A vent header is the head honcho in a venting system and always leads to outside air. Although a vent header is really not much different from a stack vent; it is named, so that it can be sized by different rules.
A vent header is the combination of two or more major sources of air to the drainage system; such as a vent stack or a stack vent. Once a vent pipe is named a vent header anything else connecting to it won't change it's name.
- Stack vent + vent stack = Vent header
- Vent stack + vent stack = Vent header....ect
Sizing a vent header
Your local code requirements for sizing may differ. Generally vent headers are sized by the total number of fixture units being vented and by the total length of the vent (from the farthest vented fixture to fresher air). If installed in cold climates that are subject to frost closure, the portion of the vent that penetrates the roof must increase one size and be a minimum of three inches.