What term describes air that is introduced into a draft hood and mixed with flue gases?

The correct term for air that is introduced into a draft hood and mixed with flue gases is "draft air." This terminology specifically refers to the air that is needed to assist in maintaining proper combustion and venting of the gas appliance. The draft hood's primary function is to ensure that sufficient air is provided to facilitate the drawing out of combustion products from the appliance.

Draft air is critical for several reasons. First, it helps prevent a negative pressure that could lead to backdrafting, which can draw harmful flue gases back into the occupied spaces of a building. Additionally, the introduction of draft air helps to stabilize chimney performance by balancing the temperature and density of the flue gases, promoting efficient venting.

The other options do not accurately describe this phenomenon. For instance, fresh air usually refers to external air that is brought into a building for purposes not specifically related to the draft hood. Return air typically refers to the air that is cycled back into the HVAC system from the conditioned spaces within the building, while indoor air simply refers to the air present within the confines of a building without specification of its origins or functions regarding combustion. Therefore, the term draft air best fits the description provided in the question.