Background

The heating capacity of a boiler is measured in British thermal units (BTUs). Homeowners or businesses should not simply use their old boiler’s BTU figure and replace it with a boiler of the same size. For hot water boilers, a heat loss calculation is needed, and for steam boilers, the building’s piping and radiators should be evaluated.

Calculation

Heating contractors calculate a building’s heating needs by taking into account the temperature difference between the desired indoor temperature (typically around 70 degrees Fahrenheit) and the outdoor design temperature, which is the average lowest recorded temperature for a location. Contractors also consider other factors like how well-insulated a building is and the age of the building to figure its heat loss in BTUs per hour. A replacement boiler must be rated to handle a building’s heat loss in colder months

Steam Boilers

A steam boiler must produce enough steam to reach all radiators in a building before it condenses back to water.