Boiler sizes are based on BTUs, and cold climates typically need about 50 BTUs per square foot; moderate climates need about 35 BTUs per square foot; and hot climates need about 20 BTUs per square foot. A BTU calculator can determine the right size boiler for the home's size.
The size is not the only consideration when choosing a boiler for a house. Other considerations include:
Many older boilers are too large for the house, which leads to lowered efficiency and increased utility costs. On top of the size, the AFUE, or annual fuel utilization efficiency rating, is an important consideration. This number rates the effectiveness of the boiler in converting the fuel to heating energy, and 85 percent or higher is considered efficient. To reach an almost 100 percent rating, use an electric boiler, which is ideal in areas with low electricity costs.
Another consideration when choosing a boiler is its venting requirements, as some utilize a chimney, and others use a roof or wall vent. Power-vent boilers need to be installed in open rooms because they use air from inside the room, so they cannot be installed in crawl spaces or closets. Condensing boilers also have different venting requirements, so take these requirements into consideration when purchasing a new boiler.