How to Prepare for Annual Legionella Testing: A Complete Guide

Introduction

Annual Legionella testing is a cornerstone of any effective water safety management programme. Whether you are a facilities manager, a landlord, or a health and safety officer, knowing how to prepare for testing — and what to expect — will help you get accurate results and maintain compliance with ACOP L8 and HSG274.

This guide walks you through everything you need to do before, during, and after your Legionella water sampling.

Step 1: Review Your Legionella Risk Assessment

Before any testing takes place, revisit your most recent Legionella risk assessment. Check that it is still current and reflects the actual state of your water systems. You should update your risk assessment if:

• There have been changes to the building layout or water system
• The building has been unoccupied or had reduced use (e.g. post-pandemic)
• Previous test results showed elevated Legionella counts
• It has been more than two years since the last review

Your risk assessment will identify which outlets and water systems need to be sampled, and at what frequency.

Step 2: Identify Your Sampling Points

HSG274 provides guidance on which outlets should be sampled. Typically, sampling points include:

• Sentinel outlets: The first and last outlets on each hot and cold water circuit — these give the best indication of system-wide conditions.
• Calorifier base: The base of hot water storage vessels, where sediment can accumulate and temperatures may be lower.
• Cold water storage tanks: Particularly important if tanks are large, infrequently used, or located in warm roof spaces.
• Showerheads and spray taps: These produce aerosols and are a key risk point.
• Any outlets that have been out of use: Stagnant water is a significant risk factor for Legionella growth.

The number of samples you need will depend on the size and complexity of your water system. Our Legionella testing kits are available in 1-sample and 6-sample variants to suit systems of all sizes.

Step 3: Prepare Your Equipment

Using the right equipment is essential for accurate results. Our UKAS-accredited testing kits include everything you need:

• Pre-labelled sterile sample bottles with sodium thiosulphate (to neutralise chlorine)
• Full sampling instructions
• A pre-paid return envelope for fast, safe delivery to the laboratory
• A chain of custody form to maintain sample integrity

Make sure you have the correct number of sample bottles for your planned sampling points before you begin.

Step 4: Follow the Correct Sampling Procedure

Correct sampling technique is critical. Errors in sample collection can lead to inaccurate results — either missing a genuine contamination problem or triggering unnecessary remedial work. Follow these steps:

1. Do not flush the outlet before sampling. For Legionella testing, you need to capture the water that has been sitting in the pipework — this is where bacteria are most likely to be present. Take the first-draw sample directly from the outlet without running the water first.
2. Remove or clean the aerator/showerhead before sampling if you are also taking a second (post-flush) sample, as biofilm on fittings can affect results.
3. Fill the sample bottle to the indicated level — do not overfill or underfill.
4. Label each bottle immediately with the outlet location, date, time, and your name.
5. Keep samples cool (between 2°C and 8°C) and dispatch to the laboratory as quickly as possible — ideally on the same day.

Step 5: Record Everything

Good record keeping is a legal requirement under ACOP L8. For each sampling event, record:

• The date and time of sampling
• The location of each sampling point
• The name of the person who took the samples
• Water temperatures at the time of sampling
• Any observations about the condition of the outlet or system

Keep these records alongside your laboratory results for at least five years.

Step 6: Understand Your Results

Once your samples have been analysed by the UKAS-accredited laboratory, you will receive a report showing Legionella counts (expressed as colony forming units per litre, or cfu/L). HSG274 provides guidance on how to interpret results:

• Below 100 cfu/L: Satisfactory. Continue your normal monitoring programme.
• 100–1,000 cfu/L: Action required. Review your control measures and investigate the cause. Retest after remedial action.
• Above 1,000 cfu/L: Immediate action required. Take the system out of use if possible, carry out a full review, and retest before returning to service.

Step 7: Act on Your Results

If your results indicate elevated Legionella counts, do not delay. Common remedial actions include:

• Thermal disinfection (raising hot water temperatures to 70°C and flushing all outlets)
• Chemical disinfection using chlorine or other approved biocides
• Cleaning and descaling of showerheads, taps, and storage vessels
• Reviewing and adjusting temperature control across the system

After any remedial action, retest to confirm that the system has returned to a satisfactory condition.

Ready to Test?

Our UKAS-accredited Legionella testing kits make compliant sampling straightforward. Available in 1-sample and 6-sample options, each kit includes everything you need to collect, label, and dispatch your samples correctly. Results are typically returned within 10 working days.

Not sure how many samples you need? Contact our team and we will help you identify the right kit for your water system.