How Often Should Water Tanks Be Cleaned?

If you manage a commercial building, keeping on top of your cold water storage tanks is one of those responsibilities that is easy to push down the list — until something goes wrong. The reality is that water tank cleaning is not just a hygiene issue; it is a legal compliance issue, and the consequences of getting it wrong can be serious.

This guide sets out what the regulations actually require, the factors that should inform your cleaning schedule, and the warning signs that tell you a clean cannot wait until the next scheduled visit.

What the Regulations Say About Cleaning Frequency

The question of how often water tanks should be cleaned does not have a single universal answer, but it does have a regulatory framework that every facilities manager needs to understand.

Under HSG274 Part 2 — the Health and Safety Executive's technical guidance on hot and cold water systems — cold water storage tanks should be inspected annually as a minimum. That inspection should assess the condition of the tank interior, the quality of the stored water, and whether any sediment, contamination, or signs of microbiological growth are present. Where an inspection identifies a problem, cleaning and disinfection should follow immediately.

ACoP L8, the Approved Code of Practice that underpins the legal framework for legionella control, is equally clear: keeping water systems clean is a fundamental control measure, not an optional extra. Paragraph 190 of ACoP L8 states that "the risk from exposure to Legionella should be prevented or controlled; precautions include keeping the system and the water in it clean."

For potable (drinking water) tanks, the standard guidance is more frequent: inspections every six months, with microbiological sampling at the same intervals. Non-potable tanks — those feeding toilet flushing systems, for example — may follow an annual inspection cycle, though this should be informed by your site-specific legionella risk assessment.

HSG274 Part 2 does distinguish between potable and non-potable tanks when it comes to inspection frequency, but in practice the vast majority of commercial cold water storage tanks are cleaned on an annual cycle. This aligns with what the guidance requires and what the industry delivers day to day. More frequent cleaning is not something the regulations demand as standard, and it is rarely necessary on a well-maintained, properly sized tank that is turning over its capacity regularly.

In short, once a year is what the guidance points to, and for most sites it is entirely appropriate — provided the annual inspection and clean is actually happening, and the records to prove it are in place.

What Happens During a Professional Tank Clean?

A thorough water tank cleaning and disinfection typically involves draining the tank, removing accumulated sediment and debris from the tank floor and walls, scrubbing internal surfaces, and then disinfecting using an approved chlorine-based solution. Water samples are taken at 48 hours post-disinfection to confirm microbiological safety before the system is returned to service. Records are generated throughout — something your auditors will want to see.

It is worth noting that this is not a job for untrained staff. HSG274 is explicit that cleaning must be carried out by competent people using the right equipment and appropriate biocides.

The Factors That Should Influence Your Cleaning Schedule

An annual clean is the right starting point for the majority of commercial sites. Whether your circumstances call for more attention than that comes down to a handful of variables that any Responsible Person should be weighing up.

Tank Material and Age

GRP (glass-reinforced plastic) sectional tanks are the most common type in UK commercial buildings and are generally straightforward to clean. Older steel tanks or concrete tanks are more prone to corrosion, scale build-up, and surface deterioration, which can harbour biofilm and bacteria even after a clean. If your tank is showing signs of internal deterioration — pitting, rust staining, or damaged lining — you may want to look at whether water tank relining or replacement is a more practical long-term solution than repeated cleaning.

Water Turnover and Stagnation Risk

Tanks that cycle through their full capacity regularly are lower risk than those where water sits for extended periods. If your building has seen reduced occupancy — seasonal closures, remote working, or refurbishment works — stagnation becomes a genuine concern. Legionella pneumophila thrives in warm, still water with a source of nutrients. A tank that was clean six months ago can deteriorate quickly if turnover has dropped significantly.

For sites with known stagnation risks, increasing inspection frequency is a sensible precaution, and your risk assessment should reflect this.

Stagnant Water Tank

Building Type and Occupancy

Healthcare settings, care homes, and buildings with vulnerable occupants have a much lower tolerance for risk. The same is true of sites where aerosols are regularly generated — spray taps, showers, or calorifier systems in close proximity to occupied areas. In these environments, more regular cleaning and sampling may be prudent, regardless of what the tank looks like on inspection.

Inlet Water Quality

Most commercial tanks are fed from the mains, which provides a degree of inherent quality control. Sites drawing from boreholes, rainwater harvesting systems, or other non-mains sources need to clean more frequently and test more rigorously, as incoming water quality is less predictable.

Signs Your Tank Needs Cleaning Now

Scheduled maintenance is important, but there are circumstances where you should not wait for the next planned visit. The following are all triggers for immediate water tank cleaning:

Visible Contamination or Sediment

If an inspection reveals sediment build-up on the tank floor, visible algae, discolouration, or any organic matter, cleaning should be arranged without delay. Sediment provides a nutrient source for bacteria and reduces the effectiveness of any residual disinfectant in the water.

Failed Microbiological Sampling

A positive sample result for Legionella or coliform bacteria is an immediate trigger for investigation and remedial action. Your water hygiene monitoring programme should be generating records regularly enough that a failed result prompts action before anyone is put at risk. If you are not yet running a structured monitoring programme, that is the first thing to address.

Physical Damage to the Tank

Cracks in tank walls, failed covers, damaged inlet or outlet pipework, or evidence of vermin ingress all compromise the integrity of the system. A tank with a damaged or ill-fitting lid is not a clean tank, regardless of when it was last washed out. Depending on the nature and extent of damage, you may need to consider tank repair or full replacement alongside the cleaning programme.

After a Period of Shut-down or Reduced Use

If your building has been unoccupied or operating at significantly reduced capacity for four weeks or more, the water in your tanks should be considered suspect. A full drain-down, inspection, clean, and disinfection — combined with flushing of all outlets — is the appropriate response before normal occupancy resumes.

Keeping Records and Staying Compliant

One aspect of water tank cleaning that facilities managers sometimes overlook is the paperwork. ACoP L8 and HSG274 both require that records are maintained for all cleaning, inspection, and sampling activities. These records should include dates, findings, actions taken, and the identity of the person or contractor carrying out the work.

Audit trails are not just a bureaucratic exercise. In the event of a legionella investigation or enforcement action, your records are your first line of defence. If you cannot demonstrate that a cleaning regime was in place and being followed, the legal and reputational risks are significant.

For a broader picture of what your ongoing water hygiene obligations look like — including temperature monitoring, sampling, and TMV servicing — the water hygiene monitoring services section of the site has a useful overview.

For the vast majority of commercial sites in the UK, an annual inspection and clean is what the guidance requires and what good practice looks like. The key is making sure it actually happens, that it is carried out by competent people, and that the paperwork is in order. Where your risk assessment identifies specific concerns — a poorly performing tank, a history of contamination, or a period of extended low use — that is the time to review whether the schedule needs adjusting. The decision should always be risk-led rather than based on a blanket rule.

The most defensible position is always one grounded in your site-specific risk assessment. If your current risk assessment is more than two years old, or if your building use has changed since it was last reviewed, it is worth having it revisited before your next scheduled clean.

If you want to understand more about what is involved in a professional tank clean and disinfection, and what the process looks like from start to finish, you can find a detailed breakdown on the water tank cleaning services page.

For further reading on keeping your water systems compliant, you may also find our article on understanding ACoP L8 a useful reference — it sets out what the code of practice requires and how those duties translate into day-to-day building management.