Choosing the right size furnace is an important part of owning a home. Not only will a suitably sized furnace efficiently heat your house, it will also keep your family comfortable and help regulate the energy bills.
But with so many different sizes and models on the market, how do you know which furnace is best for your needs? Luckily, calculating the correct furnace size for your house is easier than you may realize.
In short, the size of your house and your local climate will play the largest roles in determining the right furnace for you. However, there are also a few other factors to consider, including furnace efficiency ratings, sun exposure and the condition and construction of your home.
Below, we'll walk you through everything you should know about shopping for a new furnace, including how they're rated and which size to consider for your home.
How furnaces are measured
Before discussing furnace size recommendations, it's essential to understand the basics behind furnace ratings.
Furnace capacity is measured in British Thermal Units, which express how much heat is required to increase the temperature of 1 pound of water by 1 degree Fahrenheit. BTUs are also used to measure heat pumps, air conditioners and similar devices.
Residential furnaces come in various sizes, but most homes should be fine with a furnace in the range of 30,000 to 100,000 BTUs. We'll explain how to choose your size below.
Factors to consider when choosing a furnace size
For the most accurate furnace size estimation, you should consider a few factors, including your local climate and the square footage of your home. Here's how these factors (and others) impact which size furnace you should choose.
As you might expect, bigger houses will require larger-capacity furnaces than smaller houses. To determine an appropriate furnace size, you'll need to know the approximate square footage of all heated areas in your home.
There's a good chance you already know the square footage of your home, but if you don't, try looking up your online home listing or blueprints. If you can't find either of those, you can get a rough estimate by multiplying the length and width of each heated room in your house and then adding it all together.
When you're buying a new furnace in the US, you also need to consider your climate. Depending on where you live, your heating needs could be drastically different than someone in another part of the country. Conveniently, the US is split into several different climate zones, so it's easy to find yours.
If you live in a warmer part of the country, like Texas or Florida, you won't need as much power from your furnace. In these zones, 30 to 40 BTUs per square foot should get the job done. You might even find that a furnace isn't necessary and get a heat pump instead.
It's a different story in the colder regions. In places like Wisconsin and Wyoming, your furnace needs to work harder to provide heat, so you may need around 50-60 BTUs per square foot.
New furnaces are rated on their annual fuel utilization efficiency (AFUE), expressed as a percentage and mandated by the federal government. Furnaces with higher AFUE ratings will transfer a larger percentage of heat into your home, resulting in less energy wastage and fewer emissions.
In the past, furnaces were only about 55 to 70% efficient. Today, you can find furnaces with AFUE ratings as high as 98.5%. Although these high-efficiency furnaces are more expensive, they don't need to be as large as less-efficient models. Plus, they can help you save money on energy bills since more fuel is converted into heat.
Outside of these factors, the conditions at your home can also influence what size furnace you need. For example, if your house receives a good amount of sun exposure each day, you could buy a smaller furnace than someone whose house is in the shade. Similarly, if you have a newly-built house with solid insulation, your furnace won't need to be as powerful as it would in an old or drafty house.
Calculating the right furnace size for your home
With all of this in mind, let's walk through some examples to determine which size furnace you need to keep your home warm.
Let's say that you have a brand-new 2,000-square-foot home in Houston (which is in Zone 1). In that case, you'd multiply your square footage by the estimated BTUs in your zone. To put it together:
2,000 square feet X 30 to 35 BTUs = 60,000 to 70,000 BTUs
On the other end of the spectrum, let's say that you live in an older 1,500-square-foot home in Minneapolis (Zone 5). In that scenario, you'd need:
1,500 square feet X 50 to 60 BTUs = 75,000 to 90,000 BTUs
In these examples, you'll end up with a range of furnace sizes – which is where factors like insulation and sun exposure come into play.
As a rule of thumb, you can stay in the lower end of the range if you have a newer home, good insulation and receive quite a bit of sunshine. If not, look toward the higher end.
For the new home in Houston, that means about 60,000 BTUs, whereas the older home in Minneapolis would need closer to 90,000 BTUs.
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The bottom line
Finding a furnace that fits your home is beneficial for many reasons, including your comfort and safety, your wallet and the environment. The examples above will give you a good idea of what type of furnace to look for, but you should consult an HVAC professional for a precise furnace size recommendation based on your home size, climate and other factors.
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