Back-Siphoning: Small Risk Of A Huge Problem

By Bruce W. Maki

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When you turn on a faucet you expect clean water to come out. In North America we tend to take our clean water supply for granted.

But in water supply systems there is a rare but dangerous phenomenon called back-siphoning. This can only occur when three conditions are present:

  1. A faucet (or a garden hose) is turned on.
  2. The faucet or hose is submerged in water that has "left the system" and is therefore considered dirty.
  3. There is a sudden loss in water pressure in the water supply system, perhaps caused by a break in a supply pipe or simply the water being turned off for maintenance.

The sudden loss of pressure can allow dirty water to be sucked into the supply pipes, which would contaminate the water system. This could be just the plumbing in a single house, if it was on its own well. Or it could be a large part of a municipal water system.



A plumber once told me about a case where a man was mixing pesticide in his backyard, and his garden hose was submerged in a tank of water with the chemical. There was a disruption in the supply pressure and back-siphoning sucked the pesticide solution into the city water system. What a mess! 

(Imagine the guy in his backyard... "Hey, where'd my insecticide go? That's weird! Who's messin' around here? Dang kids..."  That kind of un-explainable event could drive a paranoid person to extremes of behavior!)

Once the water system people finally noticed that the tap water was tainted, it would have been necessary to flush a lot of water from many hydrants and homes, and perform tests on the water, and alert the public, and deal with the news media, and many other headaches.


As a consequence, there are some elements of modern plumbing codes that prevent back-siphoning:

  • Faucets must be above the level of a basin, so that  a plugged basin cannot let dirty water into a faucet.
  • Hose connections must have a special device called a "vacuum breaker" that prevents the backwards flow of water when pressure is shut off.

These things may seem like a hassle, but they are for the public good. 


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Copyright 2001

Written February 11, 2001