Step by step
How to test water for legionella
With a postal kit you collect the water yourself. The laboratory does the rest. The result is only as good as the sample, so plan which outlets to test and post the bottle the same day.
Last reviewed October 2026 against HSE guidance.
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Before you start
- Read your kit's instructions first. They take priority over any general guide, because laboratories differ on bottle size and method.
- Pick the day. Sellers ask for samples to arrive on a weekday and within 24 to 48 hours. Sample on a Monday, Tuesday or Wednesday morning.
- Choose the outlets. The usual choices are the outlet nearest to the cylinder or tank and the one furthest away, plus any outlet that is rarely used. Showers are a common choice because they create spray.
- Do not flush first unless the instructions say so. Running the tap beforehand washes away the water you are trying to test.
Taking the sample
- Wash your hands. Do not touch the inside of the bottle or cap, and do not rinse the bottle. It may contain a neutralising agent.
- Label the bottle with the outlet, date and time.
- Turn the tap or shower on gently and fill the bottle to the marked level, avoiding splashing.
- Cap it firmly and complete the sample form.
- Keep the bottle cool and out of sunlight. Do not freeze it.
- Pack it in the return packaging and send it the same day, or book the courier collection.
To avoid breathing in spray from a shower, remove the head or lower it into the bottle or a clean bag, and run the water gently.
Mistakes that spoil a result
- posting on a Thursday or Friday so the sample sits over the weekend
- under-filling the bottle. One seller states a full 250ml is required; another uses a 1 litre bottle.
- rinsing the bottle or touching the inside of the cap
- leaving the sample in a warm car or on a windowsill
- sampling just after the system has been flushed or disinfected, unless that is what you want to check
Professional sampling
HSE's guidance says sampling should follow BS 7592. A water hygiene company will choose sample points from the risk assessment and take samples in line with that standard. For larger buildings, or where a result may be relied on formally, professional sampling is the safer route. Use the form above for a quote.
After you post it
Expect a result in about two weeks. Some laboratories report a positive result as soon as they see it. See understanding your results for what the numbers mean, or compare kits if you have not bought one yet.
Questions people ask
Can I test for legionella myself?
Yes. A postal kit contains a sterile bottle, instructions and return packaging, and the price includes laboratory analysis.
How many samples do I need?
One bottle tests one outlet. Many people sample two or three outlets: the nearest and furthest from the tank or cylinder, and any that is rarely used.
Should I run the tap before taking a legionella sample?
Follow your kit's instructions. Many ask for a first-draw sample without flushing, because that shows what is sitting at the outlet.
Can I test for legionella at home without a lab?
Rapid tests give an on-site result in about 35 minutes, but they are screening tests for Legionella pneumophila only. A laboratory culture is the recognised method.
Sources
- HSE: HSG274 Part 2, The control of legionella bacteria in hot and cold water systems
- HSE: Legionella and landlords' responsibilities
Checked against these sources in October 2026. This page is general information, not legal advice.
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