Why warm, still water breeds bacteria, and the everyday habits that keep a cold water system safe.
Bacteria, including Legionella, multiply readily in water that is both still and warm. Two habits keep cold water safe: keep it cold, and keep it moving.
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.
Route cold pipework and cisterns away from heat sources such as hot pipes, boilers and airing cupboards, and insulate cold pipework that runs through a warm space, since gaining heat from its surroundings is what pushes still cold water into the risk range. A cistern that is oversized for the household turns its water over more slowly, so the water sits and warms for longer, which adds to the risk even if everything else about the installation is sound.
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 is a branch with no outlet drawing through it at all, so its water never turns over. Cut redundant pipework back to the pipe still in use rather than just capping it, because a capped branch left full is still a dead leg.
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 is used or contaminated water flowing the wrong way, back towards the wholesome supply. Pipework carrying wholesome water must never be cross-connected to pipework carrying anything else, because that connection is exactly the route backflow needs. The simplest, most reliable protection at an outlet such as a tap filling a basin or bath is an air gap between the outlet and the highest the water in the vessel could reach.
If water runs discoloured
If water suddenly runs brown, black or orange, particularly soon after work on the street main, suspect disturbed deposits rather than a fault in the house. Contact the water supplier for advice before drinking it; boiling does not show what caused the colour and cannot be relied on to make it safe.