---
title: "Hot water storage: preventing Legionella in tanks and cylinders"
source_url: https://legionella.io/articles/hot-water-storage-preventing-legionella-in-tanks-and-cylinders/
canonical_url: https://legionella.io/articles/hot-water-storage-preventing-legionella-in-tanks-and-cylinders/
pillar: "Water Temperature Control"
summary: "The flow thermometer reads 60°C; the cool, sludgy base of your calorifier may not. How to store hot water so the bottom of the tank can't breed Legionella."
primary_keyword: "hot water storage"
date_published: 2025-06-16
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Hot water storage: preventing Legionella in tanks and cylinders

Walk into most plant rooms and you can settle the hot water question in five seconds: glance at the flow thermometer on top of the calorifier, see 60°C, move on. The catch is that a storage vessel is not a single temperature. It is hottest at the crown, where that thermometer sits, and coldest at the base, where the cold feed enters and where scale, rust and sludge settle out of the incoming water. That cool, well-fed layer at the bottom is exactly where Legionella would choose to live.

So the useful question about hot water storage is not "is the water hot?" It is "is the whole vessel hot, and can I show the cold corner isn't quietly incubating anything?" Below are the questions maintenance teams actually ask about cylinders and calorifiers, answered for a UK system.

## What temperature should stored hot water actually be kept at?

The HSE benchmark most UK schemes are built around is to store hot water at 60°C and distribute it so it reaches at least 50°C at the outlet within about a minute of running — 55°C in healthcare premises, where the people exposed are more vulnerable [1]. The 60°C figure is not arbitrary: Legionella is progressively killed as water climbs past roughly that point, so a vessel genuinely held there is hostile to it. Treat these as the common starting figures your risk assessment confirms or adjusts for your plant [2], not numbers to defend for their own sake. A cylinder that holds 60°C at the top but feeds tepid water to a basin 30 metres away is failing the test that matters — the one at the point of use.

## Why does the bottom of the cylinder matter more than the top?

Heat rises, so a storage vessel stratifies: a hot layer on top and a cooler layer at the base, right where the cold feed comes in and where sediment drops out of the water. Scale, rust flakes and organic sludge gather in that bottom few inches and hand Legionella both the cooler temperature and the nutrients it needs. The flow thermometer on the crown never sees any of it. A vessel can read a comfortable 60°C on the gauge while its base sits well under that — which is why a sludged, poorly mixed cylinder is a classic blind spot rather than an exotic failure.

## Is one thermometer reading on the flow enough to prove control?

No. A single reading is one point at one moment, and on a storage vessel the most reassuring point — the flow at the top — is the least likely to reveal a problem. What gives you a real picture is a small set of readings that bracket the system: the storage flow temperature, the temperature at or near the base or drain where you can reach it, the return temperature on a circulating system, and the sentinel outlets nearest to and furthest from the vessel [3]. Your risk assessment sets which points and how often. The principle is that you are judging whether the whole vessel and its loop hold control, not whether one thermometer happened to be happy on the day.

## What does the return temperature tell you?

On a system that circulates hot water around a loop so outlets run hot quickly, water that has travelled the loop comes back to the calorifier to be reheated. If that return has dropped well below the storage temperature, somewhere on the loop is running cool enough to matter. The common expectation is a return at 50°C or above [1]; one that keeps falling short points to undersized circulation, a tired pump, or long branches the loop never really reaches. It is one of the most useful single numbers in the plant room, because it summarises the health of the whole distribution leg rather than just the tank.

## How often should a calorifier be inspected and drained?

Periodically, and on a schedule your risk assessment sets rather than a fixed calendar rule for every site [3]. Two jobs matter for storage specifically: drawing off from the drain valve at the base to see what is actually sitting there, and inspecting and cleaning the vessel internally where access allows, to clear the sediment bed that feeds growth. A calorifier that has not been opened or drained in years is carrying an unknown — and the cheapest moment to find a heavy sludge layer is during planned maintenance, not during an outbreak investigation. Where sampling is used to back this up, the frequency follows the system and the risk assessment, not habit [4].

## Does turning storage temperature down to save energy raise the risk?

It can, and this is a live tension on UK sites. Heat pumps and other low-carbon heat sources run cooler than gas boilers, and there is a standing temptation to drop storage and circulation set-points to suit the kit or trim the bill. Lowering stored hot water toward the growth range without compensating controls hands the bacteria back the conditions the 60°C regime was removing. None of this means decarbonising is off the table — it means a change to storage or circulation temperature is a change to a safety control, and it belongs in a reassessment with competent input, not a quiet tweak on the controller. If you are redesigning around lower flow temperatures, the pipework layout matters as much as the numbers; see [Avoiding stagnation](https://legionella.io/articles/avoiding-stagnation-design-tips-for-consistent-water-temperatures/).

## What about a standby calorifier or a cylinder no one uses much?

These are the quiet reservoirs. A duty/standby pair can leave the standby vessel sitting full and slowly cooling for weeks, so rotating duty keeps either one from stagnating cool. A cylinder in a barely-occupied building — a seasonal site, a void let, a rarely-used annexe — drifts to room temperature while the water in it goes nowhere. The fix is the same logic as any low-use part of a system: keep it hot and keep it moving, or take it out of service deliberately rather than by neglect. Flushing the outlets fed by these vessels is part of the same job; see [Flushing little-used outlets](https://legionella.io/articles/flushing-little-used-outlets-best-practices/).

## If you change one habit this quarter

Stop judging your main calorifier by the thermometer on top of it. Next time someone logs the storage temperature, have them also note the return temperature and, where the drain valve is reachable, what comes out of the base — then write down what they will do if either reads low, not just the figure. A reading with a decision attached is control. A reading on its own is just a number in a logbook.

## A note on the numbers

The temperatures and intervals here are the common UK starting points, not a specification for your plant. What your vessel should store at, how its base is checked, how often it is inspected and what happens when a reading is low all depend on the calorifier's design, the system it feeds and the people using the water — and only a competent, site-specific risk assessment can set them [2]. Treat any change to storage or circulation temperature, including one made for energy efficiency, as a change to a safety-critical control that goes through that assessment rather than the boiler controls alone.

## Related reading

- [Identifying Legionella hazards in your water system](https://legionella.io/articles/identifying-legionella-hazards-in-your-water-system/)
- [Thermostatic mixing valves and Legionella risk](https://legionella.io/articles/thermostatic-mixing-valves-and-legionella-risk/)
- [Flushing little-used outlets: best practices](https://legionella.io/articles/flushing-little-used-outlets-best-practices/)
- [Avoiding stagnation: design tips for consistent water temperatures](https://legionella.io/articles/avoiding-stagnation-design-tips-for-consistent-water-temperatures/)

## 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