The answer to 'difference between heat exchanger and' is that a heat exchanger is a device used to transfer heat between two or more fluids, while a furnace is the complete system that heats the air in a home. A heat exchanger is a component within a furnace that transfers the heat from the combustion process to the air that is then distributed through the ductwork.
The key differences are that a heat exchanger is a specific part, while a furnace is the entire heating system. The heat exchanger is responsible for safely transferring the heat, while the furnace includes the heat exchanger as well as the blower, controls, and other components needed to heat the home.