What drains faster, a vented or unvented drain? For every side-by-side test that I have ever conducted, the unvented drain is faster. This is contrary to the water-bottle analogy that so many plumbers and home inspectors frequently talk about. As I said in my blog post Plumbing Vents, Why Houses Need Them (forget the water bottle analogy), plumbing vents don’t make things drain faster; they protect traps.
I demonstrated this in a recent blog post and video, but I had a number of people suggest that I wasn’t conducting a valid test because I was only using small, clear tubing. I said I’d follow up with another test using a real drain and a clear trap. Here it is, enjoy! https://youtu.be/4VkOsrHhjsI
An unvented fixture will drain faster because all the water that has left the drain is actually pulling water out of the fixture. When a fixture is properly vented, this doesn’t happen.