When selecting the right boiler for your home, there are several factors to consider, such as size and efficiency. Boiler size is mainly determined by the size of your home and how many hot water outlets you require. An HVAC contractor can help you choose the most suitable boiler size for your home. Additionally, a Manual J load calculation should be done to determine the size of the boiler needed for your home, considering climate, insulation levels, construction materials, and window/door size and orientation. On average, a Manual J calculation costs between $300 and $700.