Cold-water storage tanks are common in some larger residential blocks, commercial buildings and older properties across London. They can also create conditions that need to be considered in a Legionella risk assessment, particularly where water turnover is poor, temperatures rise, or the tank and associated pipework are not well maintained.
The important point is not simply whether a building has a tank. The risk depends on how the whole water system is designed, used and maintained. The Health and Safety Executive (HSE) advises dutyholders to understand the system, identify where water may be stored or stagnate, and apply proportionate controls where needed.
Why cold-water storage tanks can affect Legionella risk
Legionella bacteria can multiply where suitable conditions exist, including water temperatures broadly between 20°C and 45°C, stagnation, and the presence of nutrients such as scale, sludge, rust or organic matter. Storage tanks can therefore become relevant when water remains in the system for long periods, the tank warms up, or contamination enters through poorly protected openings.
For cold-water systems, temperature control is one of the principal risk-management measures. HSE guidance states that cold water should, where possible, be stored and distributed below 20°C. This is one reason tank location, insulation, capacity and water turnover matter.
What should be checked on a cold-water storage tank?
A competent assessment should consider the condition and operation of the tank as part of the wider system. Typical checks include:
- Whether the tank has a close-fitting lid in good condition.
- Whether overflow, warning pipes and vents are protected against insects and vermin where applicable.
- Whether insulation is intact and helps protect the stored water from excessive warming.
- Whether the water surface and internal condition show visible contamination, sediment, corrosion or debris.
- Whether the stored volume is appropriate for actual demand rather than being substantially oversized.
- Whether water flows through the tank regularly or parts of the system experience stagnation.
- Whether redundant or little-used pipework creates dead legs or areas of poor turnover.
- Whether monitoring and maintenance records match the control measures identified by the risk assessment.
HSE’s technical guidance, HSG274 Part 2, advises an annual inspection of cold-water storage tanks to check their condition inside and outside and the condition of the water. The precise maintenance programme should still reflect the system and its assessed risk.
Water turnover matters as much as tank condition
A tank may look clean but still be part of a poorly managed system if water turnover is low. Oversized storage, vacant areas, infrequently used outlets and changes in building occupancy can all reduce movement through the system.
HSE guidance recommends keeping stored volume to what is reasonably needed and ensuring flow through connected tanks. In practical terms, a risk assessment should consider whether the building’s current water use still matches the way the system was originally designed.
Temperature checks provide evidence that controls are working
Where temperature is used as a control measure, the aim is not merely to set a target once. The system needs monitoring so that failures can be identified. HSE advises that incoming and stored cold water should remain below 20°C, and cold-water sentinel outlets should achieve below 20°C after running for two minutes.
In some London buildings, plant rooms, roof spaces and poorly insulated service areas can become warm during summer. If cold-water temperatures rise, that should trigger review of the system and the effectiveness of the existing controls rather than reliance on assumptions.
Does every tank need cleaning or Legionella sampling?
Not automatically. Cleaning, disinfection and microbiological sampling should be based on the condition of the system, the risk assessment and the applicable control regime. A visible problem, loss of control, significant contamination, system alteration or other risk indicator may justify remedial action, but routine laboratory testing is not a substitute for a suitable risk assessment and effective system management.
What London landlords and property managers should do
If you are responsible for a building with stored cold water, start by establishing what the system actually contains and how it operates. Keep drawings and records where appropriate, know which outlets are supplied from storage, identify little-used areas, and make sure responsibilities for inspection and maintenance are clear.
For rented residential property, HSE notes that landlords have a duty to assess and control the risk of exposure to Legionella. For many simple domestic systems, the assessment may confirm a low level of risk and only proportionate controls may be necessary. Larger or more complex systems can require more detailed management.
When a Legionella risk assessment is useful
A risk assessment is particularly useful when the water system is not fully understood, the property has storage tanks or complex distribution, occupancy has changed, there are little-used outlets, or previous maintenance records are incomplete. The purpose is to identify realistic sources of risk and decide what controls are appropriate for that particular property.
You can read more about our Legionella risk assessment services, review our Legionella FAQs, or request a property-specific quote for a London building.
Key takeaway
A cold-water storage tank is only one part of a building’s water system, but it can reveal important issues involving temperature, cleanliness, stagnation and water turnover. Good Legionella control depends on understanding the complete system, applying proportionate controls and checking that those controls continue to work.
Official reference: HSE guidance on hot and cold water systems and HSE practical considerations for hot and cold water systems.