What is a BTU?
A BTU or British Thermal Unit is the amount of heat required to raise 1 pound of water 1 degree Fahrenheit. All gas appliances are rated in BTU's/hr Input which is an approximation of the amount of gas consumed on full fire in one hour as expressed in terms of the heat potential of the gas.
Depending on the design and efficiency of the appliance, a certain proportion of the heat generated from burning ends up as useful heat in the room (Heat Output) while some portion is lost to vents and chimneys (Flue Loss).
How many BTU's will I need?
The amount of heat you will require will depend on a number of factors such as room size, insulation levels, amount and type of windows, climate and the type and use of other heating systems in the home. It will also vary from hour to hour, day to day and season to season because of these same factors.
To give you a general guideline, studies average winter home heat requirement was between 10,000 and 20,000 BTU/hr. Specifically, a 1300 square foot house in Vermont with average insulation required 14,000 BTU/hr in January and February.
Information provided by Valor Fireplaces.