Welcome to brain Stuff from house Stuff works dot com where smart happens. Hi Am Marshall Brain with today's question, what is an anti siphon valve? If you live in a house with a built in lone sprinkler system, there's a device called an anti siphon valve or a backflow prevent in your yard. What is this thing for and how does it work? Andi siphon valves are a great example of how building codes evolved. They also show how the code can contain rules that seem to make no
sense but actually solve significant problems. Most building codes now require ati siphon valves in several different places in a house, including all external hose connections, outdoor sprinkler systems, and sometimes even in toilet fill valves. First, let's define a siphon. A siphon is any pipe, pose or tube that's used to move a liquid from higher locations to lower locations. To use a siphon, you fill the piper hose with the liquid and you stick one end of the pipe
into a liquid at a higher location. Then you let the liquid start draining at a lower location. As the liquid drains out of the pipe at the lower location of vacuum develops that sucks the water from the higher location. Gravity and suction do all the work, so no pump is required. You can use siphons to drain ponds, empty barrels, remove gasoline from gas tanks, et cetera. Now think about your yard. Let's say you take a garden hose and you stick it into a three gallon barrel that you
have in your yard. When you turn on the water, you fill the hose with water. As the barrel fills, the end of the hose inside the barrel gets covered with water. Now imagine that the water pressure fails, for example, because someone opens a fire hydrant and significantly lowers the water pressure on your block. What will happen is that the water in the barrel will siphe been back through the hose into your water system. So the next time you turn on your kitchen sink, you're drinking water out
of that barrel. Yuck. And the problem is even worse if you're using the hose to fill a pesticide tank. In the case of a lawn irrigation system, if there's standing water in the yard when the siphon effects starts, it can suck fertilizer, weed killers, dog poop, and all sorts of other things into your home's plumbing. That's a double yuck. The anti pipheon valve prevents all of this. The anti piphon valve is nothing more than a one
directional valve. It can be as simple as a spring loaded flap that lets water flow in only one direction. A key thing to note is that many people skip the anti pipe valve. Either they don't know about it, or they don't understand it, or they think it costs too much, or they simply don't like the idea of being told what to do by the building code. In the process, they endanger their own health and the health of their name. Merse because there's an actual problem that
the anti sifon valves solves. That's why there are building codes and building inspections. Do you have any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com.