How high should the air inlet be positioned above street level when ventilating a manhole?

For the ventilation of manholes, positioning the air inlet at a height of 2.0 feet above street level is essential for several reasons related to safety and efficiency. This height helps to ensure that the ventilation system effectively draws in fresh air while minimizing the risks associated with contaminants and hazardous gases that may accumulate at lower levels.

Air tends to stratify, and gases like methane or hydrogen sulfide, which are common in sewerage systems, can be heavier than air and settle close to the ground. By placing the air inlet at 2.0 feet, the system can more effectively access cleaner air that is less likely to contain harmful concentrations of these gases, thereby improving air quality during the ventilation process.

Additionally, this height helps prevent debris or water from entering the ventilation system, maintaining its functionality and safety standards. It's also an industry-standard practice to ensure compliance with safety regulations, making it a widely accepted recommendation for manhole ventilation systems.

In contrast, other options may not capture the necessary qualities of ventilation management or could expose the inlet to more contaminants, increasing the risk of hazardous conditions.