The Vokera heat exchanger is a replacement part for Linea, Unica, Compact, Procombi, Syntesi and Sabre appliances and comes with 4x O-rings. A heat exchanger is a device that transfers heat from a boiler's burner to water. A primary heat exchanger is found in all boilers and it uses heat from a burner to raise the temperature of the water in the system, whereas a secondary heat exchanger is only found in combi boilers and it allows heat to be transferred between closed systems.