What is the function of a fixed draft hood?

The function of a fixed draft hood is to maintain a neutral or zero over fire draft condition. This is essential for ensuring that combustion air is properly supplied to the appliance while preventing excessive draft that could lead to unstable combustion or increased emissions.

A fixed draft hood allows for the proper mixing of combustion gases with the surrounding air, which is important for optimizing the efficiency of the fuel-burning process. By maintaining a neutral draft, the hood helps to stabilize the performance of the appliance, ensuring that it operates safely and effectively under varying conditions of pressure in the flue and surrounding environment.

The other options refer to different draft conditions that would not accurately represent the role of a fixed draft hood. Negative over fire draft implies a situation where the pressure is lower than the atmospheric pressure, which could cause reverse flow of combustion gases. Positive over fire draft indicates higher pressure, risking spillage of combustion products. Variable over fire draft conditions fluctuate and do not provide the consistent operation that is crucial for safety and efficiency, which is exactly what a fixed draft hood is designed to mitigate.