Heating systems are commonly rated in BTUs (British Thermal Units). One BTU is the amount of heat required to raise the temperature of one lb. of water by one degree Fahrenheit. Several factors, such as square feet, determine how large a heating system is needed to heat a building. Determining how many BTUs are required to heat a square foot is a key starting point in the process of selecting the proper heating appliances.
One BTU is sufficient to heat about 55 cubic ft. of air by one degree Fahrenheit. To figure out how many BTUs are needed to heat one square ft. of floor area, you need to determine the cubic volume of the area to be heated and the maximum temperature needed. Measure the length, width, and height of each room in a building that is to be heated. Multiply length by width to find the square footage. Next, multiply the square footage by the room height to find the cubic volume. If you are heating more than one room, add up the total square footage and the total cubic volume of all the rooms.
Estimate the maximum temperature needed to keep the area as warm as you wish. Take the lowest temperature you are likely to have outside during the year and subtract it from your target temperature. If you don't know the probable minimum temperatures where you live, consult the temperature data provided by the National Climate Data Center.
Finally, multiply the number of degrees temperature change needed by the number of cubic feet and divide by 55. Then divide the result by the square footage to find BTUs per square foot. Suppose you have a 1,000-sq.-ft. building with 8-ft. ceilings and want to be able to raise the temperature from zero to 21.1 degrees Celsius. You have 8,000 cubic feet times 70 degrees divided by 55, or 10,018 BTU. Divide by 1,000 sq. ft. to get 10.18 BTU per sq. ft.
BTU requirements depend on more than just the volume to be heated and the temperature range. Other major factors include the rate of heat loss through roof, windows, walls, and doors. Air leaks and poor insulation increase BTU requirements, as does normal ventilation. Good sun exposure and heat generated by other appliances reduce BTU needs.
Heating systems are normally rated in BTUs per hour. Due to the complexity of the variables, you need a calculator to estimate BTUs per square foot. There are several good ones available free online. Because of heat losses it takes several times as much BTU capacity to maintain a desired temperature than to simply heat the air in a building to start with. For example, using the heater shop heating calculator, you'll find you need about 75,000 BTU for a 1,000-sq.-ft. building if you have good insulation. Divide by 1,000 sq. ft. for the BTUs per square foot, which in this example equals 75. With poor insulation the BTUs needed may reach almost 300,000 (300 BTU per square foot).