The four main types of boilers are fire-tube, water-tube, cast iron, and condensing. Fire-tube boilers are the oldest type of boiler and are typically used in residential and commercial applications. They are composed of a series of tubes that contain hot gases from a combustion chamber, which pass through a water-filled chamber and then out of the boiler. Water-tube boilers are more efficient than fire-tube boilers and are typically used in industrial applications. They are composed of a series of tubes that contain water, which is heated by hot gases from a combustion chamber. Cast iron boilers are made of cast iron and are typically used in residential and commercial applications. They are more efficient than fire-tube boilers and are typically used in applications where space is limited. Condensing boilers are the most efficient type of boiler and are typically used in commercial and industrial applications. They are designed to capture and reuse the heat from the exhaust gases, resulting in higher efficiency and lower energy costs.