How to Determine the Right Size Furnace for Your Home

Marwa Hasan
Written by Marwa Hasan
Updated November 2, 2021
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When it's time to upgrade your furnace, a little bit of math goes a long way

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When it gets cold outside, there are few things as precious to a home as a furnace—it keeps you and your family warm and cozy through the long winter months. So, if it’s time to upgrade your existing heating system, determining the correct furnace size is an essential step. Let’s discuss the best way to estimate the furnace size and other key considerations that factor into a furnace’s efficiency and compatibility with your home.

Estimating the Furnace Size You Need

Buying the wrong furnace size is a common (and inconvenient) mistake. Furnaces that are too small produce insufficient heating, while furnaces that are too big cause excessive energy consumption. Plus, a furnace purchase is a substantial investment, so you’ll want to make sure you’re getting the right one.

Heating Capacity

The furnace size doesn't just mean the unit's actual physical dimensions. It also refers to the furnace’s heating capacity. The size of a furnace is estimated by how much heat it produces per hour, which is measured in BTUs (British thermal units). The BTU is the energy required to heat one pound of water by one degree Fahrenheit. 

The higher the BTU rating, the more warmth the furnace can provide. The average furnace typically produces between 80,000 to 100,000 BTUs an hour.

Your Home’s Square Footage

The size of your home and the number of square feet that need to be heated are important determining factors. Bigger homes require more BTUs to heat. The general rule of thumb is between 30 and 60 BTUs per square foot, depending on the climate where you live. 

To get the total square footage of your house, you can look up the original appraisal, listing, or lease. If you don’t have the paperwork, start by measuring the length and width of each room you’re planning to heat. Then, multiply the dimensions together to get the square footage of each room. Add all of the room totals together to get your home’s total square footage.

Here are the furnace BTUs required based on home sizes:

  • Furnace for a 1,200-square-foot house: 35,000 to 75,000 BTUs

  • Furnace for a 1,800-square-foot house: 55,000 to 110,000 BTUs

  • Furnace for a 2,100-square-foot house: 65,000 to 125,000 BTUs

  • Furnace for a 2,400-square-foot house: 70,000 to 145,000 BTUs

The BTU usage for each house size is not a single number but rather a range, so the square footage alone is not the determining factor. You'll need to consider several other factors, such as the climate zone and furnace efficiency, to get a more accurate idea of the BTUs it takes to heat your home.

Climate Zone

Where you live is an essential factor in picking the right furnace. This is why climate zones can help you determine how many BTUs per square foot you need for a furnace. Living in colder climates means you need a more powerful furnace. For example, a Texas home with mild winters requires less heating power than a Minnesota home. 

First, determine your climate zone. Then check this list to determine the BTU output necessary for your zone:

  • Zone 1 (Southern regions): This zone occupies desert and subtropical areas, including cities like Miami and Houston, and has a heating factor of 30 to 35 BTUs.

  • Zone 2 (Mediterranean): This zone includes California and Southern cities like Atlanta. The heating factor ranges from 35 to 40 BTUs.

  • Zone 3 (Oceanic and Humid Continental): This zone includes Virginia, Missouri, and Kansas and has a heating factor of 40 to 45 BTUs.

  • Zone 4 (Semi-Arid and Humid Continental): This zone includes Boston, New York, and Chicago and has a heating factor of 45 to 50 BTUs.

  • Zone 5 (Alpine and Humid Continental): The northernmost zone, including Buffalo, New York, and Minnesota’s Twin Cities, has a heating factor of 50 to 60 BTUs.

To calculate the necessary output, use your home’s square footage and multiply it by the BTUs your climate requires. 

For example, if you live in Zone 3, you will need a heater that produces 40 to 45 BTUs per square foot. The equation is figured as: 2,000 square feet (the home’s square footage) X 40 BTUs (climate requirement) = 80,000 BTU output. 

Furnace Efficiency

When shopping for a furnace, you’ll notice two numbers: an input rating in BTUs and an efficiency rating percentage. 

The second number indicates the furnace’s efficiency, which is how effective the furnace is in converting air to heat. Furnaces typically have around an 80% efficiency rating, although newer models have a standard 90% efficiency, and more expensive and powerful models have at least a 95% efficiency rating. 

Let’s refer back to the hypothetical example above. If the total is 80,000 BTUs of heat but the efficiency rating is 80%, it will only produce 64,000 BTUs. The correct furnace size, in this example, requires more heating power—a 100,000 BTU furnace with an 80% efficiency rating or higher will produce the 80,000 BTUs necessary.

Get a Second Opinion

Before buying your new furnace, you may want to get a second opinion from an expert to ensure the furnace is the right size. In fact, many municipalities require that contractors use software to determine the best furnace size for your home. A professional can also determine other factors, such as the age, the ceiling height, the construction of your home, and the insulation. A licensed HVAC technician with the proper experience and technology can help you figure out all the components to your equation, which can be part of the package of installing your furnace.

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