Cold Water Storage Tanks: Sizing, Installation, and Water Regulations Compliance

Cold water storage tanks (CWSTs) are a fixture of older UK housing stock and remain standard in many commercial, healthcare, and multi-occupancy buildings. While modern new builds predominantly use mains pressure systems, the installed base of storage tank systems is enormous — and plumbers working on extensions, conversions, and commercial premises need to understand them thoroughly.

This guide covers tank sizing, materials, installation requirements under the Water Supply (Water Fittings) Regulations 1999, Legionella risk management, overflow and warning pipe sizing, and the key differences between domestic and commercial installations.

Why Cold Water Storage Tanks Are Still Used

In a mains pressure system, all cold water outlets draw directly from the rising main — high pressure, simple, and increasingly standard in new build. So why do storage tanks persist?

• Mains pressure inadequacy — in properties where mains pressure is insufficient (typically below 1 bar), a storage tank provides a local reserve and gravity-fed pressure to the system
• Supply continuity — in remote locations, agricultural properties, and some commercial settings, a storage tank provides a buffer against temporary supply interruptions
• Backflow protection — a storage tank provides Type AA air gap protection, the highest protection category under the Water Regulations, breaking any potential backflow path between the mains and the building's hot water system
• Existing systems — the majority of UK housing built before the 1990s used indirect cold water systems with a loft tank. These remain in service and require ongoing maintenance
• Commercial and healthcare — large water storage is required for fire suppression systems, process water, and to manage demand in buildings where peak flows would exceed mains supply capacity

Types of Cold Water Storage Tank

Polyethylene (PE) Tanks

Single-piece moulded polyethylene tanks are the standard choice for domestic applications up to around 450 litres. They are lightweight, corrosion-resistant, food-safe grade (WRAS approved), and available in standard sizes from around 50 litres up to 1,000+ litres.

Older black polythene tanks (pre-1990s) may not meet current WRAS requirements — check whether replacement is advisable on older installations.

GRP (Glass Reinforced Plastic / Fibreglass) Tanks

GRP sectional tanks are the standard for commercial and industrial cold water storage above approximately 1,000 litres. They are assembled from flat panels on site, allowing very large capacities to be installed through standard doorways and access hatches. GRP is strong, durable, WRAS-approved, and resistant to algae growth when correctly insulated.

Sectional GRP tanks must be assembled on a flat, level base — typically a concrete plinth or heavy-duty GRP support stand — and the joints between panels sealed with approved sealant.

Galvanised Steel (Legacy)

Pre-1970s tanks were often galvanised steel. These are now generally considered life-expired and should be replaced — they corrode internally, introducing iron contamination, and the zinc lining can dissolve at low pH. If you encounter a galvanised steel tank on a service visit, advise replacement.

Sizing Cold Water Storage Tanks

Domestic Sizing

For a standard UK domestic property, the traditional rule of thumb was 200 litres for a 2-bedroom property, 230 litres for 3 bedrooms, and 270 litres for 4 bedrooms. In practice, modern water usage patterns and the prevalence of power showers have increased demand — 230 litres is a reasonable minimum for a family home.

If the cold water storage tank also serves a vented hot water cylinder (indirect system), the cylinder cold feed volume must be accounted for in addition to the cold distribution demand. A 120-litre hot water cylinder will draw 120 litres of cold water when fully recovered, plus any simultaneous cold draw-off. Size generously.

Commercial Sizing

Commercial cold water storage sizing is based on BS EN 806 Part 2 and the guidance in the CIBSE Guide G (Public Health Engineering). The key parameters are:

• Demand unit method — each outlet (tap, shower, WC cistern, etc.) is assigned a demand unit value based on its flow rate and frequency of use. The total demand units determine the required flow rate and storage volume.
• Storage duration — BS EN 806 Part 2 recommends storage equivalent to one day's usage for most commercial buildings, or up to 48 hours for healthcare and critical applications where mains supply interruption would be serious.
• Refill rate — the ball valve and supply pipe into the tank must be sized to refill the tank within a specified time (typically 4–8 hours) without creating supply problems elsewhere in the building.

Ball Valve Selection

The ball valve (also called float valve or ballvalve) controls the fill of the storage tank. It must:

• Be WRAS approved and rated for mains pressure on the inlet side
• Provide a Type AA air gap — the discharge from the valve must discharge at least 25mm above the overflow pipe invert (or as specified by the valve manufacturer)
• Be sized to refill the tank within the design time without restricting mains supply to other outlets

Standard domestic ball valves are available in 1/2" and 3/4" BSP, designed for mains pressure (up to 16 bar rated, operating typically at 1–5 bar). For low pressure or high flow applications, equilibrium ball valves (which are pressure-balanced and fully open at low pressure differentials) are preferred.

Plastic side-entry ball valves (Fluidmaster, Dudley, Torbeck) have largely replaced traditional brass Portsmouth and Croydon valves on domestic installations — they are quieter, more reliable, and easier to adjust. Brass ball valves remain standard in commercial applications.

Overflow and Warning Pipes

Every cold water storage tank must have an overflow pipe. The Water Regulations require the overflow to:

• Be sized to discharge at least as much water as the maximum inflow rate from the ball valve — typically at least one pipe size larger than the inlet
• Discharge in a visible location where overflow would be immediately noticed (e.g., externally, above the eaves) — this is why it's sometimes called a "warning pipe" rather than a true overflow
• Be fitted with a mosquito screen or fly mesh at the external end
• Not be connected to a drain or sewer — the overflow must discharge in a way that alerts the building occupant to a failed ball valve

Standard domestic overflow pipe is 22mm OD copper or 21.5mm solvent weld plastic. For a ball valve with a 15mm inlet, a 22mm overflow is typically adequate.

Water Regulations Requirements

The Water Supply (Water Fittings) Regulations 1999 (and the equivalent Scottish Byelaws) govern all water fitting installations in the UK. Key requirements for cold water storage tanks:

• WRAS approval — the tank, ball valve, and any other fittings in contact with water must be WRAS (Water Regulations Advisory Scheme) approved or comply with the Regulations
• Insulation — tanks in loft spaces must be insulated to prevent freezing in winter and overheating in summer. Minimum 25mm insulation all round, including the lid. Do not insulate the tank base — heat rising from the dwelling below helps prevent freezing from underneath.
• Lid — every tank must have a close-fitting lid to prevent contamination from debris, insects, birds, and vermin. Mesh screens on overflows and vents contribute to this.
• No direct connection to mains and system — the tank provides the air gap between mains and the building distribution system. Never connect the distribution pipework directly to the mains without appropriate backflow protection.
• Notification — certain plumbing work on water fittings requires notification to the water undertaker (water company). Tank replacement generally does; check the Regulations Schedule 2 for the specific requirement.

Legionella Risk Management

Cold water storage tanks are a known Legionella risk — they provide a large volume of water that can stagnate and reach temperatures in the Legionella growth range (20–45°C) in warm loft spaces during summer.

The HSE's Approved Code of Practice L8 and Technical Guidance HSG274 Part 2 set out the control measures required:

• Keep cold water cold — insulate the tank to keep water below 20°C. In warm loft spaces, additional precautions may be needed: insulating the loft space above the tank, or using a cold water booster set to eliminate the need for a tank altogether in refurbishment projects.
• Turnover — tanks should be sized to ensure regular water turnover. An oversized tank in a low-demand property creates stagnant water — a significant Legionella risk. Right-size the tank for actual demand.
• Inspect and clean — domestic tanks should be inspected annually and cleaned every 5 years (more frequently if contamination is found). Commercial tanks require risk assessment-driven inspection, typically annually or more frequently.
• Cleaning procedure — drain the tank, clean the interior with a mild detergent solution, rinse, disinfect with chlorine solution (50ppm for 1 hour, then flush through), and refill. Document the cleaning with before and after water samples where required for compliance.
• Landlords — landlords with cold water storage tanks in residential rental properties have a duty under the Health and Safety at Work Act and HSE guidance to manage Legionella risk. For most domestic systems this means an annual check and periodic cleaning.

Installation Checklist

When installing or replacing a cold water storage tank:

1. Confirm the existing floor structure can support the full tank weight (water weighs 1 kg/litre — a 230L tank fully loaded weighs 230kg plus tank weight). Distribute the load on tank bearers spanning multiple joists.
2. Confirm access — a 230L polyethylene tank will fit through a standard loft hatch; GRP sectional tanks can be assembled in position.
3. Fit tank on bearers level and stable.
4. Install ball valve — WRAS approved, correct pressure rating, with union connection for ease of future removal.
5. Install outlet connection with a servicing valve and union for isolation.
6. Fit overflow pipe — correct sizing, terminating visibly and with fly screen.
7. Fit lid — close-fitting, ventilated (small mesh-screened vent to allow pressure equalisation without allowing ingress).
8. Insulate tank on all sides except base — joints taped.
9. Insulate all pipework in the loft space — cold feed, distribution, and overflow.
10. Test — open ball valve, verify tank fills correctly, overflow is at the correct level, and ball valve shuts off cleanly.
11. Notify water undertaker if required.
12. Document installation and provide information to the property owner regarding Legionella risk management requirements.

Common Problems and Remedies

Noisy ball valve (water hammer)

Caused by high mains pressure creating vibration as the valve closes. Solutions: fit a pressure reducing valve (PRV) on the mains inlet to the property, replace with a quieter equilibrium valve (Torbeck, Fluidmaster), or fit a mini pressure reducing valve on the ball valve inlet.

Slow refill

Ball valve orifice scaled up, or inlet restricted. Remove and clean/replace the valve diaphragm. Also check the isolating valve on the mains inlet is fully open.

Contaminated water (odour, colour, taste)

Inspect the tank interior for debris, biofilm, or vermin activity. Clean and disinfect per the procedure above. Check the lid is secure and overflow/vent screens are intact. If contamination is persistent, consider whether the tank is correctly sized for turnover.

Frozen tank or pipes

Inadequate insulation. Add insulation to the tank and all pipework in the loft. Consider trace heating for extreme exposures.

Products from APM

APM Electrics Plumbing stocks plumbing components for cold water storage installations including isolation valves, pipe fittings, and pipe insulation. Browse our plumbing collection for valves, fittings, and associated products, or contact our trade counter for availability on tank accessories.

Summary

• Cold water storage tanks remain widespread in pre-1990s housing and commercial buildings — essential knowledge for any plumber
• Polyethylene for domestic; GRP sectional for large commercial — both must be WRAS approved
• Minimum domestic tank size: 230 litres for a family home; commercial sizing uses BS EN 806 demand unit method
• Ball valve must provide Type AA air gap; overflow must be visible and sized larger than the inlet
• Tanks must be insulated on all sides (not the base), have a close-fitting lid, and overflow screens
• Legionella risk requires: cold water below 20°C, adequate turnover, annual inspection, 5-yearly cleaning (domestic), and documented risk management for commercial/rental
• Check if water undertaker notification is required before installation
• Verify floor loading — a full 230L tank weighs over 230kg

Shop Plumbing Fittings at APM Electricals — Trade Counter, Acton W3

APM Electricals stocks a full range of cold water storage tanks and plumbing fittings for trade professionals. Same-day collection from our Acton trade counter.

• Fernox Central Heating Protector 500ml
• Taurus 15mm Corner TRV with Lockshield

Browse our full Plumbing Fittings range at apmi.uk. Visit us at 24 Western Avenue, Acton, London W3 7TZ or call 020 8702 8080.