Every winter you remember how thankful you are for the furnace that keeps your home toasty warm. However, most people fail to ever take the time to understand how that magical appliance actually works. Discover how your furnace keeps your home warm and what you can do to make it more effective in performing its job.

Simple Heating Explanation

Before getting into the particulars of how a furnace works, let’s start at a high level of what it does. A furnace works simply by generating heat and then transferring it to the air circulating through the system. Once that heat is transferred, it’s pushed back out into your home. That’s the simplest answer you can get, but let’s dig deeper to look at the specifics of how your furnace warms up your house.

How a Furnace Generates Heat

The first thing to understand is how the heat is generated that warms the air coming from your vents. There are two types of furnaces, one that burns fuel and one that uses electricity. The electric type has an electric resistance coil, much like what you’d find in an electric space heater.

The fuel-burning type typically runs on either natural gas or propane. Unless you have a tank that has to be filled, you likely have a natural gas furnace.

Most modern furnaces have an electric ignition of some form. Some use what’s called a hot surface ignitor, which acts like a glow plug in a diesel engine. When it’s time to ignite the burner, it uses electricity to heat up a ceramic element until it ignites the gas in the burn chamber.

The other type of ignitor is called an intermittent pilot light. Unlike standing pilot lights, it does not remain lit all the time. Rather, it uses an electrode to light the pilot which then lights the burner itself.

What most people don’t realize is that it’s not actually the flame that heats the air moving through the system. Rather, it’s the hot exhaust from the burning fuel that contains the heat the furnace uses to warm your house.

Transferring Heat to Your Air

Getting the heat from the exhaust to the air circulating through your system requires some specialized components. After all, you don’t want to just mix the exhaust into your air.

Once the fuel is burned, the exhaust flows up through a series of tubes called the heat exchanger. The air moving through your system flows over these tubes, allowing it to absorb the heat from the exhaust.

In standard furnaces, this is where the most energy is lost. This is why you don’t have a 100% efficiency rating. Rather, the exhaust retains some of the heat, which is why the exhaust pipe needs heat shielding as it vents through your roof.

The minimum efficiency rating in the United States is currently 80%. This means that 80% of the heat generated by the burning fuel is transferred to the air, while the other 20% is lost in the exhaust.

The Importance of Airflow

Airflow is the key to making your furnace run. There’s the ability for your furnace to draw in air, heat it, and push it back out again. However, there’s more to airflow than that simple cycle.

Without proper airflow around your home, your furnace would only heat the air around the supply vents. However, if the system’s working properly, your home heats evenly. It’s the proper airflow that causes the heat to circulate.

When the furnace draws air in through the return vents, it creates negative air pressure. Then the air coming from the supply vents creates positive air pressure. Air is a type of fluid, so it follows the laws of fluid dynamics. One of those laws is that it seeks to equally distribute throughout its container, which means equalizing the pressure.

When your furnace is working properly, it creates enough of a pressure difference to cause the air in your home to move. Hot air also rises, so between the pressure difference and the temperature difference, the heated air moves around your home.

Aside from issues with the furnace inhibiting airflow, restrictions around your vents can also cause problems. The best way to ensure air can flow is to keep at least 2 inches around your vents clear, including at least 2 inches above them.

Your Air Filter’s Critical Role

One of the key parts to maintaining your furnace’s ability to circulate air is the air filter. The air contains small particles of dust, dander, pollen, and many other things. Left in the air, these would settle in your furnace and clog it, preventing it from drawing and pushing the needed air volume.

Your system’s air filter removes most of those particles before they enter your system. However, because it’s collecting these particles, you need to replace the filter periodically to prevent an airflow restriction.

The common 1- and 2-inch filters typically need replacing every 30 to 90 days. Thicker filters may last several more months, depending on your air quality and the thickness of the filter. Check with your system’s owner’s manual for the recommended replacement frequency. Inspect the filter every month to keep an eye on how quickly it’s clogging. When you check it, gently vacuum the loose contaminants from the intake side to extend its service life.

Maintaining and Improving Your Unit’s Efficiency

Without some attention and maintenance, your furnace will slowly lose its efficiency and experience more wear as it runs. This happens because of airflow restrictions that will form throughout the system. It also happens because mounting hardware and electrical connections can loosen and the thermocouple and burner get dirty. It can also occur because the circulating fan gets off balance. Your furnace’s components eventually start wearing out, which adds strain to the rest of the system. This can lead to extensive repairs.

Fortunately, you can easily protect your system by properly maintaining it. The first step to maintenance is replacing their filter regularly, as we’ve discussed above. The next step is to have professional maintenance every year.

During a maintenance visit, a technician performs a deep cleaning of the system, tightens electrical connections and mounting hardware, and balances your circulating fan wheel. They also perform tests on individual components and sensors to ensure they’re working correctly. Finally, they’ll test the entire heating cycle to confirm everything is initializing and turning off at the right times. If they find anything performing sub-optimally, you have the opportunity to repair it before it causes additional problems within your system.

What’s Different About High-Efficiency Units?

Understanding how standard furnaces work, what’s the difference in a high-efficiency unit? The first noticeable difference is the use of two different heat exchangers. This keeps the exhaust in the system longer, allowing the circulating air to absorb more of the heat before it vents outside. This alone significantly improves the unit’s efficiency.

Some furnaces are multi-stage, which means it has two different burner settings. They also often have variable speed fans, so there’s more variation than just on or off. Both of these mean the furnace can run at a lower level when the heat demand isn’t as high, reducing the energy and fuel consumed.

Since 2004, people around Saint Peters have turned to Agers Heating & Air Conditioning for reliable heating and AC installation, maintenance, and repair. Our expert team also provides indoor air quality solutions to help make your home’s air healthier. Contact Agers Heating & Air Conditioning to schedule your furnace maintenance or repair with one of our NATE-certified technicians today.

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