How your plumbing pipes work
You may wonder how I got my start as a builder. The hook was set in Boy Scouts, when I obtained my home repairs merit badge. I then helped pay my way through college working on the weekends and during the summer for a man who restored old homes he bought out of foreclosure. After graduating from college with my geology degree, I decided to start my own construction business.
Within a year, I had purchased an Federal Housing Administrationrepossessed home in an auction for $8,500. The house needed lots of work, including new drain pipes for the only bathroom. I tackled this project and was instantly attracted to plumbing work. I found it fascinating how the different-sized drain pipes connected to one another and, more importantly, how each fixture had a mysterious pipe that extended up from the drain pipe to the ceiling.
This extra pipe was a vent pipe. I didn’t understand how that vent pipe worked, but after a few trips to the library, I discovered why it was a vital part of the system. I was on my way to becoming a master plumber.
I’d like to share some of what I discovered all those years ago so you don’t get sick from sewer gas. Once you understand how all the plumbing pipes in your house work together, you’ll have fewer clogs and save hundreds or even thousands of dollars on plumbing repairs.
It’s important to realize the plumbing drain and vent pipes in your home mimic what Mother Nature does with drain water. Water that falls on mountains travels through everlarger streams and rivers until it gets to the ocean. The rivers get bigger and deeper to handle the water from thousands of square miles of surface area.
The same is true in your home. A vanity sink on an upper floor of your home only needs a small-diameter pipe such as 1 ¼ inch or 1 ½ inch. Toilets, because of the size of the solid waste they accept, require a 3-inch pipe. By the time all the fixtures in your home are connected, a 4-inch pipe transports the water and waste on its way to your septic tank or the city sewer. The pipe buried in your yard that connects to a city sewer is almost always 6 inches in diameter.
Have you ever seen a waterfall? You have them in your home at many fixtures. When the water leaves the P-trap under a drain, it almost always travels horizontally through a pipe just as a river flows across land. When the horizontal pipe connects to a vertical one, you get a small waterfall within the pipes. Plumbers call these vertical pipes stacks. Typically the stack ends in a basement or under a slab connecting to a new horizontal pipe that leaves your home. The same is true with waterfalls: The water starts to flow horizontally at the base of the falls.
By now you’re probably wondering about the vexing vent pipes. These pipes allow air from outside your home into the plumbing system.
You may think the pipe up on your roof acts like your fireplace chimney that sends smoke outdoors. It’s the opposite.
Before you turn the water on at a fixture or flush a toilet, the drain pipes beyond the water seal in your P-traps are filled with air. As soon as you flush your toilet and 1.6 gallons of water rushes into the pipes, it pushes the air in front of it. You’ve experienced this rush of air if you’ve stood next to a tall elevator in a high-rise building. As the elevator comes racing down the shaft, it rams the air in front of it and some blasts out of the closed doors.
The vent pipe in your roof allows the air to enter the system to replace the air being pushed through the pipes by the drain water. Atmospheric pressure, around 14 pounds per square inch, helps get the air into the vent pipes. The building code calls out the minimum size for vent pipes, but they can work if smaller.
One day I wanted to test this. I created a small bathroom with normal fixtures but instead of using the code-required vent pipe sizes, I vented the entire bathroom with just ½-inch-diameter pipe. I filled the fixtures with water to the brim, pulled the drain plugs and flushed the toilet at the same time. Everything worked perfectly because plenty of
air could get through the small tubing. After satisfying my curiosity, I installed the correct-sized vent pipes.
Plumbing departments want to make sure all the drain pipes and vent pipes in your home are sized properly. This is why they
require you to supply a riser diagram or isometric plumbing drawing to obtain a permit. I’ve always loved drawing these three-dimensional drawings each week for architects and plumbers.
You want the pipes to be
sized correctly so you minimize the possibility of clogs in your system. Pipes that are sized too small can get clogged. You can also create clogs by using the wrong fittings. You want the water flowing through the drain pipes to make gentle turns
when the water is flowing horizontally.