Backflow is a term in plumbing for an unwanted flow of water in the reverse direction.

Backflow can be a serious health risk if the mains water supply is comes into contact  with foul, stagnant or contaminated water.

In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl.  For this reason, the Water Supply (Water Fittings) Regulations 1999 require installers to undertake a series of safety measures and install backflow prevention devices.

Causes of backflow
Backflow can occur when the mains pressure becomes lower than the pressure in the supply pipe.

• Incorrectly installed plumbing systems
• Burst mains
• Pumps on supply pipes
• Using hydrants for firefighting

Preventing backflow

• Install a storage cistern to 'break' the mains supply at a float operated valve
• Install mechanical backflow preventers, such as non-return valves
• Ensure physical air gaps are maintained at taps and top-up pipes