Ask the client: what are air vents and why do I need them?

Question: F. Hey Tim, I had the opportunity to tour a house under construction. All the drain lines were in, but I also saw all sorts of other additional pipes that connected to the drain pipes and eventually merged and went through the roof. It got me thinking. What are these pipes and why were they installed? Do I have the same pipes in my house? It seems like a lot of wasted pipe to me. – – Sharon A., Lake Wales, Fla.

A. It can be really revealing to see the frames, wires, pipes, and other service items in a home before any drywall or plaster skin is applied. You would be stunned to discover what is on the other side of that smooth coating if you had x-ray vision.

I’ve been a master plumber since I was 29. When I first got into the construction business after graduating in geology, I got interested in carpentry, plumbing, electrics and roofing. All of these things were of great interest, but the installation cast a lasting spell over me. I really enjoyed the three dimensional problem solving challenge that you need to perform to route all of the drain and vent pipes in a typical house.

I wondered how it all worked and was lucky enough to work in many old houses where I saw old master plumbers installing huge cast iron pipes for a long time. Often times, you could still smell the hemp soaked in oak oil that was used to wrap the joints before the molten lead was poured to make the joints waterproof.

It’s easy to fall into the trap of thinking that indoor installations have been around for centuries. The truth is that indoor plumbing is one of the newer innovations in construction. It wasn’t until the late 19th century that people made the connection between common illnesses and the lack of sanitary conditions in crowded cities. Once that happened, plumbers took on the stature of doctors because they – the plumbers – could keep you healthy by making it possible to move harmful waste away from your home.

It wasn’t long before the plumbers figured out something you might not have paid much attention to in high school physics class. I’m talking about the venturi effect. When water flows through a pipe and passes an empty pipe, the moving water can create a vacuum.

When you don’t install vent pipes on every fitting in a plumbing system, the Venturi effect comes into play. Bad things can happen when you flush toilets, drain a tub, or do laundry. The water rushing and cascading through the pipes can suck the water out of the U-shaped traps under your sinks, tubs, floor drains, and showers. In this case, vermin can crawl into your house and sewage gas can also enter.

The plumbers of the past quickly discovered that just like us, a plumbing system needs to breathe air. You can quickly demonstrate this with a simple, disposable plastic water bottle. Fill one of these cheap bottles with water and turn it upside down. The water gurgles, gurgles, gurgles from the bottle. There is a fight between water and air outside the bottle. The air struggles to get into the bottle to replace the drained water.

Now do the same, but poke a very small hole or cut a thin slit in the bottom of the bottle and turn it over. The water will drain freely from the bottle as air can enter the bottle through the small vent hole or slit you made in the bottle.

The vent pipe you see on your roof does the same thing as the tiny hole or slit you cut in the water bottle. Here, the air is drawn into the plumbing system every time you drain water from a device.

You can connect all of the vent pipes connected to every faucet in your house to a simple pipe that comes out of the roof, or you can have multiple vent pipes run through the roof to minimize the amount of pipes used.

Plumbing ventilation pipes must be properly sized, sloped to drain any condensate back into the drain pipes, and installed with the utmost care. All of this is governed by installation codes and you always want to make sure that the vent pipes are installed with the same skills as the drain pipes.

As I speak on the subject, I’m not at all a fan of the modern day air intake valves that seem to be the beauty of the ball. You have moving parts – and we all know that things with moving parts will fail sooner or later. Conventional vent pipe systems have no moving parts. Period.

I recorded a great video for you showing all of the vent pipes in a new home. I think you will be amazed at how many there are and how they are all connected with just one pipe coming out of the roof in this very large house. To view the video, go to

Subscribe to Tim Carter’s free newsletter and listen to his new podcasts on

Comments are closed.