Posted: 26/04/2014 at 19:27
With plumbing systems it is important to prevent contamination of the water for obvious reasons. Dirty water getting back into the system poses a health risk and steps must be taken to prevent this happening. Backflow (as it is called) is when water which could be contaminated is drawn back into the plumbing system.
Regulations require that in order to prevent contamination of domestic water systems, hose taps must be fitted with non-return valves to prevent contamination to domestic supplies through back-flow.
A scenario that is quite feasible is when filling a garden pool. When the pool is full and the still submerged hose is turned-off, contaminated water can easily be siphoned back into the domestic supply.