Water sampling plays an important role in verifying that domestic hot and cold water systems are being managed safely and effectively. Now that we are in 2026, many businesses will be reviewing their water safety arrangements and asking a more focused question:
What water sampling is actually required for our hot and cold water system?
This guide focuses on domestic hot and cold water systems, including taps, showers, cold water storage tanks, stored hot water heaters and associated pipework commonly found in commercial buildings. It does not cover cooling towers or evaporative condensers, which are subject to separate and more prescriptive requirements.
A Risk-Based Approach to Water Sampling
There is no single sampling regime that applies to all domestic water systems.
UK guidance is clear that water sampling should be risk-based, proportionate, and justified by a suitable and sufficient Legionella risk assessment. Sampling should support, not replace, control measures.
Factors influencing risk include system design and condition, temperature control, water usage patterns, history of issues, and the vulnerability of building occupants.
Legionella Sampling
What Health Risk Does Legionella Present?
Legionella bacteria can cause Legionnaires’ disease, a potentially serious form of pneumonia. In some cases, exposure may also lead to a milder, flu-like illness known as Pontiac fever.
How Are People Exposed to Legionella?
Legionella exposure occurs when tiny contaminated water droplets are breathed in, usually produced by outlets such as showers, spray taps and other outlets that create fine water droplets.
Legionella is not normally transmitted through ingestion of drinking water.
When Is Legionella Sampling Required Or Expected?
Routine Legionella sampling is not required for all domestic hot and cold water systems.
Sampling may be required where:
- Safe hot and cold water temperatures cannot be reliably maintained.
- The water system is complex, poorly understood or has known deficiencies.
- Low use, stagnation, or dead-leg risk is present.
- Control measures are repeatedly failing.
- Occupants may be more vulnerable.
- Previous incidents or sample results raise concerns.
Any decision to sample or not should be clearly justified and documented in the risk assessment.
Important exception: This guide does not address hot tubs, spa pools, and certain water features that recirculate water and produce aerosols. Instead, these systems are subject to stricter, routine microbiological testing because they have a higher potential for aerosol generation and conditions that are particularly favourable for bacterial growth.
What Action Is Required If Results Are Positive?
A positive Legionella result does not automatically indicate immediate danger, but it does require investigation.
Actions typically include:
- Review of system control measures and records
- Investigation into temperature performance and outlet usage
- Corrective or remedial works where deficiencies are identified
- Follow-up sampling where appropriate
All responses should be proportionate and risk-based.
General Bacteria (Total Viable Counts)
What Health Risk Does General Bacteria Indicate?
General bacterial testing (often referred to as total viable counts or TVC / TACC) does not target a specific pathogen. Instead, it provides an indication of overall water quality and system hygiene.
Elevated counts may suggest stagnation, poor turnover or hygiene issues that could allow harmful bacteria to grow.
How Are People Exposed?
General bacteria are not linked to a specific disease pathway. However, poor microbiological water quality may increase the likelihood of exposure to harmful organisms through normal water use.
When Is General Bacteria Sampling Required or Expected?
General bacterial sampling may be required:
- Following new installations, commissioning or alterations
- Where water quality is in doubt
- After prolonged stagnation or building closures
- When investigating complaints or system concerns
Sampling should be targeted and justified by risk.
Important Exception: Private water supplies
Where outlets are supplied by a private water supply, such as a borehole, routine microbiological sampling is typically required to demonstrate that the water is wholesome and suitable for use. This commonly includes general bacteria testing, regardless of building type.
What Action Is Required If Results Are Elevated?
Actions depend on the level and trend of results, but may include:
- Review of flushing and usage regimes
- Cleaning or disinfection of affected outlets
- Investigation of hygiene or design issues
- Repeat sampling to check improvement.
E.Coli and Coliform Sampling
What Health Risk Do E.Coli and Coliforms Present?
- Coliform bacteria indicate that the system may be vulnerable to contamination.
- E. coli indicates faecal contamination and represents a serious health risk.
How Are People Exposed?
Exposure typically occurs through ingestion of contaminated drinking water, which can cause gastrointestinal illness. Their presence indicates a failure in water quality protection.
When is E.Coli and Coliform Sampling Required Or Expected?
Sampling may be required:
- Following new installations or system alterations
- Where drinking water safety is in question
- After contamination incidents or supply interruptions
Important Exception: Private water supplies
Outlets supplied by boreholes or other private water supplies require routine monitoring for E.coli and coliform bacteria. This is necessary to demonstrate ongoing drinking water safety, as these supplies are not subject to the same controls as mains water.
What action is required if the results are positive?
- E.Coli: immediate investigation and corrective action are required.
- Coliforms: investigation into potential contamination routes and system vulnerability.
Actions may also include:
- Restricting the use of affected outlets
- Flushing, cleaning or disinfection
- Remedial works to address defects
- Repeating sampling to confirm resolution
Pseudomonas Sampling
What Health Risk Does Pseudomonas Present?
Pseudomonas aeruginosa is an opportunistic pathogen that can cause serious infections in immunocompromised or highly vulnerable patients, particularly in healthcare settings.
How Are People Exposed?
Exposure is typically associated with direct contact at the outlet, such as taps and showers, rather than whole-system aerosol exposure.
When Is Pseudomonas Sampling Required Or Expected?
Routine Pseudomonas sampling is primarily associated with healthcare environments, particularly in areas with highly vulnerable patients. Sampling requirements are driven by HTM 04-01 and local water safety plans.
Outside healthcare, routine Pseudomonas sampling is not normally required and would need a clear risk-based justification.
What Action Is Required If Results Are Positive?
Actions are usually outlet-specific and may include:
- Restricting or removing the use of affected outlets
- Cleaning, disinfection or replacement of fittings
- Increased monitoring and follow-up sampling
- Review of local water safety controls
How Often Should Domestic Water Sampling Be Carried Out?
There is no fixed frequency for sampling most domestic hot and cold water systems.
Sampling frequency should be determined by:
- Risk assessment findings
- System performance and compliance history
- Previous sample results and trends
- Changes to building use or occupancy
Note: In healthcare settings, Pseudomonas sampling may be carried out at defined intervals as part of local water safety plans, in line with healthcare-specific guidance.
Sampling Must Link Back to the Risk Assessment
Water sampling should never exist in isolation.
A suitable and sufficient Legionella risk assessment should:
- Identify where sampling is necessary.
- Justify which tests are required and why
- Define sampling locations and frequencies.
- Be reviewed following changes or incidents.
Clear reasoning and good records are just as important as the results themselves.
Getting Water Sampling Right in 2026
Used correctly, water sampling provides valuable evidence that domestic hot and cold water systems are being managed safely.
A structured, risk-based sampling programme helps to:
- Verify control measures
- Identify emerging risks
- Support compliance decisions
- Demonstrate competence during audits and inspections.
Start with risk – not routine.
Need clarity on your water sampling obligations for 2026?
Understanding what sampling is required starts with a clear, up-to-date risk assessment that reflects how your system is actually used.
