A furnace BTU is a unit of measurement that calculates the amount of heat energy, or thermal energy, that a furnace can deliver. BTU is short for British thermal unit, and it is used through North America as a way to measure energy and heat value. The furnace BTU number measures the capacity of a furnace to deliver heat output, and a larger output number represents a larger furnace.
The official definition for one BTU is the amount of heat required to raise the temperature of one pound of 60-degree water by one degree at a pressure equal to one atmosphere. In most countries that use metric measurements, the BTU has been replaced by the joule (J). MBTU represents 1,000 BTUs, while MMBTU is equal to 1 million BTUs. A therm is the term for 100,000 BTUs, and the therm is equal to the energy created when burning about 100 cubic feet of natural gas.
When measuring a furnace BTU, manufacturers list the numbers in tens of thousands or hundreds of thousands, and 80,000, 100,000, and 120,000 BTUs are common measurements. Looking at the furnace BTU measurements of various heaters and furnaces is a good way to compare performance levels when shopping. Be certain to look at the furnace BTU output number when trying to determine heat output.
In addition to the BTU measurement on a furnace, you might see an efficiency measurement, such as 80% or 90%, which highlights the heat output of the furnace versus the amount of energy it requires. A more efficient furnace requires less energy to heat the home. All newer furnaces should include a furnace BTU input number and an output number, which reflects the efficiency of a furnace. The difference between these two numbers represents the amount of heat produced (input) to the amount of heat actually delivered to your home (output). Every furnace "wastes" a certain amount of heat.
Several sites on the Internet provide calculators for determining the heating needs of a home or a room. Such calculators work well for determining the furnace BTU measurement required to provide adequate heating. Any company installing a furnace should perform a calculation of the heating needs for a home, called a load calculation. Keep in mind, however, that load calculations can be a little tricky because of several factors, including the amount of insulation in a home, the types of windows installed, and the home's location.