As the heating season approaches, you’ll want to ensure your furnace is in working order before the temperatures drop. Part of taking a proactive approach to your furnace service is knowing how your furnace functions and all of its components that work together to provide heat. It’s also important to understand the different types of furnaces to know what works best for your home and your preferences.

Then, if something goes wrong, you will know if you can DIY a solution or if you need to call in a professional for furnace repairs or service. So, do you know what’s inside your furnace? Let’s find out what makes your furnace tick.

Key Takeaways:
  • Knowing how your furnace works is a key part of DIY furnace maintenance.
  • Different furnaces have different components and operating mechanisms that are important to understand for furnace service.

What Is a Furnace?

Simply put, your furnace is a heating unit that generates and distributes heat throughout your home. It is the original type of heating system that is widely still used in homes today. There are many different types of furnaces and various kinds of fuel that are used to make these systems work.

Each furnace works with a specific fuel source and has different parts and components that allow it to function properly, all with different ways of operating. There are also different costs, energy-efficiency ratings, and characteristics for furnaces.

Parts Of Your Furnace

Before you can learn how your furnace works, you have to learn about the components that make it up. Having a clear understanding of the parts will help you realize how they work together to make your furnace tick, and how small issues can cause major repairs.

Heat Exchanger

When a furnace burns to create heated air, toxic gasses are emitted that can be hazardous to human and animal health. The heat exchanger’s purpose is to keep the heated air that enters your home free of toxic gasses.

This part consists of long, s-shaped metal tubes. The heat exchanger is where the gas burns inside the tubes and vents any toxic gasses out of the home, which is typically through the roof. Once the gas is burning inside the tubes and they have been heated, the blower motor will push cold air past the outside of these tubes, resulting in the air being heated. This is how the toxic air is kept away from the air that flows into your home.


The burner in your furnace is the area where air and gas are combined, which will result in a flame. This flame then becomes the heat source that warms the air that circulates in your home.

Pilot Lights and Igniter

If you have a gas furnace, your furnace will need a way to ignite the gas to create a flame. Newer furnace models will typically use a hot surface igniter (HSI) that will get red hot and light the gas. However, older furnace models will usually have a pilot light that is constantly burning which will light the gas.

Gas Valve

A gas valve is the component of the furnace that allows gas to flow into the furnace. It’s also responsible for shutting off the gas if there’s a problem, such as a mechanical failure that can present a safety risk.


The thermocouple allows the furnace to detect whether the pilot light has been lit. Once the thermocouple senses heat, it will send a signal to tell the gas valve to let gas flow into the area.

Flame Sensor

When an HSI is used in your furnace, it needs a flame sensor present. The flame sensor acts as a safety device to detect when the HSI has been used. When a flame sensor doesn’t detect any heat, it will shut the gas off so that there are no problems.

Draft Inducer Motor

As you have already learned, when gas combusts, it creates toxic fumes. The draft inducer motor’s purpose is to push those toxic fumes out of your home through a vent pipe instead of allowing them into your home’s heated air. It turns on before the gas begins burning and creates a kind of vacuum to remove those toxic fumes.

Pressure Switch

The pressure switch in your furnace has an intricate purpose and setup. It is used to ensure that the draft inducer motor is fulfilling its job and pushing the toxic gasses out.

When the draft inducer motor is on, it will create a vacuum that causes the pressure switch’s diaphragm to pull in and activate. Then, when the pressure switch is activated, gas will flow into the system. If the pressure switch does not feel the pull from the vacuum, it will refrain from allowing gas into the unit.

Blower Motor

When all components of your furnace are working correctly, one thing you’ll need to have functioning properly is the blower motor. This component is the part of your furnace system that blows the hot air into your home. When the blower motor pulls air in from the return air filter, it runs it past the heat exchanger to be heated, and then the hot air enters your home through the vent system.

Blower Motor Capacitor

Your blower motor will need to be started and run at a steady pace. That is where the blower motor capacitor comes in. This part of the furnace system will help start up the blower motor and keep it running steady while the heating system is on.

Furnace Limit Switch

A furnace running too hot can become a problem very quickly, sometimes leading to fires. The furnace limit switch is a safety feature that turns off the gas as a safety measure if the furnace begins to overheat. That is why your furnace limit switch should always be in the best working condition.

Return Air Filter

All furnaces should have clean air filters for many reasons. The return air filter is used to help keep debris and dirt from entering the system, which could end up back in your indoor air. When these are not replaced and are very dirty, it could cause the furnace to stop working correctly or compromise your air quality.

Types Of Furnaces

Now that you’ve learned the different parts that make your furnace operate, you should know the different types of furnaces and fuel options. This can not only help you understand your current furnace but also determine what kind you want if you need a replacement.

Single-Stage Furnace

Single-stage furnaces have only a single speed. They run on either full power or turned off. This allows them to constantly distribute hot air into your home because the furnace is working at maximum capacity.

Two-Stage Furnace

A two-stage furnace works differently than a single-stage furnaces. Instead of only having one speed, they operate at two speeds. This allows them to function more efficiently because they’re not operating at full capacity constantly. Because they are more efficient, two-stage furnaces are usually the top pick for residential homes.

Fuel Used

There are a few different options for the kind of fuel that is used for a furnace. These fuels include:

  • Electricity
  • Natural Gas
  • Oil

How Your Furnace Works

You might have a general idea of how your furnace works, but knowing the full process can help you decide if a furnace service is required to help your system function properly. The steps below are how a gas-powered furnace works.

  1. A thermostat is used to activate the furnace.
  2. The burner will drive gas to the heat exchanger once the gas valve is switched on.
  3. The air will be ignited by the gas.
  4. The heat exchanger will then complete its job by pushing the toxic gasses out of the home.
  5. The blower motor will then push the cold air through the system to heat it.
  6. The air will be pushed throughout your home to heat it.
  7. All dust and debris will be caught by the air filter.

Measuring How Energy-Efficient Your Furnace Is

If you’d like to know how energy-efficient your furnace system is, you’ll need to know your furnace’s Annual Fuel Utilization Efficiency (AFUE). This is what tells you the amount of fuel that you have used to heat your home and how much has been lost due to combustion.

If your AFUE is 90% or above your furnace is considered to be highly efficient. AFUE ratings that are 50% to 70% have a lower efficiency.

Tips For Furnace Maintenance And Safety

Furnace maintenance is one of the most important aspects of keeping your furnace running optimally and safely. Here are some tips for DIY furnace maintenance as a homeowner:

  • Every 30 days check your carbon monoxide and smoke detectors.
  • Keep an eye on the color and characteristics of your furnace flame.
  • Never operate a system if you smell gas.
  • Seal your ductwork to prevent air loss.

How To Keep Your Furnace Running Smoothly

The best thing that you can do to ensure your furnace will continue running properly throughout the colder months is to schedule a furnace service. A quick tune-up can identify any potential problems and make sure that you avoid large and expensive ones, especially in the middle of the heating season. Routine services are recommended twice a year to keep your furnace in the best shape.

The JW Plumbing, Heating and Air team are experts in furnace service and tune-ups. Give us a call to schedule an appointment today!

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