Does Water Ever Flow Backwards?

It is a logical assumption that because water in a water delivery system is always under pressure, it can only flow in one direction. However, can it flow the opposite way from its intended direction? The answer is yes, and when it does it can sometimes cause disastrous results. Water will always flow toward the point of lowest pressure.

If a mainline in Avion's water system should break, or if a fire occurred and the fire department opened several hydrants, the pressure in the water mains could drop dramatically, causing a reversal of flow. The potential for this reversal of flow is why Avion is concerned about the possibility of backflow contaminants getting into the water system. If household plumbing, carrying potable water, is connected to other piping carrying another fluid or gas, such as an air conditioner containing chemicals to kill algae, the contaminant could be drawn back into the water mains. Incidents such as this have happened all too often and have been documented throughout the country.

This is why state regulations require water systems to have Cross Connection Control Programs in place for preventing backflow incidents.


"Cross connection" and "backflow" are words frequently used in the water distribution field today. It is a proven fact that over 90% of contaminated drinking water is the end result of a backflow incident caused by a cross connection.

A "cross connection" is an actual or potential link between drinking water and non-drinking substances. This link could enable non-drinking substances to enter the drinking water supply. A backflow prevention assembly, installed correctly, can prevent this from happening.

Common areas of cross connection are irrigation systems, hot tubs, swimming pools, ponds or water features, solar water heaters, and photo developing, to list a few.

In 1989, Redmond High School had ethylene glycol antifreeze from an air conditioner backflow into the water piping, sending eight teachers to the hospital. Several incidents have occurred where a car wash cross connected their plumbing and pumped dirty, soapy water through several city blocks. In a town in Arkansas a worker hooked up a hose to a nearly empty propane tank to flush out the tank. The residual pressure of the propane was greater than the water pressure and several homes exploded and burned.

Cross Connection Specialists can help the water user identify these potential problems and suggest ways to eliminate them or recommend the proper backflow prevention assembly as required by the State of Oregon.