What type of air is mixed with flue gases in a draft hood?

The correct choice is dilution air. In the context of heating systems and draft hoods, dilution air refers to the fresh air that is introduced to mix with flue gases to decrease their concentration before they are vented into the atmosphere. This process helps to ensure that harmful substances and pollutants in the flue gases are diluted to safe levels, thus enhancing safety and reducing the potential for harmful emissions.

Using dilution air is crucial for maintaining proper ventilation, ensuring that the combustion process is efficient, and reducing the risk of backdrafting, which can allow dangerous gases to enter living spaces. The use of dilution air also aids in preventing the buildup of toxic exhaust products, making it essential for safe operation.

Other options such as fresh air, return air, and exhaust air do not specifically relate to the function of the draft hood in mixing with flue gases. Fresh air is typically unconditioned air brought into a space for ventilation, return air refers to air that is cycled back into a heating or cooling system after having been conditioned, and exhaust air is the air that has already been used in a space and is being expelled. In contrast, dilution air specifically addresses the mixing process with flue gases to ensure their safe discharge.