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If you’re installing a new house heating system—be it a furnace, a boiler, or a heat pump—it’s essential to take your time in properly sizing the system. Believe it or not, furnaces are not a one-size-fits-all kind of appliance. On the contrary, your home may need a different size of heating system depending on numerous factors, including the size of the house and where you live.

The good news is that working with an HVAC contractor will help make this process much easier. Your HVAC contractor will know how to calculate the right heating system size based on relevant variables. However, if you want to give your HVAC contractor a basic ballpark idea of the system you’ll need, or if you’re trying to shop for furnaces or heat pumps online, it’s good to know how to figure out heating system size.

Figuring out the Variables

Before you can do any calculations or arrive at any concrete figures regarding the size of your house heating system, you need to figure out the various variables that will affect your system size. These variables tell you how hard your furnace or heating system will have to work to retain a consistent temperature in your home. The harder the furnace must work, the larger your system will likely need to skew. If the furnace has a relatively easy job to do, you can rely on a smaller system.

There are four main variables you will need to consider:

  • The size of your home: It goes without saying that heating a larger home requires more energy than heating a smaller one. As such, your square footage will be a crucial part of the calculations for the size of the heating system that you’ll need. Luckily, you probably either know the square footage of your home or can easily look it up on your property deed. Write this figure down and keep it close, because you will need to use it in future calculations.
  • Where you live: The United States is split into several different “climate zones,” based on average temperatures and typical weather plans. Zone 1 encompasses much of the southern United States—hot, humid places that rarely see winter weather or overly cold temperatures. As you move north, you stray into Zones 2, 3, 4, and 5. Zone 5 includes most of the far northern states—places such as Montana, Minnesota, Wisconsin, Michigan, and Maine—that tend to see long winters and cold temperatures. Your zone will determine how many BTUs of energy are required to heat your home, which can in turn answer the question of what size of furnace you need.

Note: You can find a map outlining the various climate zones by looking at this furnace sizing calculator. The BTU requirements for each zone are as follows.

  • Zone 1: 30-35 BTUs per square foot
  • Zone 2: 35-40 BTUs per square foot
  • Zone 3: 40-45 BTUs per square foot
  • Zone 4: 45-50 BTUs per square foot
  • Zone 5: 50-60 BTUs per square foot
  • Insulation: As you can see, there is a range of BTUs for each zone. Based on the chart, if you own a 2,000-square-foot home in Zone 5, you could need between 100,000 and 120,000 BTUs to heat your home effectively. Obviously, this is a fairly big range. A furnace that is rated to deliver 110,000 BTUs would be enough for heating a house on the low end of the scale, but insufficient for a home on the higher end. How do you determine where your home lands? The big factor here is insulation: how well insulated your house is will affect where on the scale you fall. If your home is new (or newly renovated) and has excellent insulation and energy efficient windows, you can opt for a furnace toward the lower end of the spectrum. If your home is older, is known for being drafty, or has a lot of windows, then you’ll want to select a furnace toward the higher end of the spectrum.
  • Furnace Efficiency: Finally, you need to pay attention to the efficiency ratings of the furnaces or heating systems you consider. For instance, you might be looking at a 100,000 BTU furnace that is rated to 93 percent efficiency. That furnace will actually produce 93,000 BTUs of heating, rather than 100,000. If your home needs 100,000 BTUs of heating, you will want to go up to the next tier of furnaces to be sure you are getting enough heat.

Start Sizing Your House Heating System Today

Using the above variables and calculations will help you land in the ballpark of the size of the house heating system you need. It’s still not a bad idea to work with an HVAC pro to figure out the exact right furnace for your home. However, now you will at least know how furnace sizes vary and how you can navigate the market to find a sufficient heating system for your home.