NATE Gas Heating Practice Exam

What are secondary heat exchangers designed for?

1. Optional heat exchangers used when necessary

2. Constructed so return air passes over the secondary heat exchanger first

3. Used to add more heat to the supply air before entering ducting

4. Now rarely used on higher efficiency furnaces

The correct answer is: Constructed so return air passes over the secondary heat exchanger first

Secondary heat exchangers play a crucial role in enhancing the efficiency of heating systems. They are specifically designed to utilize residual heat from combustion gases after the primary heat exchanger has extracted the main portion of the heat. This process allows for additional heat transfer to the supply air. The design of a secondary heat exchanger ensures that the air is maximally heated before being distributed into the ducting system. Placing the return air over the secondary heat exchanger first optimizes the heat transfer process because it allows the air to absorb more warmth from the gases that are still carrying useful heat before the gases are expelled through the flue. This effectively improves the overall efficiency of the system, as it helps in maximizing the heat extracted from combustion and reducing wasted energy. In contrast to this, other options may misconstrue the function and design intent of secondary heat exchangers. While optional functionalities or rare usage could be relevant in some contexts, the essence of secondary heat exchangers lies in enhancing heat recovery and efficiency in heating systems.