BTU is an acronym for "British thermal unit", and it is a measurement of heat produced by burning fuel. The general rule of thumb is 35 BTUs heats one square foot; therefore 35,000 BTUs = 1,000 square feet of heating area for a 100% efficient appliance. Because direct vent appliances vary in efficiency, the heat output will vary as well. For example, if you have a direct vent appliance with a BTU input rating of 28,000 and an efficiency of 78%, you will need to divide that input rating by .78 to determine the actual BTU output. The output for this appliance is then 21,840 BTUs in total, which then leaves us with the ability to heat approximately 620 square feet.
Tyler M. - Fireplace Specialist
on November 19, 2012