Plumbing vent pipes need to be straight in order to keep the proper air flow into the plumbing system.
Do Plumbing Vent Pipes Need to be Straight?
Plumbing vents need to be straight on the vertical. This is necessary to avoid any vapor locks from occurring. When there is a bend in the vent pipe, moisture accumulates and is trapped. Because the vent pipe is the warmest pipe in the house all year long, the moisture that is trapped in a bend may turn to vapor and cause a vapor lock. This vapor lock will prevent the vent pipe from working properly.
Plumbing vent pipes are used to allow fresh air into the plumbing system and help the other drains in the house work properly. This means that there are bends or connections in the other piping and this piping needs to attach to the venting system. The 90's or 45's used to attach to the plumbing vents do not cause vapor locks if the plumbing vent is at the proper 90-degree straight essential for proper venting.