How does a chimney heat exchanger system efficiently transfer heat from the flue gases to water for heating purposes?

A chimney heat exchanger system efficiently transfers heat from flue gases to water for heating by using metal tubes or coils that run through the chimney. As the hot flue gases pass through the chimney, they heat up the metal tubes, which in turn heat the water circulating through them. This process allows for the transfer of heat from the flue gases to the water, increasing the overall efficiency of the heating system.