What You Need To Know About Backflow

By "Apollo" Valves
May 17, 2017

If a driver travels the wrong direction on a one-way street, bad things usually happen. The same can be said if the water in a plumbing system flows in the opposite direction it was intended. This phenomenon is called backflow. 

There is a well-documented case of a fatal backflow problem from 1933. During the Chicago World’s Fair, nearly 100 people died and hundreds more got sick when drinking water was contaminated by waste water. Fortunately, most instances aren’t quite so serious, but we can’t take the chance.

Backflow can happen in one of two ways: through backpressure or backsiphonage. Let’s explore the two.


Plumbing systems are designed and installed so that the water pressure in a building only travels in one direction, from high pressure to low. Backpressure occurs when the water utility's supply pressure is overtaken by greater pressure in the building.


Not all water moving backward is caused by backpressure; water also can travel backward through negative pressure or a vacuum. This is known as backsiphonage. The water in the plumbing system at some point drops below that of atmospheric pressure, which causes water to travel in the opposite direction, thereby creating a siphon.

Flow of water

Imagine if the tank that stores water from your toilet comes in contact with the water that you cook with. When the potable water comes into contact with non-potable water, you get a cross connection. Plumbing codes mandate that cross connections be identified, eliminated, or contained so that health incidents do not take place.

There are many opportunities for these cross connections to occur. We need fire systems to help protect our buildings, but there is stagnant legionella water in those systems. Homeowners demand green lawns, but there are pesticides all across your yard that can travel back into the water system.

The biggest offender of cross connection is actually your common garden hose. If you're filling up your pool and submerge your hose in it, you now have a cross connection between the drinking water and the pool water. If the pressure in the water main drops, the contaminated water in the hose could be sucked back into the pipes connected to your drinking water.

How do we prevent it?

A backflow preventer is an assembly, device, or method that helps prevent the flow of water from moving backward. Choosing the correct type of preventer helps ensure that backflow incidents do not take place. Some backflow preventers protect against low hazard and some against high hazard. Some assemblies only protect against backsiphonage, while others protect against both backsiphonage and backpressure. Fully understanding the application you are using is as important as knowing which backflow preventer to select.

Consult with the local jurisdiction regarding code requirements in your area. Better yet, seek advice from someone who has been trained in backflow prevention. This subject is too serious to be taking chances.

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