In a gas furnace, the heat exchanger is the critical part that safely transfers heat from the burning gas to the air inside your home without letting the gas fumes mix with the indoor air.

Here’s how it works step-by-step:

1. Burning Gas Creates Heat:
   Natural gas (or propane) is burned inside a combustion chamber. This creates very hot combustion gases (mainly carbon dioxide and water vapor).

2. Heat Transfer in the Heat Exchanger:
   The hot gases flow through the inside of the heat exchanger, which is a sealed metal chamber (often made of stainless steel or aluminized steel).

   • The metal walls of the heat exchanger absorb the heat from the hot gases.

3. Air Circulation:
   A blower fan pushes cooler indoor air across the outside of the heat exchanger (but the air never touches the combustion gases).

   • As the indoor air flows over the hot metal surface, it absorbs the heat.

   • The air warms up and is then blown through ducts into the rooms of the house.

4. Exhaust of Combustion Gases:
   After giving up their heat, the combustion gases exit the furnace through a flue or vent pipe safely to the outside.

Key point:
The heat exchanger keeps the flames and combustion gases separated from the air you breathe, making the system both safe and efficient.