There are two kinds of heat exchanger that can be present, the primary heat exchanger and the secondary heat exchanger. The primary is present in almost all boilers and it allows the heat to be transferred from the burner to the water within the system. The secondary heat exchanger is only present in combination boilers and allows the heat to be transferred from one closed system to another. The Primary Heat exchanger is sometimes known as the heat engine or "main" heat exchanger. The secondary heat exchanger can also be known as the domestic hot water heat exchanger or the plate heat exchanger .