A cold water storage tank has one job that matters for Legionella control: hold the stored supply below the temperature where the bacteria start to multiply. Simple enough on paper. The catch is that tanks live out of sight — a loft, a boxed-in plant room, a rooftop enclosure — and out of sight is exactly where temperature drifts up without anyone noticing.

A reading taken at half past eight on a February morning tells you almost nothing about the same tank at four o’clock on a July afternoon. That gap, between the comforting number and the real condition, is where most cold-storage problems hide. The scenario below is a composite, built from patterns that recur across UK sites rather than one real building, but every step in it is ordinary.

A summer the logbook never saw

Picture a three-storey 1980s office, cold water fed from a single storage tank in an uninsulated roof void. The water hygiene contractor visited monthly and logged the cold sentinel temperatures. Every reading sat comfortably below 20°C. The logbook ran clean for two years.

The visits were always mid-morning, and usually in the cooler half of the year, because that was when the contract’s other tasks clustered. Nobody had decided to avoid summer afternoons; it just fell out that way. Meanwhile the roof void, under a flat felt roof with no ventilation, climbed well into the thirties on hot days. The tank, oversized for a building whose occupancy had halved since it was built, turned its contents over slowly. Stored water sat for long stretches, warming towards the air around it.

The first hint came not from the records but from a tenant on the top floor, complaining that the “cold” tap ran lukewarm first thing. A spot check on a warm afternoon found stored water at 24°C. The clean logbook had been true and useless at the same time.

What the clean readings were hiding

Nothing here was sabotage or neglect in the obvious sense. Three ordinary things stacked up.

The tank was in the worst possible place: an unventilated void that behaves like a greenhouse in summer, with nothing between the warm air and the stored water. Cold storage sitting in warm surroundings will follow those surroundings unless something stops it. HSE’s guidance on hot and cold water systems frames the target plainly — cold water should be stored and distributed so it stays below 20°C where practicable, because roughly 20–45°C is where Legionella multiplies happily [1].

The tank was also too big. An oversized tank feels like prudent resilience, but low turnover means water lingers, and lingering water has time to warm and to stagnate. Sizing is a control measure, not just a plumbing choice.

And the monitoring missed the moment that mattered. A sentinel reading is only as honest as its timing. A cold-water sentinel checked when the building and the weather are cool will pass on a day the same outlet would fail. The risk assessment, not the contractor’s diary, should decide when and how often temperatures are taken [2][3].

The decisions that turned it around

The site did not need a new tank on day one. It needed to see the system clearly, then act in order.

First, they re-timed monitoring. A summer round of cold-water checks, taken in the afternoon on warm days, replaced the assumption that any clean reading was representative. That one change exposed the real picture inside a fortnight.

Second, they insulated the tank and its nearby pipework and looked hard at the void itself, to see whether there was any cheap way to vent the trapped heat. Insulation does not turn warm storage cold, but it slows the drift and gives the rest of the system room to work. For the pipe side of that, see Pipe insulation and its role in Legionella control.

Third, they tackled the oversizing. Rather than replace like-for-like, they specified a smaller tank matched to actual demand, restoring turnover so stored water no longer sat for days.

Each step went into the records with the reasoning attached, not just the date. “Tank downsized to restore turnover after low-occupancy review” is a line an auditor, a successor, or a future version of you can actually use.

Lessons you can carry to your own tanks

The transferable points are not exotic. They are the things a clean logbook never tests.

  • Judge the tank by its worst conditions, not its average. If you only ever check on cool mornings, schedule at least one round on a hot afternoon and compare the two.
  • Treat tank location as a risk factor in its own right. An uninsulated, unventilated roof void is a known weak spot; record it in the assessment even when today’s reading passes.
  • Size for turnover, not just storage. Oversized cold tanks trade a little resilience for a lot of stagnation. Where occupancy has fallen, the old tank may now be part of the problem.
  • Check the things that let heat and dirt in. A close-fitting lid, intact insect screens, and clean internal surfaces all belong in the same inspection as the thermometer.
  • Record the why, not only the what. A reading is data; a reading plus the decision it triggered is control.

If you want the monitoring side in more depth, Temperature checks: the cornerstone of Legionella monitoring covers how to take and read sentinel temperatures properly, and BS 8580-1: what a good Legionella risk assessment should include sets out how all of this should be pinned back to the assessment itself.

Before you treat this as a template

This is a worked illustration, not a specification for your building. The temperatures, sizing and timing in it are common patterns, but your tank’s correct target, how often it is checked, and what counts as an acceptable result are set by a competent, site-specific risk assessment, carried out and acted on by people competent to do so. Where a reading sits outside the expected range, the answer is investigation and corrective action through your written scheme, not a one-off tweak. Sampling can support that picture, but HSE is clear that testing should follow the system and the risk assessment rather than a fixed calendar [4].

FAQ

What temperature should a cold water storage tank stay below?

General HSE guidance points to keeping stored and distributed cold water below 20°C where practicable, because that is around where Legionella starts to multiply [1]. The precise target, and how it is verified for your tank, come from your risk assessment rather than a universal number.

My monthly readings are always fine — why check the tank again in summer?

Because a cold reading taken in cool conditions can pass when the same outlet would fail on a hot afternoon. Roof voids and unventilated plant spaces heat up, and an uninsulated or oversized tank warms with them. Timing checks to catch the worst case, not the average, is the whole point [3].

Is a bigger cold water tank safer than a small one?

Usually not. Oversized tanks turn their water over slowly, so stored water lingers and warms, which is the opposite of what you want. Matching tank capacity to real demand keeps water moving and is itself a control measure, especially where occupancy has dropped since the system was installed.

Sources

[1] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems — Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm