A 900-square-foot space can represent one large room, such as a loft or great room, or a small home with several rooms. For the former, one heater or fireplace may be enough for a cold day, while the latter requires a furnace that can force hot air through heating ducts. In either case, correct sizing maximizes efficient energy use and comfort.
British Thermal Units are the measurements used to compare energy use and heating capacity among furnaces. They define the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit, under normal atmospheric pressure. A furnace might have a capacity of 50,000 BTUs per hour, which represent the input energy. Furnaces also have an efficiency rating, which is expressed as a percentage, such as 90 percent. Multiplying both numbers gives the heat output. In this case, 50,000 BTUs times 90 percent equals a heat output of 45,000 per hour.
A quick way to estimate the BTU needs of a 900-square foot space is to use the estimated capacities listed at the U.S. Environmental Protection Agency's Energy Star website. It states that a room with 700 to 1,000 square feet needs a furnace with a capacity of 18,000 BTUs. If more than two people regularly use the room, add 600 BTUs per person. If the room is a kitchen, increase the capacity by 4,000 BTUs. If the room is sunny, reduce the capacity by 10 percent. If it is heavily shaded, increase the capacity by 10 percent.
According to AC4Life, a distributor of residential air conditioning and heating products, a quick way to calculate furnace size for 900 square feet is to multiply that number by a BTU estimate per hour based on climate. Southern states such as Texas and Florida have an estimate of 30 to 35, which requires a furnace with 27,000 to 31,500 BTUs. Northern states such as Maine and Minneapolis use an estimate of 50 to 60 BTUs per hour, for a furnace size of 45,000 to 54,000 BTUs. The lower number applies to well-insulated spaces, while the higher is for spaces with poor or older insulation.
Other factors that can influence the size of a furnace include the number of walls, doors, skylights and other openings where heat can escape; the local climate; the design of the house and its orientation toward the sun; the location and tightness of the ducts; and that amount and type of ventilation such as exhaust fans. Only Manual J of the Roofing Contractors of America takes these considerations into account when calculating furnace size. And only trained heating contractors can use this document to accurately specify the furnace size for a 900-square-foot area.