What must the entire length of the vent section of a horizontal branch drain be sized according to?

The correct understanding is that the entire length of the vent section of a horizontal branch drain must be sized according to the total drainage discharge. This is based on the need to ensure that the venting system can adequately handle the air necessary to maintain proper drainage flow.

When a drainage system operates, it produces a certain amount of discharge. The vent system provides the air necessary to prevent vacuum conditions from occurring, which can disrupt flow and cause drainage issues. Sizing the vent according to the total drainage discharge ensures that it can accommodate the maximum volume of air that may need to enter the system to replace the water exiting through the drain.

While flow rate, potable water pressure, and ventilation requirements are important considerations in plumbing design, they do not specifically inform the sizing of vent sections in horizontal branch drains like total drainage discharge does. Thus, proper sizing based on this discharge ensures optimal performance of the plumbing system and prevents problems like slow drainage or sewer gas intrusion.