What temperature should cold water be distributed at?
The short answer
Cold water should be delivered at 20°C or below, and should get down to that within a couple of minutes of running the tap. Above 20°C, cold water is in the range where bacteria including Legionella can multiply — so cold pipework should be kept away from heat sources, and lagged where it runs alongside hot pipes or through a warm space.
The cold side gets far less attention than the hot side, and it shouldn't. The whole safety logic is that hot water is kept too hot for bacteria to grow, and cold water is kept too cold. A cold pipe that has been run through an airing cupboard, or clipped tight against a hot flow pipe under a floor, sits in the warm middle ground and undoes half of that.
It's also why a stretch of pipework that nobody uses is a problem — a spare bathroom, an outside tap that's forgotten for months, a shower in a guest room. Water sitting still and warming up gently is exactly what you don't want. If part of your house genuinely isn't used, running those taps through for a couple of minutes now and then is a sensible habit.
If your cold tap runs noticeably warm at first pull, and keeps running warm, it's worth mentioning to your plumber — it usually means a pipe is picking up heat somewhere it shouldn't.
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