What Does Anti-siphon or Backflow Prevention Mean?

Mar 12, 2019 by

What Does Anti-siphon or Backflow Prevention Mean?

The water supply in our homes is normally under pressure, but occasions can arise when the pressure drops. For example, if there is a fire in your neighborhood and the fire department begins to pump large amounts of water; the water pressure in the surrounding area can drop. This low pressure or no pressure can contribute to conditions that allows water to flow backwards. If at the same time, a fixture in the house is open and submerged in water, water can be siphoned back into the water supply.

To continue the example a bit further, if at the same time as the fire in the neighborhood, you were using a garden hose with a sprayer jar attached to the end of the hose filled with fertilizer, that fertilizer (or whatever was in the jar) could be sucked back into the household water supply. The next time water was drawn from a fixture inside the house that fertilizer would be in the water. This is the reason that modern plumbing codes require anti-siphon protection to be installed on the water lines that lead to garden hose connections.

Builtin Anti-siphon Protection

What does this have to do with toilets? The refill valves in many older toilets do not have builtin anti-siphon protection, and it is possible for the water stored in a toilet tank without anti-siphon protection to be drawn back into the water supply. About the only time you’d even know this was happening is if you use a “toilet tank cleaner” that turns the water blue. When I was teaching, I had a number of students tell of turning the water on in the bathtub and finding that it was blue. Their water supply had experienced low pressure or no pressure just before this event, and the blue water in their toilet tank had been sucked back into the supply.

Today plumbing codes throughout the U.S. and Canada mandate the installation of anti-siphon toilet fill valves in all gravity flow toilets (new installations, retrofits, and replacements). However, it is not easy to determine if an existing refill valve (ballcock) in a toilet tank meets local plumbing codes; many of them are not marked with any identification. However, when you replace a refill valve, you should only purchase one that is clearly marked “anti-siphon” or carries a UPC shield showing the valve is “code-approved”. When in doubt, ask your plumber or hardware store for assistance. Photo courtesy of Fluidmaster, Inc.

Cutaway of Toilet Tank

This tank in the photo was assembled for demonstrating how to replace a refill valve, and it was never intended for actual use. However, the height of the parts illustrate two problems that occur in some installations. Either the refill valve is too short, or the overflow pipe is too tall for this tank. It would allow the water level to rise over the top of the refill valve which would nullify any code approved anti-siphon feature the refill valve might have. In addition, the pipe appears to be about even with the hole for the handle. In which case, the water level could rise above the hole for the handle, which could under certain circumstances cause a flood.

Should you fix a refill valve when you don’t know if it is or isn’t a code approved anti-siphon valve? I leave that to you to decide, but before you do, you may want to do further research on Backflow Prevention.

Another occasion when siphoning is a problem in a toilet tank occurs when the refill tube drops too far down into the overflow pipe. Then the water is siphoned from the tank into the overflow pipe and down the drain. While this scenario won’t harm you, it will wreak havoc on your water bill, because you have water constantly recycling through your toilet. This problem often arises when a new refill valve is installed. They usually come with extra long refill tubes that are meant to be cut to size, but instead a do-it-yourselfer just drops the long tube down inside the pipe. The refill tube should be cut to just reach the top of the overflow and then be clipped to the top edge of the pipe.

Backwater Prevention

Another closely related issue is that of a toilet installed in a basement. In the event of a sewer backup sewerage will back out of the basement floor drain and the toilet. A backwater valve can prevent or greatly reduce the possibility of a sewer backup in a basement. A backwater valve is a fixture installed into a sewer line, and sometimes into a drain line, in the basement of your building to prevent sewer backflows. A properly installed and maintained backwater valve works on a one-way system, sewage can go out, but cannot come back in. For more information see the following sites:

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