Heat exchangers can be split into two types, primary and secondary. The primary is present in most boilers and can sometimes be referred to as the main heat exchanger or heat engine. The secondary heat exchanger is only found in combi boilers and is sometimes called the plate heat exchanger or hot water heat exchanger. The primary heat exchanger allows the water in the system to be heated by transferring the heat produced by the burner. The secondary heat exchanger is used to transfer heat between two closed systems.