Cold Water Storage Tanks — Header Tanks, Cisterns, and Indirect Cold Water Systems for UK Plumbers

Cold water storage tanks (CWST) supply indirect cold water to bathroom fittings in gravity-fed plumbing systems and feed the primary fill to older vented hot water cylinders. While combination boilers and unvented cylinders have largely replaced gravity-fed systems in new residential builds, millions of UK homes retain indirect cold water systems — and the cold water storage tank is a regular source of plumbing service work, replacement requests, and Water Regulations compliance issues.

This guide covers tank types and materials, Water Regulations requirements, sizing calculations, best-practice installation, insulation, common defects, and when a storage tank is no longer required.

Why Cold Water Storage Tanks Exist

In a traditional UK indirect plumbing system, the rising main supplies only the kitchen cold tap (and sometimes a garden tap or outside tap). All other cold taps, the WC cistern, and the cold feed to the hot water cylinder are supplied from the cold water storage tank in the loft. This arrangement:

• Provides a reserve of water in the event of supply interruption
• Reduces demand surges on the mains supply
• Allows gravity-fed pressure to bathroom fittings (though this is low pressure — typically 1 bar at 10 m head)

In contrast, a combi boiler or unvented system supplies all outlets directly from the mains, eliminating the need for a storage tank altogether.

Tank Types and Materials

Polyethylene (PE) Tanks

The most common type in modern UK installations. Black MDPE or food-grade polyethylene tanks are UV-resistant, lightweight, and manufactured to Water Regulations Advisory Scheme (WRAS) approval. Available in circular (pot-style) or rectangular sectional configurations. Standard domestic sizes: 114 litres (25 gal), 182 litres (40 gal), 227 litres (50 gal), 455 litres (100 gal).

Black polyethylene inhibits algae growth — always specify black tanks over clear or white for roof-space installations open to daylight.

Fibreglass (GRP) Tanks

Glass-reinforced polyester tanks are specified for large commercial or industrial installations. Available from 250 litres to several thousand litres. Sectional GRP tanks can be assembled through standard access hatches and loft apertures.

Galvanised Steel Tanks

Now obsolete for potable water storage. Galvanised tanks corrode internally, contaminating the water with zinc and heavy metals. If a galvanised tank is found in service, advise immediate replacement. Water from a galvanised tank may carry a risk of legionella growth in the corrosion products.

Sectional Tanks

For large-capacity installations or lofts with restricted access, sectional tanks (typically GRP or PE panels bolted together) can be assembled in situ. Used in commercial buildings, apartment blocks, and where a single-piece tank cannot be manoeuvred into position.

Water Regulations Requirements

Cold water storage tanks for potable water must comply with the Water Supply (Water Fittings) Regulations 1999 (WSR 1999). Key requirements:

WRAS Approval

The tank itself must be WRAS-approved or constructed from materials on the WRAS approved products list. All fittings installed on the tank — ball valve, overflow, outlet connections — must also be WRAS-approved.

Cover and Insulation

The tank must be fitted with a close-fitting lid that:

• Excludes light (prevents algae growth)
• Excludes insects, birds, and vermin
• Is not airtight (allows ventilation through insect-proof mesh)

Sides and top of the tank must be insulated to prevent freezing and to maintain water temperature below 20°C (above which legionella risk increases). Base insulation is not required as the floor of the loft provides some protection, but in very cold lofts a fitted base pad is good practice.

Float Valve and Overflow

The ball valve (float valve) must be type AA, AB, or AD air gap to prevent backflow of stored water into the mains supply — a critical Water Regulations requirement. The overflow pipe must be:

• Larger bore than the inlet supply (typically 22 mm minimum where the inlet is 15 mm)
• Directed to discharge in a visible location (to alert the occupant to overflow)
• Not connected to the foul drainage system
• Screened at its termination point against insects

Legionella Risk

Under the HSE's Approved Code of Practice (ACOP) L8 and HSG274 Part 2, domestic cold water storage tanks should maintain water temperature below 20°C. Tanks above 500 litres in commercial or residential buildings with multiple occupancies are subject to formal legionella risk assessment and may require thermostatic controls or dosing. For domestic tanks, the risk is lower but tank temperatures above 20°C or stagnant water in low-use pipework should be flagged to the owner.

Sizing Cold Water Storage Tanks

The traditional UK rule of thumb for domestic storage is 90–115 litres per person. For a 4-person household, a minimum 360-litre tank is appropriate if the system is fully indirect. In practice, most modern retrofits use a 200–230 litre tank as the bathroom-only demand is significantly lower in homes with a combi boiler (which supplies the kitchen directly from the mains).

For commercial and multi-occupancy buildings, CIBSE Guide G and the Water Industry Research guidance provide detailed demand calculations based on fixture units.

Installation Best Practice

Structural Support

A 200-litre tank full of water weighs approximately 200 kg. The loft joists may not be rated for this point load. Install a spreader board or platform across a minimum of 3 joists using 18 mm structural plywood. Check joist centres and span against BS 8103-3 or seek structural advice for older properties with undersized joists.

Access

The tank must be accessible for inspection, cleaning, and maintenance. Leave minimum 350 mm clearance above the lid for ball valve access. Do not enclose the tank in a void with no inspection access.

Connection Arrangement

Standard outlet configuration:

• Cold feed outlet: 25–50 mm above the base of the tank, fitted with an isolating valve (gate valve or ball valve)
• Vent pipe from hot water cylinder: Terminates above the tank lid (open-vented systems)
• Float valve inlet: BS 1212 Part 2 (Portsmouth pattern) or diaphragm type; 15 mm for domestic supply
• Overflow: 22 mm minimum, 50 mm above the ball valve shut-off level
• Warning pipe (optional): 15 mm at 25 mm below overflow level to give early warning before the overflow discharges

Pipework Insulation in Loft

All cold water pipework in the loft void must be insulated to prevent freezing. Use 25 mm foam lagging on 15 mm pipe as a minimum; in exposed or uninsulated lofts use 38 mm lagging or trace heating. Do not lay loft insulation beneath the tank — the small amount of heat rising from below helps prevent the tank base from freezing.

See our guide to frost protection and trace heating for loft pipe freeze prevention.

Backflow Prevention

The inlet ball valve must incorporate an air gap. Never use a simple float valve that allows the inlet to be submerged below the water surface — this constitutes a fluid category 5 risk (potential contamination of drinking water mains) under the WSR 1999. Use a BS 1212 Part 2 Portsmouth valve or a diaphragm-type float valve with integral air gap. All work on mains-connected equipment is notifiable to the local water undertaker.

See our full guide to backflow prevention and fluid categories.

Common Defects and Repair

Overflow Running Continuously

The ball valve is not shutting off correctly. Check:

• Float arm bent or submerged — adjust arm angle or replace float
• Diaphragm washer split or degraded — replace diaphragm assembly
• Portsmouth valve seat corroded or worn — replace valve
• Inlet water pressure too high for the valve rating — install a pressure reducing valve on the rising main

See our guide to water pressure problems and PRVs.

Tank Contaminated or Discoloured Water

Check for:

• Unsealed lid allowing insects or bird droppings to enter
• Algae growth (green water) — tank exposed to light, replace with opaque (black) tank
• Sediment from corroded galvanised tank — replace tank immediately
• Black particles — degraded butyl liner (some older tanks used lined steel) — replace

After any tank contamination, drain the system, disinfect the tank with sodium hypochlorite solution, flush thoroughly, and retest before returning to service. Record all disinfection work.

Tank Cracked or Leaking

Polyethylene tanks cannot be effectively repaired — replace. A leaking tank in a loft can cause catastrophic ceiling damage. Drain immediately via the outlet valve if possible; use a submersible pump if the outlet valve is inaccessible or blocked.

Airlocks in Cold Feed Pipework

Airlocks in the cold feed from the storage tank typically occur when the system has been drained and refilled without adequate venting. Symptoms: no flow from bathroom cold taps despite the tank being full. Cure: connect a hose between the mains kitchen cold tap and the affected fitting and allow mains pressure to push the airlock back through to the tank. Alternatively, blow through with a foot pump or use a hydraulic pump to clear the airlock.

When to Replace vs Remove the Storage Tank

Tank Replacement

Replace when: tank is galvanised steel, cracked, or >20 years old and showing corrosion in fittings. Sectional GRP commercial tanks should be inspected every 5 years and replaced if panel delamination or joint leaks are found.

System Conversion to Combi

When converting from a vented cylinder/gravity system to a combi boiler, the cold water storage tank is no longer required. The tank and all associated loft pipework should be drained and removed to eliminate legionella risk from stagnant water. Decommissioning includes:

• Isolating and capping the rising main branch to the tank
• Draining and removing the tank
• Capping or removing loft distribution pipework
• Re-running cold supplies to bathroom fittings direct from a new mains distribution point

System Conversion to Unvented Cylinder

Replacing a vented hot water cylinder with an unvented cylinder (G3 notifiable work) also allows removal of the cold water storage tank if all cold draws are rerouted to the mains. The G3 installation must be carried out by a competent person holding a current G3 qualification.

See our guide to unvented hot water cylinders and G3 regulations.

Related Topics

• Ballvalves and float valves — tank replacement and Water Regulations
• Stopcocks, gate valves, and isolation valves
• Pressurisation units and automatic system filling
• Pipe lagging and insulation — preventing frozen pipes
• Gravity-fed, combi, and unvented shower systems

Recommended Products from APM

While APM does not currently stock cold water storage tanks (polyethylene tanks are typically sourced directly from plumbing merchants), we stock the essential fittings for tank installation and maintenance: