---
title: "Water tank inspections: checking for Legionella risk"
source_url: https://legionella.io/articles/water-tank-inspections-checking-for-legionella-risk/
canonical_url: https://legionella.io/articles/water-tank-inspections-checking-for-legionella-risk/
pillar: "Monitoring, Flushing & Sampling"
summary: "A field checklist for inspecting cold water storage tanks: the lid, light, sediment and temperature signs that flag a Legionella risk, plus what to do next."
primary_keyword: "water tank inspection"
date_published: 2025-07-27
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."
---

# Water tank inspections: checking for Legionella risk

Lift the lid on a neglected cold water storage tank and you can often smell the problem before you see it. A film of debris floating on the surface, a tide-mark of scale around the waterline, daylight coming through a cracked lid and feeding a green bloom on the wall. None of that shows up in a temperature log. It shows up in an inspection.

A tank inspection is the one routine check that looks at the system instead of a single reading. Flushing proves water moved. A sentinel temperature proves one point was cold on one morning. Opening the tank tells you whether the conditions Legionella needs are quietly building up where most of your stored water actually lives.

Cold water storage tanks sit on the list of systems most likely to create risk for a reason [3]. They hold a large volume, often more than the building now uses, and they are easy to forget because they are usually tucked in a roof void or a plant room nobody visits. In UK practice the control of legionella in these systems runs off two references — the Approved Code of Practice L8 and the technical guidance in HSG274 [1][2] — and both expect physical inspection as part of the scheme, not just numbers in a logbook.

## What to check when the lid comes off

Treat the inspection as a structured walk over the tank, top to bottom, recording what you find rather than ticking a box. Group it the way the risks group.

**The cover and how things get in**
- Check the lid is close-fitting, intact and seated flush. A warped, cracked or propped lid lets in dust, insects and light.
- Confirm insect and vermin screens are present on the overflow and warning pipe, and are not torn, blocked or missing.
- Look for light getting in. Daylight inside a tank drives algae and feeds the biofilm that Legionella shelters in.
- Note anything that has already got in: dead insects, leaves, bird droppings, building dust, a stray tool.

**The water itself**
- Measure the stored water temperature and, where you can reach it, the incoming cold feed. Cold water is generally expected to be stored and distributed below about 20°C [4]; write down the actual figure, not "OK".
- Judge clarity. Cloudiness, a tint of colour, or an oily surface film all earn a closer look.
- Assess turnover. If the tank is far bigger than current demand, water can sit for days. That is the stagnation Legionella prefers, and it is a design problem a temperature check will never reveal.

**The inside surfaces**
- Inspect the base for sediment, sludge, scale or rust. A layer of debris is both food and shelter for biofilm.
- Scan the walls and waterline for slime, scale build-up or corrosion.
- Note the tank material and its condition: splits, distortion, or a sagging plastic cistern that no longer drains cleanly to the outlet.

**The fittings and the flow path**
- Check the float (ball) valve operates and is not held part-open, which lets the level drift and the water age.
- Confirm the inlet and outlet are positioned so water crosses the tank rather than short-circuiting one corner. The patch of water furthest from the outlet is where a dead zone forms.
- Check the overflow and warning pipe discharge correctly and stay screened.

**Where the tank lives**
- Check the insulation is intact and the lid is not being used as a shelf for warm plant or stored boxes.
- Note nearby heat sources — uninsulated pipework, plant, a hot roof void in summer — that nudge stored water up towards the 20–45°C growth band.

**Before you walk away**
- Record each finding with a reading or a photo, not a single tick.
- Raise any out-of-range temperature or visible contamination as an action with a named owner and a date.
- Decide whether the finding calls for cleaning, disinfection, sampling or a risk-assessment review, and write down the reason.

## How to run it so the record actually counts

The value of a tank inspection lives in the write-up, not the visit. A line that reads "tank inspected, satisfactory" is worth almost nothing six months later when someone asks what the water temperature was. A line that reads "stored water 17°C at 09:40, base clear, lid screen intact, photo attached" is evidence you can stand behind.

Two habits separate a real inspection from a box-tick. First, capture numbers and images at the point of inspection — a stored-water temperature, a photo of the base, a photo of the lid and screens. Scanning a tag at the tank to open the right record beats chasing paper across a roof void, which is the case [makes for QR-coded logging](https://legionella.io/articles/using-qr-codes-to-streamline-maintenance-logging/). Second, record the decision alongside the finding. "Sediment on base, cleaning raised as action AC-214, due within four weeks, escalates to the responsible person if not closed" turns an observation into managed control.

The tank temperature you take during the inspection is often one of your cold-water monitoring points, so it should reconcile with your routine sentinel readings rather than contradict them — [covers how sentinel outlets fit the wider monitoring picture](https://legionella.io/articles/sentinel-outlets-what-they-are-and-how-to-monitor-them/). If the stored water is warmer than expected, the inspection becomes a starting point for diagnosis, not a closed item.

## The bits people skip

Even diligent teams tend to miss the same handful of things.

The **warning and overflow pipe screen** gets forgotten because it sits outside the tank; a missing screen is an open invitation to insects and vermin. People measure **only the stored temperature** and never the incoming feed, so they cannot tell whether a warm tank is a storage problem or an upstream one — and a persistently warm cold feed is its own investigation, the kind covered in [on temperature troubleshooting](https://legionella.io/articles/troubleshooting-temperature-problems-in-plumbing-systems/). A **redundant tank that is still connected** is a dead leg the size of a cistern, yet it rarely makes the inspection round because nobody thinks of it as live. And almost everyone photographs the **surface** of the water but not the **base**, where the sediment that actually matters settles.

The most common omission of all is recording the task instead of the decision. Cleaning and disinfection, when an inspection shows they are needed, are a separate piece of specialist work with their own method and verification — the same discipline that applies to [showerhead cleaning and descaling](https://legionella.io/articles/showerhead-cleaning-and-descaling-schedules/) at the other end of the system.

## A note of caution before you open anything

Opening a tank is not always a quick favour. Larger tanks can be confined spaces, and even a hatch inspection can contaminate the water you are trying to protect if done carelessly — so this is work for a competent person following a safe method. The temperature figure quoted above and how often you inspect are set by your written scheme and the person who assessed your site, not by a checklist found online, which is why both are flagged for verification rather than stated as rules. And a tank that looks clean on the day is reassuring, not conclusive: inspection is one strand of control, read alongside your temperatures, your flushing, and — where the risk assessment calls for it — sampling carried out to a recognised method such as BS 7592 [5].

## FAQ

### How often should a cold water storage tank be inspected?
Frequency follows your risk assessment and written scheme rather than a fixed calendar. In common practice a tank gets a visual inspection at least annually, with stored-water temperature checked more often as part of routine monitoring; after any disturbance, repair or a use-pattern change, inspect sooner. Confirm the interval that applies to your system through your assessment.

### Do I have to clean the tank every time I inspect it?
No. An inspection is a look; cleaning and disinfection are triggered by what the look reveals — significant sediment, scale, corrosion, fouling or contamination — or by an interval set in the risk assessment. Cleaning a sound, clean tank on a fixed schedule for its own sake is usually effort better spent elsewhere.

### If the tank is clean, is the rest of the system fine?
Not necessarily. The tank is the start of the cold water system, not the whole of it. Downstream pipework, calorifiers, long runs to far outlets and any low-use fittings each carry their own risk, and a spotless tank tells you nothing about a shower nobody has run for a month.

## Related reading
- [Sentinel outlets: what they are and how to monitor them](https://legionella.io/articles/sentinel-outlets-what-they-are-and-how-to-monitor-them/)
- [Troubleshooting temperature problems in plumbing systems](https://legionella.io/articles/troubleshooting-temperature-problems-in-plumbing-systems/)
- [Showerhead cleaning and descaling schedules](https://legionella.io/articles/showerhead-cleaning-and-descaling-schedules/)
- [Using QR codes to streamline maintenance logging](https://legionella.io/articles/using-qr-codes-to-streamline-maintenance-logging/)

## What to do next
Before your next scheduled visit, pull the last tank inspection record. If it has a stored-water temperature and a photo of the base, you have an inspection. If it just says "tank OK", you have a tick. Book a proper look — hatch open, thermometer in hand, camera ready — and write down what you actually see.

## Sources

[1] 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
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[4] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[5] BSI, "BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice". https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1