Boilers are heating systems that typically heat water and then distribute it through radiators in a building for comfort space heating. Boilers can be powered by a variety of energy sources such as gas, oil, electricity, coal or wood. Most boilers can be replaced with high-efficiency tankless boilers that require much less space in your home or business. Combi-boilers, which are capable of heating domestic water for tap and space comfort heating, have gained popularity in recent years. These boilers turn off when not in use and often boast 98% efficiency.