What Size Furnace Do I Need? Your Guide to Finding the Perfect Fit

Shopping for a new HVAC system can conjure up quite a few questions—especially when you start digging into the specifications of the system. Furnace sizing, in particular, is one area that often causes people to scratch their heads. For example, you might be questioning what size furnace you need. If so, look no further—we’ve covered the basics of furnace sizing below.

What Are the Common Sizes for Furnaces?

When you’re shopping for a furnace, you’ll notice that sizes are expressed in British thermal units (or BTUs), which measure how much heat an appliance can produce in an hour. In short, furnaces with higher BTU ratings can generate more warmth than those with lower ratings.

The right furnace size for you depends on several factors, including the size of your home, its condition, and your local climate. With that said, residential furnaces often range from 30,000 to 120,000 BTUs.

Furnace Size by Home Size

Your home’s square footage is one of the main factors to consider when calculating what size furnace you need. Basically, bigger homes need larger furnaces because homes require 30 to 60 BTUs per square foot. 

Here’s a look at how many BTUs you need for a range of home sizes.

Additional Factors That Influence Furnace Size

While important, your home’s size is only one factor when it comes to calculating the right furnace size. Here’s what else to consider.

Your Home’s Age and Insulation Quality

Well-insulated homes don’t need as large of a furnace because the insulation acts like a warm blanket that traps heat inside your home and keeps it toasty. But if you have poor-quality or old insulation, it’s easier for heat to escape from your house. 

In particular, many older houses were built with weaker insulation than modern homes, so they tend to lose more heat and get colder during winter. To compensate, older and poorly insulated homes will require a more powerful furnace that can provide a bigger, more consistent flow of heat.

Your Region’s Climate

As we’ve mentioned, your local climate also plays a significant role when sizing a furnace. It’s pretty simple: If you live in a colder area (like the Midwest or Northeast), you’ll need a furnace with enough heating capacity to warm you up when it’s freezing outside. In milder or warmer climates, your furnace can be smaller since it doesn’t have to produce as much heat.

Furnace Efficiency

Like a car with good gas mileage, high-efficiency furnaces can create more heat with fewer BTUs. If you have a furnace with a high Annual Fuel Utilization Efficiency (AFUE) rating (rather than a low-efficiency model), it can be smaller but still produce the same amount of heat.

Other Home Features

There are other factors that impact your home’s ability to distribute and retain heat and, therefore, the furnace size you should choose. These include: 

• Number and quality of windows: With more windows, there’s more potential for heat to escape from your home. However, you can prevent some heat loss by installing high-quality, energy-efficient windows.

• Home layout: Heat distribution may work differently in homes with open floor plans versus closed ones. Also, if you have multiple stories, you may need zoned heating.

• Ceilings and walls: High ceilings and indoor walls can impact heat flow.

• Sunrooms: Due to their unique design, sunrooms may need special consideration when it comes to heating.

How to Calculate Furnace Size

Speaking with a local furnace installer is the most accurate way to figure out what size furnace you need. However, you can use the following formula to estimate the right furnace size for your home:

Home’s square footage x Heating factor = Recommended furnace size range

To use this formula, start by calculating your home’s square footage. Then, determine your region’s heating factor—or how many BTUs you need based on your climate zone.

BTUs Needed for Each Climate Zone

Each climate zone has its own recommendations for BTUs per square foot. Don’t forget that these aren’t set-in-stone numbers and will still be impacted by other factors like home size, insulation, and the number of windows you have. The real-world recommendations for your furnace’s BTUs may be higher or lower than these, which is why you should only rely on an HVAC professional’s evaluation for what size furnace you need.

What to Do if You Have the Wrong Size Furnace

When your furnace isn’t the right size for your home, you could encounter several frustrating or expensive problems. If it’s too small, it will have a hard time heating your home properly because it isn’t big enough to do so. In turn, it will run for longer cycles, which can increase your energy costs.

On the other hand, if your furnace is too big, it may provide more heat than necessary, resulting in overly hot areas within your home. When this happens, the furnace will turn on and off more quickly than normal (known as short cycling). The constant switching between modes can also lead to higher energy bills.

Both of these situations indicate that your furnace isn’t operating as it should. As a result, it could break down faster and need replacing earlier than it would if it were the right size. In either case, it’s best to have a furnace repair pro take a look.