A heat exchanger allows the transfer of heat from a liquid or gas to another liquid or gas without requiring direct contact of the fluids. Within heat exchangers, heat is transferred from the warmer fluid to the colder fluid as both streams flow through the pipe network.
In a boiler heat exchanger, heat is transferred from the hot gases of a combustion process to water moving through the exchanger's internal piping system. As a result, the water heats up as the gas cools down. Other appliances that utilize a heat exchanger include refrigerators and air conditioners, though the direction of heat transfer in these systems is opposite that of a boiler.