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What size of furnace do I need in my home?

Your home’s furnace keeps you and your family comfortable throughout the winter. When it comes time to replace your older model, you might think getting a new furnace is as easy as picking one out online. In reality, the size of furnace matters, and buying the wrong-sized furnace—either too large or too small—can lead to efficiency and performance issues in your home.

In this article, we’ll provide you with a quick-and-easy guide to furnace sizing and address your top questions about finding the right size of system. Let’s dive in.

What is the average size of a furnace?

First, let’s discuss what “furnace size” means. In most cases, this term does not refer to the actual length-to-width, physical size of the system. Instead, your furnace’s “size” is how much heating output it creates, measured in British Thermal Units (BTU). The higher a furnace’s BTU, the more heating it can provide over a predetermined period of time.

Furnaces are not one-size-fits-all. They’re not even one-size-fits-most! A single furnace make and model can come in several different sizes, each designed for a different home. The average size of a furnace in your area depends on a number of factors: the size of your home, where you live, and the type of home you live in.

Square Footage

By far, the most significant factor in furnace sizing is your home’s total square footage. A larger home requires a larger furnace with a higher BTU output. But, it’s not just that simple. There are a number of other factors you’ll need to consider when sizing your home for a new furnace.

Climate

Where you live plays a major role in what size of furnace you’ll need. The colder your winters, the more BTU output your furnace requires. On this front, the United States is divided into five distinct climate zones. Here’s a breakdown of each:

Zone 1 | 30-35 BTU (Per-Square-Foot) | Regions: Desert Southwest, Humid Southeast, Florida, Texas

Zone 2 | 35-40 BTU (Per-Square-Foot) | Regions: California, High Desert, Sun Belt

Zone 3 | 40-45 BTU (Per-Square-Foot) | Regions: Mid-Atlantic, Oregon Coast, Middle Rio Grande Valley

Zone 4 | 45-50 BTU (Per-Square-Foot) | Regions: Colorado Plateau, Mountain West, Midwest, Northern New Mexico

Zone 5 | 50-60 BTU (Per-Square-Foot) | Regions: Alaska, Rocky Mountain States, New England, Minnesota

Homeowners in Albuquerque typically fall in Zone 3, while homes in Santa Fe and at similar elevations may be part of Zone 4. As we’ll discuss below, you should talk to a locally trusted HVAC company for an assessment of what climate zone your home falls in.

Other Variables

There are many other variables that can impact furnace sizing. These include:

Insulation: If your home has insufficient attic or wall insulation, it could lead to serious heating loss through your attic. This will need to be accounted for when installing a new furnace or—better yet—more insulation will need to be installed.
Windows: This includes both the total number of windows and the type of windows. Single-pane windows are less energy-efficient than dual-pane windows. Larger windows are more inefficient than smaller framed ones.
Stories: Even at equivalent square footage numbers, multi-story homes have different heating needs than single-story properties. This includes homes with basements.

As you’ve no doubt inferred by now, finding the right furnace for your home isn’t as easy as just knowing your square footage. We’ll get into this below, but the complexity of furnace sizing is one of the key reasons why you should always consult with a professional HVAC contractor.

Is a higher-BTU furnace better?

Not necessarily. But, this is a common misconception for understandable reasons. After all, why would anyone want less heating power? If a higher-BTU furnace means more heating, that’s a good thing—right?

It’s not a perfect analogy, but think about this in reference to your car or truck. A larger vehicle may provide more horsepower, but it also uses more fuel and costs you more when you fill up at the pump. If you only need a vehicle for your daily commute, a large pickup truck or SUV might not be the most-efficient choice. The same basic principle applies to your heating system.

As we’ll review below, it’s more important to find the right-sized furnace—not the largest one possible.

When it comes to furnace size, can a furnace be too big?

Yes. If your new furnace is too large for your home, you’re likely going to run into efficiency, performance, and comfort issues almost right away.

When the temperature in your home drops, the thermostat turns on the furnace, which starts up and blows heated air through your air ducts and into your home. An oversized furnace can do this really quickly—good, right? Not exactly. As a homeowner, you want a furnace that heats your home effectively over a long period of time. As the oversized furnace finishes heating the home, it turns off. The temperature drops, and it has to start back up.

This problem is known as “short cycling”, and it’s the root cause of many furnace issues. First, short cycling puts your home’s comfort on a rollercoaster ride. Your indoor temperatures swing wildly, depending on when the furnace is on. Second, since furnaces use the most energy when initially starting up (much like your vehicle!), the constant starting-and-stopping drives up your energy consumption and costs. Finally, this leads to unnecessary wear-and-tear on the furnace itself, which—over time—can cause breakdowns and shorten its lifespan.

Paradoxically, a more powerful furnace can actually result in a less-comfortable home. But, going in the opposite direction also leads to problems.

What happens if a furnace is undersized?

On the other end of the spectrum is a furnace that’s too small for your property. This typically occurs one of two ways: either the HVAC contractor made a mistake in measuring the home and accounting for its heating needs, or there was a home addition or renovation that significantly increased the square footage of the home.

If your furnace is undersized, every day and night is going to be a war of attrition for the system as it just barely keeps up with your home’s heating needs. You’ll likely hear your furnace running constantly, only to see the temperature in your home rise by 1-2 degrees. If you don’t hear the problem, you’re sure to notice it when you open your next utility bill.

Just as with short cycling, a constantly running system is incurring far more long-term stress than it was designed for. Too many years of this, and your furnace will inevitably break down, leading to you having to replace your heating system prematurely.

Is it better to undersize or oversize your furnace?

Honestly: neither. You don’t have to settle for either outcome. Finding a new furnace should remind you of the classic Goldilocks fairy tale: your furnace shouldn’t be too small or too large—just right.

Work with a licensed and experienced HVAC contractor or company. Let them know you’re in the market for a new furnace; many companies offer free in-home estimates on new systems. During this in-home estimate, they’ll take measurements of your home, weigh all the variables listed above, and determine what size of furnace is the perfect fit for your home.