What is the primary purpose of the secondary heat exchanger in a gas furnace?

The primary purpose of the secondary heat exchanger in a gas furnace is to condense products of combustion. In a high-efficiency gas furnace, the design includes a secondary heat exchanger that captures and utilizes the heat from the flue gases that would otherwise be vented out.

As the flue gases pass through this secondary heat exchanger, they cool down, allowing for the condensation of water vapor in the exhaust. This process not only extracts additional heat, making the furnace more efficient, but it also reduces the temperature of the gases being released into the atmosphere. By condensing these gases, the furnace can achieve better energy efficiency and reduced emissions, leading to a more environmentally friendly operation.

Using this approach allows the system to maximize heat transfer to the living space while minimizing energy waste, which is a key feature in modern condensing gas furnaces.