What temperature should cold water be kept at?
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Key points
- Cold water should reach the outlet at below 20°C within two minutes of running the tap.
- Below 20°C legionella effectively stops multiplying, so a cold system that stays cold is its own control measure.
- Cold water almost never arrives warm from the mains — it warms up inside the building, in loft tanks, against hot pipework, and in long runs through heated spaces.
- Run the tap for the full two minutes before judging: the first draw is the water that was standing in the branch.
- A cold outlet that stays warm is a fault to trace and fix, and recurring warmth at a sentinel outlet should be recorded and investigated.
Cold water should be kept below 20°C, measured at the outlet within two minutes of running the tap. That is the figure in HSG274, and it exists because legionella effectively stops multiplying below 20°C — which means a cold water system that genuinely stays cold is controlling the bacteria by itself, with no chemicals and no intervention beyond keeping it that way.
Why the two-minute rule matters
The first water out of any tap is the water that has been standing in the branch pipe feeding it, and standing water warms toward room temperature. Judging a cold system on the first draw would fail almost every tap in the building. The two minutes is the allowance for flushing that standing water through so you are measuring the water the system actually delivers, not the water that happened to be sitting in the last few metres of pipe. If the temperature is still at or above 20°C after two minutes of full flow, the problem is real and upstream: somewhere between the mains and that outlet, the water is picking up heat faster than it should. The temperature checker is built around exactly this measurement — outlet, time, and stabilised reading.
Where cold water warms up
Cold water enters a building from the mains at a temperature that is comfortably below 20°C for most of the year, so warmth in the cold system is almost always gained inside the building. The usual suspects:
- A cold water storage tank in a warm loft. Roof spaces get hot in summer, and an uninsulated or poorly lidded tank can sit at 25°C or more for weeks — no longer a cold system in any meaningful sense, and with a large stored volume at growth temperature. The cold water storage tank inspection guide covers what to check.
- Cold pipes touching hot pipes. Where cold and hot pipework runs in contact or shares uninsulated trunking, heat conducts straight across. Pipework should be separated and insulated.
- Long runs through heated spaces. A cold pipe crossing a warm ceiling void or a plant room arrives warm at everything downstream of it.
- Dead runs and little-used branches. Water that never moves never gets replaced with fresh cold mains water — the same stagnation problem as a dead leg, on the cold side.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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What to do about a warm cold supply
The fix follows the cause. Insulate the tank and its lid, fit a proper tank screen, insulate or separate the pipework, reroute a run that crosses a hot space, or remove a length of pipe that serves nothing. Where a tap is simply too far from the source and always runs tepid, the honest options are to shorten the run or accept that outlet needs more frequent flushing and monitoring — a decision that belongs in the risk assessment and the written scheme of control, not in someone's head.
Cold water is a control measure only while it is cold. The moment a loft tank or a shared pipe run lifts it past 20°C, the system has quietly lost one of its two main defences.
Monitoring the cold side
Cold water temperature is a routine monitoring point, not a one-off check: sentinel outlets — the nearest and farthest on the cold system — monthly, with the stabilised below-20°C reading recorded each time. In summer, when mains temperatures rise and lofts heat up, it is worth paying closer attention to stored cold water in particular, because that is when tanks drift. Every reading goes in the log book with the date and outlet, kept for at least five years, alongside the hot-side figures described in legionella temperature control. A cold reading that keeps creeping upward across the summer is a trend, and trends are exactly what the record exists to reveal.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What temperature should cold water be in a building?
Cold water should be below 20°C at the outlet within two minutes of running the tap. Below 20°C, legionella effectively stops multiplying, so a genuinely cold supply is itself a control measure. If a cold outlet runs above 20°C even after two minutes, the water has warmed up somewhere between the mains and the tap, and the source of that heat gain needs finding.
Why does cold water temperature matter for legionella?
Legionella multiplies fastest between about 20°C and 45°C, and below 20°C it effectively stops growing. Cold water enters the building from the mains well below that threshold, so a cold system that stays cold is self-controlling. The risk arises when the system lets the water warm up — in a loft tank in summer, in pipes running alongside hot pipework, or in long runs through warm spaces — and crosses into the growth range.
What should you do if the cold tap runs warm?
First run it for the full two minutes, because the first draw includes water that has been standing in the branch. If it is still at or above 20°C after two minutes, trace where the heat is coming from: common causes are an uninsulated cold water storage tank in a warm loft, cold pipes running in contact with hot pipes, and long pipe runs through heated ceiling voids. The fix is usually insulation, rerouting, or removing an unnecessary section of pipe, and recurring warmth at a sentinel outlet should be recorded and investigated.
Can legionella grow in cold water?
Legionella survives in cold water but does not multiply below about 20°C, which is why keeping the cold supply genuinely cold is a control measure in itself. Survival is not the same as growth: the bacteria can persist in a dormant state and resume multiplying if the water later warms up. That is why cold systems are monitored rather than ignored — a cold tank that reaches 25°C in a summer loft is no longer a cold system.
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