Heat exchangers can be split into two types, primary and secondary. The primary is installed in most boilers and is sometimes called the main heat exchanger or heat engine. The secondary heat exchanger is generally only found on a combi boiler and can also be known as the domestic hot water heat exchanger or plate heat exchanger. The primary heat exchanger is used to increase the temperature of the water in the system by transferring the heat from the burner. The secondary heat exchanger allows heat to be transferred between one closed system and another.