When I’ve been asked what backflow prevention is, the question has usually come with a comment that it sounds gross. Backflow prevention actually involves clean water and keeping that water clean. To explain what backflow prevention is, we’re best to explain what backflow is. It goes without saying that backflow prevention is the endeavour to prevent backflow, and from there you’ll understand the need for backflow testing.

Backflow is defined as the unintended flow of water from a potentially polluted source into the potable water supply. For this to happen, water must travel through pipework in the opposite direction to normal hence the name backflow. Backflow can be caused by back siphonage or back pressure.

A backflow condition is regarded as simply being any arrangement whereby backflow may occur.

A backflow event can be prevented by incorporating an air gap or a backflow prevention device into the plumbing system where a backflow condition exists.

What is back siphonage?
Back siphonage is the most usual cause of a backflow event. Back siphonage occurs when there is unexpected loss of pressure in the supply and a low tap is opened or a low lying pipe is cut. The opening of a low tap or pipe enables the water in the plumbing system to escape through such an opening. This usually involves the water which is ordinarily downstream of such an opening travelling backwards through the pipework.

What is back pressure?
Back pressure occurs in a situation where one plumbing system has two different pressures. One example of this is where a house has gravity fed hot water and mains pressure cold water. It’s possible for a hot tap jumper valve to stick open while the cold tap at the same outlet (ie a bath) is open allowing the cold water to push the hot water backwards and begin filling the hot water tank with cold water. I’ve heard of this happening once where the hot water tank ended up overflowing in the ceiling and making a mess. Of course, in this situation the backflow is not dangerous because the water causing the back pressure is safe to drink. Hazardous back pressure events would usually happen at industrial sites and rarely at a domestic site.

Where can I expect to find a backflow condition?
Most commonly, backflow conditions involve hoses. For example, a dog wash with a tub for immersing fido in flea shampoo, and a spray rinse hose for wetting the parts of the dog that aren’t submerged. It’s possible with this arrangement that the spray hose could be left in the tub of water dosed with flea shampoo with the tap open during a time of unexpected pressure drop in the supply.

Another common example of a backflow condition is where a subsurface irrigation system is fitted with a chemical dosing arrangement. The chemical used to inhibit root growth in the irrigation pipes is not safe for human consumption so a backflow prevention device is used to keep it isolated from drinking water.