How Many Square Feet Will a 100,000 BTU Furnace Heat?

If you are considering a new furnace, you probably want to know how large a furnace to buy based on the square footage of your home.  In this furnace guide, you’ll find the answer you’re looking for.  We approach it by asking, “How many square feet will a 100,000 BTU gas furnace heat?”

Gas Furnaces and Climate Zones

As you would expect, furnaces can adequately heat more square feet when outside temperatures are warmer. If you’ve got a 2,000 square foot home in Vermont, you’ll need a larger furnace to heat it than if the home is in Florida.

When HVAC professionals size a furnace to match the demands of the home, they take into account the local climate.  To simplify, let’s divide the country into 5 climate zones.  If you do an online search for U.S. climate zones, you’ll find numerous maps that divide the country into 5-8 climate zones.  Here’s a rough sketch of the zones, with zone 1 the coldest and zone 5 the warmest:

Zone 1: Upper Great Lakes, Northern Plains, New England.

Zone 2: Lower Great Lakes, Central Plains, Rocky Mountains, upper Mid-Atlantic.

Zone 3: Upper Ohio Valley, lower Mid-Atlantic, Pacific Northwest.

Zone 4: Lower Ohio Valley, most of Southwest, Mid-South states including northern TX, MS, LA, GA, NC, SC.

Zone 5: Southern CA, AZ except mountains, south TX, FL.

Heating by Square Feet

Here is a basic estimate of the number of BTU’s required per square foot of living space for each zone.  When HVAC contractors size furnaces, they use a complex process known as a load calculation.  The climate zone and the square footage of the house are key factors, but not the only ones.  See the section below on other factors that affect furnace size.

Zone 1: 55 Btu per square foot; Zone 2: 50 Btu per sq. ft.; Zone 3: 45 Btu per sq. ft.; Zone 4: 40 Btu per sq. ft.; Zone 5: 35 Btu per sq. ft.

Home Sizes Heated by a 100,000 BTU Furnace

If the 100,000 Btu furnace was 100% efficient – and none of them are – here are the homes it would heat, by zone.  See the next section for how furnace efficiency affects furnace size.

Zone 1: 1,800 square feet.

Zone 2: 2,000 square feet.

Zone 3: 2,200 square feet.

Zone 4: 2,500 square feet.

Zone 5: 2,800 square feet.

Furnace Efficiency and Sizing

Furnaces range in efficiency from 80% to about 98%.  Here’s what this means.  If a furnace creates 100,000 Btu of heat, a 98% efficient furnace will push 98,000 Btu into your house and lose 2,000 Btu out the flue with the exhaust gases.  The 100,000 Btu is known as the furnace input and the 98,000 Btu is known as the furnace output. You’ll see those figures when you examine furnace specifications. Output is the amount of usable Btu created. Furnaces are always referenced by their input – the number of Btu they create, not the number they actually use to heat your home.

A 95% efficient furnace will output 95,000 Btu and the 80% efficient furnace will output 80,000 Btu.  So, if you need 100,000 Btu output to heat your home, you’ll need a furnace with an input higher than that. To create 100,000 Btu output, here are furnace sizes an HVAC professional would recommend for common furnace efficiencies.

98% Efficient: 100,000 to 110,000 Btu furnace.

95% Efficient: 105,000 to 115,000 Btu furnace.

90% Efficient: 110,000 to 120,000 Btu furnace.

80% Efficient: 125,000 to 130,000 Btu furnace.

Now, let’s turn this around and see how big a home the 100,000 Btu furnace would heat in each zone.  We’ll use a 90% efficient furnace and you can do the math from there.

Zone 1: A 90% efficient, 100,000 Btu furnace would make 90,000 usable Btu and heat a 1,650 square foot home.

Zone 2: 1,800 square foot home.

Zone 3: 2,000 square foot home.

Zone 4: 2,250 square foot home.

Zone 5: 2,575 square foot home.

Other Factors that Affect Furnace Size

When an HVAC contractor does a load calculation, the house’s size and climate are major factors in how many Btu are needed to heat it.  However, there are other important factors including the amount of insulation the home has, the number and quality of its windows and doors, the type of materials used in construction, and the layout of the home. A load calculation is extremely detailed, a room by room evaluation of the home’s ability to hold heat. A load calculation is sometimes called a heat loss calculation.  The more heat that is lost through insufficient insulation, poor-quality windows and doors, the amount of wall space it has, etc., the more Btu will be needed to heat it.  These factors can affect the size of the furnace needed by as much as 20%, possibly more.

This furnace guide will give you a good idea of what size furnace you’ll need for your home. Once you get a rough idea, the furnaces price articles on this site will be of greater benefit to you.  Be sure to consult a qualified contractor to determine the exact size you need.  Keep in mind, too, that proper installation is required for a furnace to perform as it should.  It makes sense to have your furnace installed by a furnace professional who can size it properly, install it correctly and tune it for maximum efficiency and performance.

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One Comment

  1. Mike Vogt
    Posted December 11, 2011 at 12:05 am | Permalink

    Would like pricing for a York Model TM9V 120,000 btu