What Size Furnace Does My Home Need?

Pipes of a heating system

Autumn is fully underway and most homes have had the heat turned on at least once. Checklists are being printed out and homeowners are starting to winterize their homes  as the weather cools. If you haven’t already, now is a good time to make sure your furnace is working—you don’t want to get caught out in the cold!

Whether you’re building a home or replacing an outdated furnace, it’s important to find one that provides enough heat for your home. Of course, the furnace size depends on the size—or square footage—of your home, as well as the quality of insulation. 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 calculate how much energy your home needs to stay warm and then buy the appropriate furnace for your needs.

What is BTU?

When it comes to determining what size furnace your home needs, you first need to find out how much energy it will take to warm your home. That’s where BTU comes in. 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. For this post, we’ll apply BTUs to home furnaces, but just be aware that they can also be used when determining the energy it will take to cool your home.

Factors That Affect Furnace Size

Determining the size of your furnace requires a little more than choosing small, medium or large, but it doesn’t have to be very complicated. A basic rule of thumb is that it takes 30 BTUs for every 1,000 square feet. At the most basic level, 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 factors to take into consideration:

  • The age of your home. 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.
  • The quality and number of windows in your home. 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.
  • Size of your home. The larger your home, the more energy it will require to heat.
  • Location of your home. The energy your furnace requires definitely depends on where you live in the United States. A home in Minnesota will depend on their furnace much more than a home in Louisiana. If you live in Minnesota, you’ll want a furnace with a higher BTU output, possibly double what the average Louisians resident installs in their home.
  • Energy efficiency rating of the furnace. 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 Size Furnace Does My Home Need?

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—and remember, if you live in a moderate climate your BTU measurement will be closer to 30. If you live in a colder climate, or have less-than-stellar insulation, your BTU could be closer to 80. 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.

Technician looking over a gas furnace with a flashlight before cleaning it.

Will I Need a Furnace Contractor?

A trusted HVAC contractor will know about the latest technology and energy efficient furnaces on the market. They can also determine if your furnace just 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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