A cold water storage tank is the part of a building's water system most likely to go wrong quietly. It is often in a roof void where nobody goes, it warms up in summer, and if the lid does not fit, insects, dust and worse get in. Inspecting it is part of the risk assessment; cleaning and chlorinating it is what happens when the inspection finds something.
When a tank needs cleaning
When inspection shows sediment, scale, biofilm, corrosion, debris or insect ingress, or where the water temperature in the tank is higher than the scheme allows. It is inspection led rather than calendar led: cleaning a tank annually whether it needs it or not is a common contract and is not what the guidance asks for. What is calendar led is the inspection itself.
What the work involves
Isolate and drain. Remove sediment and clean the internal surfaces. Rectify the defects that let contamination in, most often a lid that does not seal, a missing insect screen on the overflow or warning pipe, and missing insulation. Refill and disinfect the tank and the distribution system to the recognised method. Flush through. Then confirm the system is fit to return to use, which for a system where sampling is warranted means a post disinfection sample.
What a chlorination certificate actually certifies
That a disinfection was carried out on a stated date, by a stated method, at a stated concentration and contact time, on a defined part of the system. It is a record of work done. It is not a legionella clearance certificate and it does not make the system compliant on its own, because compliance rests on the assessment and the written scheme that the disinfection is one action within.
Getting it quoted properly
Ask what is included: tank cleaning only, or cleaning plus disinfection of the distribution system. Whether defects found will be quoted separately or fixed on the day. Whether a post work sample is included and how long the results take. And whether the building has to be off supply, because that is the part that costs the client most and is the part least often mentioned in a quote.
Questions people ask about water tank cleaning
How often should a water tank be cleaned?
When inspection shows it needs it. The inspection is scheduled; the cleaning follows what the inspection finds rather than a fixed calendar.
What is a chlorination certificate?
A record that a disinfection was carried out on a stated date by a stated method, concentration and contact time. It is evidence of work done, not a compliance certificate.
Does cleaning the tank make the system compliant?
No. Compliance rests on the risk assessment and the written scheme of control. Cleaning and disinfection are actions within that scheme.
Should the tank have a lid?
Yes. A tight fitting lid, insect screens on overflows and warning pipes, and insulation are basic controls, and a missing lid is one of the commonest findings in a domestic and small commercial assessment.