Capacity

Furnace capacity is measured in BTUs, or British Thermal Units. One BTU is equal to the amount of heat needed to raise a pound of water 1 degree Fahrenheit in temperature. Your furnace may also have input and output capacity ratings. The input capacity refers to the amount of fuel it burns, without considering any heat lost through the flue or up the chimney. Output capacity, sometimes called bonnet capacity, is how much usable heat the furnace generates. When replacing your old furnace with a higher-efficiency one, it's important to match the output capacity, not the input capacity. Furnace capacity, especially for wood boilers, may also be expressed in square feet.

Gas Furnaces

Gas furnaces are extremely common home heating devices. These are usually sold in input capacities of tens of thousands of BTU. Gas furnaces vary significantly in efficiency, however. Older gas furnaces can be as little as 50- to 60-percent efficient, while new energy-saving models can have efficiencies as high as 95 percent. Some gas boilers have efficiencies up to 99 percent. Energy Star–qualified gas furnaces must provide at least 90-percent efficiency. AC 4 Life, a Florida furnace installer, recommends 25 to 30 output BTU per square foot in warm climates and 40 to 45 output BTU per square foot in cold climates.

Oil Furnaces

Oil boilers and furnaces work much the same way gas models do. Calculate furnace size using the same method as for gas. According to the American Council for an Energy-Efficient Economy, oil furnaces range from around 78-percent efficient to 95-percent efficient, with boilers coming in between 80- and 90-percent efficient. To qualify for an Energy Star certification, oil furnaces must have at least an 83-percent efficiency rating, and boilers must rate at least 85 percent.

Electric Furnaces

Electric furnaces tend to have higher fuel efficiency than gas, oil or wood burners. According to the U.S. Department of Energy, all-electric furnaces are between 95- and 100-percent efficient. Outdoor furnaces are the least efficient, since they lose some heat through the furnace jacket, while indoor furnaces have little to no heat loss. If you're considering buying an electric furnace, you can skip calculating the output capacity and use only the input capacity to figure furnace size. Hamilton Home Products recommends a furnace capacity of 10 kilowatts per 1,000 square feet for cooler climates and 10 kilowatts per 1,000 to 1,400 square feet for warm climates. Very cold climates may require as much as 10 kilowatts for every 600 square feet.

Wood Boilers

Outdoor wood boilers burn wood and other cellulose products to heat water. This water is circulated inside the home to provide heat and household water. Due to wood's inconsistent heating value, it can be hard to size a wood boiler using the BTU method. Outdoor Wood Furnaces recommends buying based on your home's square footage and purchasing a boiler with a maximum heating capacity equal to 50 percent of the square footage in your home. Include basement, attic and other nonliving-space square footage to ensure adequate furnace capacity.