Cleaning a Cold Water Storage Tank: Procedure and Frequency

A cold water storage tank should be inspected at least annually. It should be cleaned and disinfected when inspection finds it necessary — debris, biofilm, scale, water above 20°C, broken lid or insect screens, or a positive Legionella result. The standard chemical disinfection is 50 mg/l free chlorine for 1 hour (or 20 mg/l for 2 hours) under HSG274 Pt2.

For background reading, our blog post how often should water tanks be cleaned covers the homeowner-level overview. This guide is the technical procedure.

When a tank needs cleaning

HSG274 Pt2 doesn't impose a fixed cleaning frequency — it says inspect annually and clean when needed. In real terms, the triggers are:

• Visible debris, sediment, scale or biofilm on inspection.
• Discoloured water (yellow, brown, milky) or any taste / odour from cold outlets.
• Insects, larvae or vermin evidence — usually with a failed lid or missing insect screens.
• Tank water temperature above 20°C.
• Positive Legionella sample from the cold side of the system.
• After major works on the system or a long period of disuse.

Typical frequency in practice

The clean & chlorination procedure

1. 1

   Risk assess

   Confined-space and working-at-height assessments, chemical COSHH (sodium hypochlorite or similar), neutralisation plan, controlled discharge plan. Two-person job.

2. 2

   Isolate and drain

   Close the inlet, isolate or notify users, drain the tank via the wash-out or by syphon to a suitable discharge point. Capture and remove sediment for safe disposal.

3. 3

   Clean

   Brush down walls and base, remove all sediment, scale and biofilm. Replace damaged float valves, screens or lids while access is open.

4. 4

   Refill and chlorinate

   Refill to working level and dose to 50 mg/l free chlorine (or 20 mg/l for the longer contact). Open every downstream outlet in turn to draw the chlorinated water through the whole system.

5. 5

   Maintain contact time

   Hold at the target concentration throughout the system for 1 hour at 50 mg/l (or 2 hours at 20 mg/l). Verify residuals at sentinels with a test strip or photometer.

6. 6

   Neutralise and discharge

   Neutralise with sodium thiosulphate (or equivalent) before discharge if discharging to surface water; check with the local sewer / water company for high-volume discharges.

7. 7

   Flush

   Flush the system until residual is below 1 mg/l at all outlets. Restore to service.

8. 8

   Sample if required

   Post-clean Legionella and TVC samples per the scheme of control — typically required in healthcare and after an outbreak.

9. 9

   Record

   Photographs before/after, residual readings with times, neutralisation, discharge route, completion. This is your compliance evidence.

What a good annual inspection looks like

Even when a clean isn't needed, an annual inspection is. We do the following:

• Lid: sealed, in good condition, not warped or letting daylight in.
• Insect screens on overflow, warning pipe and any vent — fitted, intact, not blocked.
• Water temperature: below 20°C (we take a probe reading at the time of inspection, ideally repeated in summer).
• Inside: photograph through the access hatch. Look for sediment, biofilm, scale, insects, discoloration, plastic degradation.
• Float valve and ball: serviceable, sealing properly, no constant trickle.
• Lagging / insulation: complete, dry, no rodent damage.
• Location: not adjacent to warm flues, hot pipework or in a baking hot loft cupboard.

Design fixes that reduce cleaning frequency

• Sealed, lockable lid with screened vent.
• Insect-screened overflow and warning pipe.
• Insulation on the whole tank and on inlet/outlet/overflow pipework.
• Tank position in the coolest practical part of the building; never above a calorifier or hot flue.
• Drain-down valve at the base for clean maintenance.
• Modular GRP tanks with a smooth inner surface (don't shed material like older steel/asbestos cement tanks).

Frequently Asked Questions

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