How Does a Cold Water Storage Tank Work?
- Marc Fitzpatrick
- Jul 18
- 3 min read
Updated: 3 days ago
A cold-water storage tank, sometimes called a header tank or cistern, is a container that stores water for later use. In most commercial and residential buildings, it provides a steady supply when the mains supply is interrupted, when pressure drops, or when large volumes are needed quickly. Facilities managers rely on these tanks to keep water available for daily use, safety systems, and to ensure compliance with hygiene regulations.
How does a cold-water storage tank work in simple terms?
A cold-water storage tank fills automatically from the mains via a float valve. When water is used in the building, the water level drops, triggering the valve to open and refill the tank. Gravity or pumps then distribute the stored water through the system. Most tanks also include an overflow pipe, vented lid and insulation to keep the water safe and at the correct temperature.
Key Components of a Cold Water Tank
Tank construction, lid and insulation
Tanks are typically made of GRP (Glass-Reinforced Plastic), plastic, concrete or galvanised steel.
A tight-fitting, vented lid with an insect screen prevents debris and contamination while allowing air movement.
Insulation is essential to keep water below 20 °C and reduce bacterial growth, especially legionella.
Overflow and warning pipes
The overflow pipe safely channels excess water outside the building when a failure occurs in the ball / float valve. A dripping overflow is a clear sign that the supply needs attention.
A warning pipe serves as an early indicator of potential overflow or malfunction. Positioned below the overflow level, it discharges water to a visible area when the ball valve or other control mechanisms fail, alerting maintenance staff to investigate and rectify the issue before flooding occurs.
Level Control and Water Regulation in Water Tanks
Float (ball) valves
A float or ball valve controls the inflow. When the water level drops because of usage within the building, the float falls and opens the valve; as the tank refills, it rises to shut the valve. A range of valves are available to suit all types of water tanks. Check out our dedicated blog on choosing the right ball valve for your tank: Choosing the Right Ball Valve for Your Cold Water Tank: A Comprehensive Guide
Pipework, backflow protection and pressure
Incoming water enters via the rising main. Check valves prevent contaminated backflow into the mains. The inlet of the tank should be on the opposing side to the outlet, to promote the flow of water within the tank. In buildings with low pressure, a booster set can be installed to increase pressure to the required levels.
Why Have Cold Water Storage Tanks in Commercial Buildings
Reliable supply
Provides a buffer during mains supply interruptions or pressure fluctuations.
Essential for high-demand times, particularly in offices, hotels and healthcare facilities.
Safety and compliance
Tanks supply water for fire sprinklers, plantroom operations and HVAC systems.
Correct design and maintenance, following Water Regulations and WRAS-approved standards, help keep water safe for drinking.
Cost-effective and proven
Unlike complex unvented systems, tanks fed systems are often easier to install, maintain and repair, making them a practical choice for many commercial sites.
Maintaining Compliance in Cold Water Tanks
Inspect annually, clean and flush as recommended by the HSE.
Keep water below 20 °C, and check regularly for stagnation, sediment or rust.
Regular inspection
Facilities managers should check:
Float valves – leaks or dripping overflow pipes often mean worn rubber diaphragms.
Insulation – damaged insulation can lead to temperature increases.
Lids and screens – must be secure and intact to prevent contamination.
Regulatory standards
Use tanks that comply with UK Water Fittings Regulations (look for the WRAS or Byelaw 30 approval).
Commercial sites should follow BS EN 806-5 and BS 8558 for inspection and sampling schedules.
Choosing and Sizing the Right Tank For Your Building
Factors to consider When Choosing a Water Tank
Usage vs capacity – ensure regular turnover to avoid stagnation. Your tank should hold no more than 24 hours of water.
Material – GRP is durable and hygienic; plastic is cost-effective; steel is robust but needs more maintenance.
Installation space – sectional tanks are ideal for plantrooms with restricted access.
Final thoughts for facilities managers
A well-maintained cold water storage tank is the backbone of a reliable water supply. For facilities managers, regular inspections, temperature control and compliance with UK water standards aren’t just box-ticking, they protect building users, prevent costly repairs and maintain trust in your water system.
Explore more on our site
For detailed advice on keeping your system compliant, read our guide on water tank maintenance. If you’re looking to renew an existing tank, then our dedicated relining page offers interesting options that prevent the need for full replacement. Conversely, if you are looking for a replacement, then check out our dedicated section - Water Tank Replacement Services | Titan Water
