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What Size Furnace Does My Home Need?

Whether you are replacing an outdated furnace or need one for a new-build home, you need to find one that provides enough heat for your square footage. Working with an experienced HVAC contractor is one way to make sure your home gets the right size furnace installed. If you’re interested in doing it yourself, you can find your unique furnace size calculation to see how much energy your home needs to stay warm.

Pipes of a heating system

Table of Contents

What is BTU for Furnaces?

BTU stands for British Thermal Unit, and is a measurement that determines the thermal energy output of various devices. To put it more scientifically, BTU measures the energy it takes to warm one pound of water by one degree Fahrenheit. More simply, it means how much energy it will take to warm your home. For this post, we’ll apply BTUs to home furnaces, but be aware they are also be used to determine the energy it will take to cool your home.

What Size Furnace Do I Need?

A basic rule of thumb for furnace size calculation is that it takes 30 BTUs for every 1,000 square feet. So, if you have a 1,000 square foot house, you’ll look for a furnace that has 30,000 BTU output. However, there are several other factors besides the size of your home that might change that:

Your Home’s Age

Typically, older homes are less energy efficient and may require more BTUs to stay warm.

Quality of Insulation

Similarly, if your home has poor insulation you may need a larger furnace.

Quality and Number of Windows

If you have old windows, the window frames may be drafty and allow heat to escape. Newer windows will have better insulation. Windows facing the south will also passively heat your home, as they allow for sunlight to enter the home and retain the warmth.

Top Furnace Brands

When you've calculated what size furnace you need, click here to see which brands we rank as the best of the best for furnaces.

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Your Home’s Location

The energy your furnace requires definitely depends on where you live in the United States. If you live in Minnesota, you’ll want a furnace with a higher BTU output than someone in Louisiana. If you live in a cold climate, you may want to use a BTU of about 80 instead of 30 to calculate your furnace needs.

The Furnace’s Energy-Efficiency Rating

Newer furnaces will have a tag that shows their energy efficiency ranking. It’s best to choose a furnace with the highest ranking within your price point. Even if your home isn’t especially energy efficient, having an efficient furnace will keep your bills as low as possible while you stay warm.

What Else to Know for a Furnace Size Calculation

If you don’t know the square footage of your home, it’s fairly easy to calculate it. To measure the square footage for your home, multiply the length and width of each room that you wish to heat. Add the measurements for each room together and you will come up with the square footage of your house. Then multiply the square footage by BTU — 30 is for a moderate climate, but remember to use a BTU of closer to 80 if you live in a cold climate or have inefficient insulation. So this would mean that if your 1,000 square foot home was in Minnesota, your calculation would look like this:

80 X 1,000 square feet = 80,000 BTUs

Will I Need a Furnace Contractor?

A trusted HVAC contractor will know about the latest energy-efficient furnaces on the market. They can also determine if your furnace needs to be repaired or replaced completely. Working with an HVAC contractor will also help to guarantee you get the best furnace for you and your family’s needs. In addition to making sure it is properly installed, a contractor will present a few furnace options and explain the pros and cons of each type. If you’ve never replaced a furnace before, I suggest consulting with an HVAC contractor before you undertake a furnace replacement project, mostly for safety reasons. They can help show you what to do and then suggest basic maintenance tips for prolonging your furnace’s life.

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