What is a good rule of thumb for the sizing of ejector vents?

The correct answer is to assign a size that is one-half the size of the subdrain discharge pipe. This guideline is based on standard practices in plumbing design to ensure that the ejector vent effectively facilitates the removal of air and any potential gases that can accumulate in the system.

Sizing the ejector vent to be one-half the size of the subdrain discharge pipe allows for adequate venting while minimizing the risk of back-pressure issues. This size ratio helps maintain proper air circulation within the system, ensuring that the ejector operates efficiently. It also serves to prevent the potential of siphoning, where the water flow can inadvertently create a vacuum, which might disrupt the overall operation of the ejector system.

Other options would not provide the same level of functionality. For example, sizing the ejector vent equal to the pump discharge pipe may lead to excessive size and airflow that does not correspond with the needs of the system. Similarly, using the volume of the ejector basin or the width of the basin for sizing does not take into account the necessary airflow specific to the discharge requirements, which can affect performance and efficiency.