The answer to 'what does heat exchanger do' is the heat exchanger is a critical component in your HVAC system that transfers heat from the furnace or boiler to the air or water that circulates through your home. It allows the hot gases produced by the combustion process to heat up the air or water without the two ever coming into direct contact.
The heat exchanger is designed to maximize the transfer of heat while keeping the combustion gases completely sealed off from the air or water that is circulated through your home. This ensures the air or water is heated efficiently and safely, without any risk of combustion byproducts like carbon monoxide entering your living space.