A condensing boiler is a high efficiency boiler which runs at a lower temperature than older, traditional style boilers. It recovers energy through, what is termed, 'Latent Heat'. When the burned fuel products cool they turn in to a liquid and create condensation. A Combi boiler provides both heating and hot water instantly. A system boiler is designed to work with a cylinder to provide hot water as it does not have the hot water heat exchanger. It contains an expansion vessel and pump allowing the boiler to be installed in a sealed system. The Heat only boiler is the simplest type of boiler. It can be used on either an open vented system, which has a tank in the loft, or in a sealed system with the addition of an external pump and expansion vessel. Hot water is provided by using a cylinder.