British Thermal Units, or BTUs, are used to measure the boiler's capacity. The amount of energy shown here is what it would take to raise the temperature of one pound of water by one degree Fahrenheit. Based on its location and climate, the number of windows and doors, the quality and quantity of insulation in the walls and ceilings, and other factors, each building has a specific BTU requirement.
A simple guideline for calculating BTU needs is to estimate that in a cold climate, interior space needs 50 BTU per square foot, in a moderate climate 35 BTU per square foot, and in a hot climate 20 BTU per square foot.