A heater with a BTU rating of 75,000 is capable of supplying a considerable area with comfortable temperatures. Exactly how much varies depending on both the local climate and the quality of the insulation. Such a heater can warm a much larger room in Virginia than it can in Vermont.
British Thermal Unit
BTU is an abbreviation for British Thermal Unit, a measurement of heat energy. One BTU is approximately equal to the heat given off by a lit match. The rating of a furnace, fireplace or other heater actually applies to the number of BTUs given off each hour. A 75,000 BTU heater produces 1,250 BTUs per minute and just under 21 per second.
Heating a Poorly Insulated Room
Sheds, pole barns and other buildings lacking proper insulation require a great deal of energy to keep warm. A 75,000 BTU heater can heat a room of between 1,900 and 3,700 square feet in a cold climate if there is little to no insulation. In more temperate climates the heater fares better, heating an uninsulated room of any size from 2,400 to 4,600 square feet.
Heating a Room with Average Insulation
Most rooms have an acceptable amount of insulation, making them more heat-efficient than outdoor buildings but not as much as is possible. An average room with dimensions falling between 2,400 and 4,600 square feet in a cold environment can be sufficiently heated by a 75,000 BTU furnace. In locations where the temperature rarely falls below freezing such a heater can supply a room with dimensions of 2,900 to 5,700 square feet.
Heating a Well Insulated Room
Rooms with high-quality insulation require much less energy to comfortably heat, meaning that a 75,000 BTU boiler can supply a very large room provided that the insulation is excellent. In cold climates it can handle rooms ranging from 3,100 to 6,200 square feet, while warmer regions allow the heater to support rooms with a square footage of anywhere between 4,100 and 8,300.