What does a cold water storage tank inspection involve?

A cold water storage tank inspection is a periodic check that the tank stays sealed, cold and clean. In practice it covers three things: the physical condition of the tank and its lid and screens, the temperature of the water inside compared with the incoming mains, and whether the tank is sized and used so the water turns over rather than sitting still.

It is a monitoring task under HSG274 Part 2, which sets out how to manage hot and cold water systems. The point is not to certify the tank as safe, but to record its condition and catch the specific faults that let legionella multiply: warmth, contamination and stagnation.

Why a cold water storage tank is a legionella risk

Cold water should sit below 20°C, because legionella multiplies across roughly the 20 to 45°C range. A stored tank works against you here in a way a direct mains feed does not. Loft tanks bake in summer. Tanks sited in warm plant rooms, airing cupboards or next to heating pipework pick up heat through the walls of the tank. Once the water drifts above 20°C, the conditions for growth are in place.

The second problem is contamination. A tank is an open vessel of standing water. Without a close-fitting lid and screened vents, it collects dust, insects, dead flies, bird droppings and building debris, all of which feed the bacteria and the biofilm they live in. The third problem is stagnation: a tank sized far larger than the building's daily demand holds water that turns over slowly, giving it more time to warm and settle.

The visual inspection: what an inspector checks

Most of the inspection is done by eye and by lifting the lid. A competent person will look at:

• The lid. Is it present, close-fitting and opaque? A tight, light-excluding lid keeps out warmth, light, dust and animals. A missing or ill-fitting lid is one of the most common findings.
• Screens and vents. Insect screens on the overflow, warning pipe and any vents should be intact and unblocked, so nothing can crawl or fly in.
• Insects, animals and debris. Dead insects, nesting material, sediment or foreign objects in the water all point to a tank that is not properly sealed.
• Sediment on the base. A layer of silt or scale is a feeding ground for biofilm and a sign the tank needs cleaning.
• Corrosion, scale and biofilm. Rust on a metal tank, heavy scale, or a slimy film on the walls all need noting and, usually, remedial work.
• Materials and condition. The tank and its fittings should be sound, with no cracks, and made of materials suitable for drinking water.

Anything found is written down and turned into an action. A missing lid gets replaced. A blocked screen gets cleared. A silted base triggers a clean. The value is in recording the finding, not in declaring the tank fine. The inspector also checks that the inlet float valve keeps its air gap above the water line, since that gap is the tank's backflow prevention and stops stored water being drawn back into the mains.

Temperature checks: stored water against the incoming main

The temperature check is where the inspection earns its keep. The inspector measures the temperature of the water in the tank, remote from the ball valve, and compares it with the temperature of the water arriving from the mains. The incoming supply is usually cool. If the stored water sits well above it, the tank is gaining heat from its surroundings and the cause needs finding: poor siting, no insulation, a warm plant room, or low turnover leaving the water to warm up.

The target is straightforward. Cold water should stay below 20°C at the tank and at the taps it feeds. Sentinel outlets are checked to confirm the cold stays cold after the water has travelled through the pipework. The full set of figures sits on the legionella temperature chart, and each reading belongs in the log book so trends over the year are visible.

Tank sizing and turnover: the hidden fault

Oversized tanks are a quiet risk that a straight visual check can miss. It is tempting to assume a bigger tank is safer because it holds more reserve. For legionella the opposite is true. A tank far larger than daily demand holds water that turns over slowly, so it warms and stagnates before it is drawn off. Part of a thorough inspection is a sense-check of the tank's capacity against how much water the building actually uses, and whether the water is moving through as it should.

Where a tank is plainly too big for the building, the answer is often to reduce the stored volume or fit a smaller tank, rather than to keep managing an oversized one. This is a design point the assessor should raise, and it belongs in the risk assessment. Guidance on sizing storage correctly, and on the tank's lid, screens and overflows, is set out in BS 8558, the water supply hygiene and fittings design standard.

Inspection and cleaning are not the same job. The inspection tells you the tank's condition. Cleaning and disinfection is the corrective task that follows when the inspection finds sediment, biofilm, corrosion or contamination. There is no fixed calendar date for a clean; it is condition-based, driven by what the inspection turns up.

The move away from stored cold water

The most reliable way to control the risk from a cold water storage tank is to remove the tank. Modern mains-pressure systems feed cold outlets direct from the incoming supply, so there is no open vessel of standing water to warm up, collect debris or stagnate. Where the building's pressure and flow allow it, taking out the tank removes an entire category of legionella risk at source.

Stored tanks are still common, and many buildings genuinely need one for storage capacity, boosted supplies or feeding a hot water cylinder. Where a tank stays, it needs the annual inspection above. Where it can go, removing it is usually the better long-term answer, and it is worth raising when the system is next reviewed.

Recording it and tying it to the guidance

A tank inspection is one line in a wider monitoring scheme. HSG274 Part 2 sets the checks and their frequencies; the ACOP L8 Approved Code of Practice sits above it as the legal duty to assess and control the risk from legionella in water systems. The inspection findings, the temperatures and the remedial actions all go into the log book so there is a written record of what was checked, when, and what was done about it.

If a finding points to a wider problem, for example warm stored water feeding shower heads or other spray outlets, that may prompt sampling. If a sample comes back positive, the response follows the steps in what to do if legionella is detected. The tank inspection is the routine check that keeps small faults from becoming that problem.