What Is a Furnace? An Introduction to the Most Active HVAC System in Your Home

Furnace, air handler, heater, “that thing in the basement” - I’ve heard it all. But no matter what you call it, your furnace is the most active HVAC system in your home.

Though some people know that furnaces provide heat for your home, few know that furnaces also play a much larger role.

As a residential sales professional, I’ve helped countless customers understand the role furnaces play in their home. And that’s what we’re going to talk about today.

In this article, we’ll discuss what a furnace is, what a furnace does, and why a furnace is one of the most important HVAC systems in your home.

What Is a Furnace?

The short answer: your furnace is an HVAC system that heats your home.

For a more thorough answer, let’s talk about what your furnace does year-round.

What Does a Furnace Do?

During colder months, a furnace heats your home by pulling air across its heating components. From there, the heated air travels through your ductwork and to individual rooms of your home.

Throughout the year - even during warmer months - a furnace circulates conditioned air around your home.

But even beyond that, your air conditioner can’t work without a furnace.

A furnace houses two components that your air conditioner relies on: the evaporator coil and the blower motor.

An evaporator coil is essential for cooling your home.

A blower motor controls air circulation.

So your furnace plays a larger role in keeping your home comfortable. And supports other systems throughout the year.

Because of this, I usually recommend that our customers upgrade their furnaces before they upgrade their air conditioners. It helps that furnaces generally have a lower initial cost than air conditioners.

We discuss more in-depth how different types of furnaces work here.

Types of Furnaces: Fuel Sources

Furnaces run on a variety of fuel sources.

In the United States, the most common fuel sources for furnaces are:

Natural gas
Electricity
Oil
Propane

Here in Central Ohio, gas and electric furnaces are the most common. But if you don’t have access to a natural gas line, then your furnace may run on propane or oil.

Gas furnaces can be the most cost-effective because of the low cost of natural gas.

Electric furnaces are an alternative to gas furnaces. But since they use electricity to heat your home, they tend to be expensive to run.

Propane furnaces can be an alternative to gas furnaces, especially for homeowners in rural areas.

Oil furnaces can typically be found in older homes now. As these furnaces age, they especially need more maintenance visits. Oil burns dirtier, which means your system will require more frequent visits.

Oil can also be a volatile fuel source. If it becomes contaminated, it can harm the system.

The type of fuel also affects what type of cooling system your furnace can pair with.

Gas furnaces are typically paired with air conditioners. But if you have a propane furnace, you may have a heat pump.

Check out how a gas furnace works in the video below.

Furnace Efficiency Ratings

A more efficient furnace saves you money.

A furnace’s efficiency is scored with an Annual Fuel Utilization Efficiency, or AFUE, rating.

An AFUE rating describes how much of the fuel your furnace uses actually goes towards providing heat for your home.

A standard efficiency furnace is usually around 80% efficient, which gives it an AFUE rating of 80. This means that 80% of the fuel your furnace consumes actually heats your home. The other 20% is vented as byproducts like waste gas.

High-efficiency furnaces are usually at least 90% efficient. The best gas furnaces can get up to 98%.

Electric furnaces are the only furnaces that are 100% efficient. But because of the cost of electricity, electric furnaces may not be the most cost-effective option.

The U.S. Department of Energy requires furnaces to be at least 78% efficient.

How Do I Know What Type of Fuel My Furnace Uses?

You usually don’t need to consult your owner’s manual for this one.

If you aren’t sure about the type of fuel your furnace uses, don’t sweat it! Your HVAC partner should be able to help you figure it out.

These are the questions I usually ask when a customer and I are trying to figure out what type of furnace they have:

Are there pipes coming out of your furnace? If there are, you have either a gas, oil or propane furnace. If not, you have an electric furnace.
Do you have a natural gas bill? If you do, your furnace is likely a gas furnace.
Do you have a tank in or outside your home? If you do, then your furnace is probably either propane or oil.

Types of Furnaces: Staging

Have you ever noticed that some furnaces run louder or longer than others? This is generally due to staging.

Staging refers to the number of capacity settings a furnace can operate at.

A furnace can be:

Single-Stage
Two-Stage
Modulating or Variable-Speed

A single-stage furnace only has one setting: 100% capacity. This means a single-stage furnace is either on or off.

A two-stage furnace has two settings: 100% capacity and a lower setting that’s usually around 60-70%.

A modulating furnace can have 5-100 settings that range from 100% capacity to 40% capacity.

A furnace’s staging mainly affects its runtime. A longer runtime can better heat your home.

When your furnace runs longer, it can better circulate air and therefore heat. And because a multi-stage system can adjust its output, you save money on energy costs.

How Do I Know What Staging My Furnace Has?

The owner’s manual for your furnace should be able to tell you what your current furnace’s staging and AFUE rating is. If you don’t have access to the manual, you also look up the model number online.

How do you find your furnace’s model number? Good question.

Your furnace should have a data sticker, which includes information like the model number. But its location may vary.

If you have a newer furnace, it may have a data sticker on the exterior. Check the top of the furnace and the right and left sides.

Don’t be deceived - you may also notice a data sticker for your system’s evaporator coil. The evaporator coil sits on top of your furnace.

But if you don’t see your furnace’s data sticker, you’ll have to remove the front panel. Look for the data sticker (and model number) on the left and right near the front of the furnace.

If your furnace has two front panels, you may need to remove both.

Getting the Most Out of Your Furnace

Regardless of the type of furnace you have, the best way to get the most out of your system is to maintain it.

In general, your HVAC system should be serviced twice a year: once for your heating system and once for your cooling system.

You should also regularly change your furnace filter.

Filters clean both the air you breathe and the air that goes through your system.

Filters can neutralize odors and remove particulates like dust and pollen from the air in your home. But filters that are clogged or too dense can lead to static pressure.

Static pressure is resistance to airflow within your system. In some cases, static pressure can stop air circulation completely.

Clogged filters are most commonly the culprit, which is why regularly changing your filters is important for both your system’s performance and your comfort.

Some filters need changed monthly; some only need changed once a year. Your filter will have a recommended replacement schedule on its packaging.