Why warm, still water breeds bacteria, and the everyday habits that keep a cold water system safe.
Water that stands still and warms up lets bacteria such as Legionella grow. Two controls cut that risk: keep cold water cool, and make sure it is used and replaced often enough.
Keep cold water below about 20°C
Cold water should be stored and distributed below roughly 20°C. Above that, and especially in the warmer range approaching body heat, bacteria such as Legionella grow much more readily.
Keep cold pipework and cisterns away from heat sources such as hot pipes, boilers and airing cupboards. Insulate cold pipework that runs through a warm space: heat picked up from its surroundings is what pushes still cold water into the risk range. A cistern too big for the household turns its water over more slowly, so the water sits and warms for longer, even if everything else about the installation is sound. To check, HSE's HSG274 Part 2 has the sentinel taps (usually those nearest to and furthest from the cold tank) run each month with a thermometer in the flow. The water should be below 20°C within two minutes, and watching the reading as it runs shows any heat picked up along the way.
Do not let water stand
Any branch, outlet or appliance connection that is used rarely lets its water stand, warm up and lose quality. Flush infrequently used outlets, such as a spare bathroom or an outside tap, at least weekly so fresh water replaces what has been standing.
A dead leg can be a branch that only flows when its outlet is used; between uses, water stands in that branch. The term is also used for redundant pipework. A capped blind end has no draw-off at all. Keep the branches you need short, and flush little-used outlets. Cut redundant pipework back to the live supply; never leave a capped branch full of water.
Screening stored water
Where a cistern's overflow or warning pipe needs an insect screen, the guidance gives a mesh with openings no greater than 0.65 mm, fine enough to keep insects out while still passing the pipe's fault flow.
Backflow: keeping used water out of the supply
Backflow means water flowing the wrong way. The health worry is what it can carry back towards the wholesome supply. Prevent cross-connections, and choose protection to suit the fluid connected and the pressures involved. At a tap, an air gap between the outlet and the spillover level of the basin or sink gives that protection, as long as nothing blocks it.
If water runs discoloured
Brown, black or orange tap water may follow a change in flow through the public main, which can stir up sediment. Discolouration can also start in the service pipe, the household plumbing or a storage cistern, so roadworks nearby do not prove the cause. Run the cold tap to see whether it clears, and ask the water supplier before treating suddenly discoloured water as safe to drink. Boiling does not tell you the cause or remove every possible contaminant.
Drinking water standards at the tap
The Water Supply (Water Quality) Regulations 2016 set the standards drinking water must meet at consumers' taps in England. Three worth knowing: lead at no more than 10 µg/l (micrograms per litre), copper at no more than 2.0 mg/l (milligrams per litre), and a pH between 6.5 and 9.5. A pH of 7 is neutral; lower values are acidic and higher ones alkaline.
Spotting an old lead pipe
Homes built before 1970 may still have lead pipes. The Drinking Water Inspectorate's check is made at the internal stop tap. Unpainted lead looks dull grey, often has a swollen joint next to the tap, and is soft, so a coin scraped gently across it shows shiny, silver-coloured metal; wear disposable gloves and avoid breathing any dust. Copper is bright and hard and dulls to brown, iron is dark, very hard and may be rusty, and plastic is usually blue, although older plastic can be grey or black. Lead pipes should be replaced. Until they are, run off water that has stood in them before drinking or cooking with it, and after any work on a lead pipe, flush the tap for at least 10 minutes.
The DWI notes that warmer water dissolves more lead, so levels are higher in summer and autumn; how long the water stood, the pipe's condition and the hardness matter too. When you run off water that has stood overnight, a drop in temperature shows that fresh mains water has reached the tap. About a washing-up bowl's worth may clear a short service pipe, but longer pipework needs more. Water that is cooler or looks clear can still carry lead.
Tell the water company when planning replacement of a lead supply pipe. DWI says it must replace its communication pipe if that pipe is lead and risks contributing lead to the drinking water. Flushing is a temporary precaution while the source of lead is removed.