What is an anti-siphon valve?

In houses with a built-in sprinkler system, anti-siphon valves prevent pesticides, pet waste, and other contaminants from polluting the home's water source. Check out this HowStuffWorks podcast to learn more about anti-siphon valves.

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Episode Transcript

Speaker 1 (00:00):
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

(00:23):
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

(00:47):
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

(01:09):
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

(01:31):
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

(01:55):
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

(02:17):
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

(02:38):
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

(02:59):
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.