside of the heat exchanger.
Gas flames burn inside of these tubes and the toxic gases are vented through the roof and out of your home. A blower motor inside of the furnace blows the heated air into your ductwork from the heat exchanger and is circulated throughout your home.
The heat exchanger is a crucial component that makes up gas furnaces. A cracked or damaged heat exchanger can seriously impact your home’s heating. As gas burns, it emits toxic fumes like carbon monoxide and its’ the heat exchanger’s job to keep these harmful gases separate from the air blown into your home. Damage that occurs to the heat exchanger will allow exhaust gas to escape and contaminate your home with carbon monoxide.
Therefore, your gas furnaces’ heat exchanger should be inspected on a regular basis to fix these issues before they occur. Also, carbon monoxide detectors should always be used in a home with a gas furnace to help you spot potential problems easily.
Blower Motor
This brings us to the blower motor, which, as previously mentioned, is the component that blows air into your living spaces via your vents. Put simply, the blower motor pushes the warm air that is generated in the heat exchanger through your ductwork and into your home.
Blower Motor Capacitor
The blower motor capacitor is the component that starts up the blower motor. In some cases, the blower motor capacity also helps to keep the blower motor running at a consistent speed.
Gas Burners
The burners are where gas and air are combined to generate flame, therefore becoming the heat source of your gas furnace. They heat up the air inside of the heat exchanger. Fuel is sent to these burners, which are ignited by the hot surface igniter or the pilot light, sending heat to the heat exchanger.
Igniter and Pilot Light
A gas furnace must have a way to ignite the gas in order to start a flame. Depending on the age of your gas furnace, it will either have an igniter or a pilot light that serves this purpose. In older gas furnaces, a pilot light burns constantly which lights the gas. The moment the gas furnace is turned on, the pilot light is ready to ignite the gas and start the heating process.
If your gas furnace has a pilot light, it will also have another essential component – a thermocouple. The thermocouple works in conjunction with the pilot light to sense when it is lit. It has two small metal wires inside of a tube that conducts electricity when they’re heated, sending a signal to the gas valve to allow the gas to flow. If the thermocouple does not sense heat from the pilot light or the component has gone bad, it will shut off the gas.
The more modern furnaces, instead, use what is known as a hot surface igniter (HSI). This is a relatively new technology that involves an electric current passing through the igniter, raising the temperature, and initiating combustion when the gas supply is switched on.
With an HSI, your gas furnace will also have a flame sensor. The flame sensor serves as a safety to detect heat from a hot surface igniter. If the flame sensor does not sense heat, it will turn off the gas.
Gas Valve
Natural gas is distributed throughout the country into cities and into your own home. As it travels from one stage to another, it changes pressures. By the time it reaches your home, the pressure is generally 8-12 inches of water column (the measurement used to measure gas pressure).
When this gas goes through the gas valve in your furnace it is converted to the correct pressure needed to operate the unit – approximately 3-5 inches of water column. In short, the gas valve controls the gas that enters a furnace, including shutting it off when there is a failure with a safety switch.
Draft Inducer Motor
When gas is burned, it gives off toxic fumes. As a result, it is absolutely crucial that these fumes are vented out of your home. Before the gas in your furnace is allowed to burn, the draft inducer motor switches on. It creates a small vacuum, pushing the toxic fumes out of your home, through the vent pipe.
Pressure Switch
A gas furnaces’ pressure switch is in place to ensure that the draft inducer motor is on and is pushing harmful gases out of the vent pipe. It functions by using a small diaphragm that pulls in when the inducer motor is on, resulting in a vacuum.
If a vacuum is present and the diaphragm can pull it, a switch is activated that allows gas to flow. If no vacuum is sensed, no gas will be allowed to enter the system.
Flue
Also known as the vent pipe, the flue is what carries the harmful byproducts of combustion, such as carbon dioxide, out of your home. It also removes the not-so-harmful materials like water vapor.
Limit Switch
If your gas furnace becomes too hot, the limit switch will initiate to shut off the gas as a safety precaution. It detects the temperature inside of the furnace and if it senses that it’s too hot, the limit switch will turn off the gas.
Parts of an Electric Furnace
Like a gas furnace, electric furnaces have several different components. While you may not need to be familiar with every single part, you should be aware of the important ones. That way, you’ll know exactly what to look for if your furnace breaks down. An electric furnace can be broken down into four main parts:
- The Contactor: The contactor is essentially a control that is used to get the voltage switched to your furnace heating element. It works directly with your thermostat, meaning if the thermostat doesn’t have a very high voltage, the contactor will be energized by it. Once your thermostat is satisfied with the temperature, the contactor shuts off by opening the connection. The contactor is a crucial component to keeping your home comfortable during the winter season.
- The Heating Elements: The heating element inside an electric furnace consists of several, tightly-wound metal coils. When your thermostat is turned on, electric relays will begin the process of ignition. However, not all of the coils will heat up at once.
- The Sequencer: On an electric furnace, a sequencer turns the heating elements on or off. It controls when different sets of coils are permitted to heat. The reason why it does not allow all of the coils to heat at once is because the power draw required would be too large for your electrical system to handle. Electrifying the coils in a particular sequence allows them to heat in a pattern, producing sufficient heat without overloading the electrical panel.
- The Transformer: The purpose of the transformer is to supply the power for the furnace. It transfers all the electrical energy from one circuit to another. In an electric furnace, you’ll have multiple currents that go through it.
Unlike the gas alternative, electric furnaces do not have gas burners, pilot lights, or hot surface igniters. Instead, the electrical ignition system is what ignites the furnace’s heating elements inside. As the electrical current passes through the coils, the air absorbs the heat created. Then, a blower motor distributes the heated air through the ducts and into your living area.
Additional Furnace Components
Ductwork
While the ductwork isn’t exactly a part of either a gas or electric furnace, it is an essential component that allows your home to be properly heated. Without ductwork, all the heated air that is created inside of the furnace will not be distributed throughout your home. Also, without ductwork that is properly sized, you will end up with rooms that are either too color or too hot.
Therefore, properly fitted and sized ductwork is a crucial part that contributes to the functionality of your entire furnace system, whether it’s gas or electric.
Return Air Filters
The return air filter is a necessary component to keep dust and debris from getting inside of the furnace. They should be inspected and replaced frequently, as a dirty air filter will prevent the unit from functioning properly.
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The Thermostat
The thermostat measures the temperature inside of your home and by setting it, it communicates with your igniter to turn on. It is essentially used to control the temperature in your home. You can use the thermostat to adjust the temperature in your home, based on your desired comfort level.
By turning the temperature on the thermostat up, the flame of the gas burner will get larger to increase the temperature in your home. On the other hand, when you turn the thermostat down, the flame will shrink to lower the temperature in your living spaces. The thermostat “speaks” to the heating elements on electric furnaces to control the heat dispersed.
Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.
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