British Thermal Units
BTU is an abbreviation for British Thermal Unit, a measurement of heat. A single BTU is defined as the heat it takes to raise the temperature of a pound of water by one degree Fahrenheit, roughly the heat put off by striking a match. Furnace ratings are given in BTUs generated per hour, though this is generally written simply as "BTU," not "BTU/h." A 100,000 BTU furnace gives off 1,667 BTUs of heat per minute, or roughly 28 BTUs a second.
Annual Fuel Utilization Efficiency
While BTU output is an important measurement of furnace power, this number must be tempered by its annual fuel utilization efficiency (AFUE). This is expressed as a percentage and indicates how much of the generated heat is actually used to raise the temperature of your home. A 100,000 BTU furnace with an AFUE of 70 percent only heats a house as well as a hypothetical 70,000 BTU furnace of perfect efficiency.
100,000 BTUs Heating Capacity
The area a given furnace can heat is thus largely dependent on the efficiency percentage as well as the insulation used and the local climate. A good 100,000 BTU furnace with a AFUE of 85 percent can consistently raise the temperature of a 2,000 square foot home by 40 degrees Fahrenheit. An energy-efficient furnace of the same strength with a 95 percent AFUE can do the same for a house of 2,200 square feet.
The Right Size for Your Home
Choosing a furnace with the correct actual BTU output for your home is imperative. A heating unit that is too small will be unable to raise the temperature sufficiently, leaving you shivering during the coldest weather. An overly powerful furnace, on the other hand, will short-cycle, wasting energy and possibly even accumulating harmful moisture. Consult an HVAC technician before buying a furnace for your new home to find out exactly what the building's heating needs are.