Cold Water Storage Tanks: Sizing, Installation Rules & Legionella Prevention

Summary

Cold water storage tanks have been a standard feature of UK plumbing for over a century. In a traditional (indirect) system, a cold water storage cistern (CWST) in the loft feeds all cold taps except the kitchen (which takes direct mains water) and supplies the hot water cylinder. Modern direct systems (particularly with combi boilers) have no CWST — but millions of UK properties, particularly older housing stock, still have them.

The primary concern with CWSTs from a compliance standpoint is Legionella. Legionella pneumophila, the bacterium responsible for Legionnaire's disease, thrives in water between 20°C and 45°C. A cold water storage tank that warms up in a poorly ventilated loft, stagnates due to oversizing, or is contaminated through a missing cover is a potential Legionella risk. The Health and Safety Executive's L8 ACOP (Approved Code of Practice) and HSG274 Part 2 provide detailed guidance on managing this risk.

For most domestic properties with a single CWST supplying a normal family, the practical Legionella risk is low provided the tank is properly installed and maintained. For commercial properties, HMOs, care homes, and any building housing vulnerable people, a documented risk assessment and control programme is legally required under the Control of Substances Hazardous to Health Regulations (COSHH) and the Health and Safety at Work Act 1974.

Key Facts

• Standard domestic tank size — 225 litres (50 Imperial gallons); adequate for a 3-bedroom house
• Larger properties — May need two tanks (typically 2x225L plumbed in series or parallel)
• Material — Modern tanks are WRAS-approved polyethylene (olefin copolymer); old asbestos cement or galvanised steel tanks should be replaced
• Cover requirement — Must be covered (vermin-proof, UV-resistant if not in a dark loft); cover should allow access and vapour escape via screened vent
• Insulation — Tank sides and top must be insulated; NOT the base (allows heat from below to prevent freeze); minimum 25mm insulation
• Inlet — Float valve (ball valve) to mains; WRAS-approved
• Overflow — Overflow pipe (minimum 22mm) must discharge safely outside and be screened to prevent insect entry
• Service valve — Isolation valve on the inlet to allow maintenance without draining the system
• Warning pipe — BS 6700 requires a warning pipe (smaller than overflow) to alert when the float valve is failing before overflow is reached
• Access — Tank must be accessible for inspection and maintenance (not permanently enclosed in insulation)
• Temperature — Legionella risk — Water must remain below 20°C; water above 20°C increases Legionella risk significantly
• Stagnation prevention — Size correctly so water turns over in use; don't fit an oversized tank
• Cleaning frequency — Annual inspection and cleaning recommended; commercial premises require documented schedule
• BS EN 805 — Requirements for water supply systems outside buildings
• Lead pipe risk — Old lead supply pipes feeding the CWST can contaminate stored water; lead pipes should be replaced

Quick Reference Table

Detailed Guidance

Sizing the Tank

The traditional rule of thumb is 1 day's storage capacity. For a domestic property:

• Allow approximately 90-130 litres per person per day
• A family of 4 = 360-520 litres storage (suggesting two standard 225L tanks)
• In practice, most 3-4 person households function adequately on a single 225L tank

Oversizing is a Legionella risk. A tank that stores more water than the household uses means water sits in the tank for days or weeks, warming up and potentially stagnating. Don't fit the largest available tank "just in case" — right-size to actual demand.

Installation Location

Ideal position:

• Above all outlets it is to supply (minimum 1m head for reasonable pressure; 3m+ for better flow)
• Cool location — loft is typical but must be insulated properly to prevent freeze and temperature rise
• Accessible for inspection and maintenance
• On a suitable platform or joist arrangement capable of supporting the weight (225L water = 225kg + tank weight)

Structural support: A 225 litre tank full of water weighs approximately 230kg. This must be spread across joists, not resting on one or two. A boarding platform of at least 18mm thick exterior-grade plywood covering at least 3 joists is standard. Larger tanks or multiple tanks require structural assessment.

Winter freeze protection:

• Cold lofts can freeze the tank and feed pipe; insulate the tank (sides and top, not base)
• Lag all cold water pipes in the loft
• Pipe entering from outside should be adequately insulated
• A tank thermostat with a 240V frost heater is an option for severely cold loft spaces

Water Regulations Compliance

The Water Supply (Water Fittings) Regulations 1999 require that:

• Storage cisterns must be covered (Regulation 16)
• All inlets must be through WRAS-approved float valves
• Overflow pipes must discharge safely without risk of contaminating the mains
• The cistern must be accessible for inspection and maintenance

Non-compliant installations (uncovered tanks, missing warning pipes, incorrect overflow discharge) are a Water Regulations offence and a potential liability if a contamination event occurs.

Legionella Management — Domestic

For a standard domestic dwelling (owner-occupied or let), the landlord or homeowner has a duty to manage Legionella risk. The HSE guidance distinguishes between:

Low risk domestic: Owner-occupied or small private lets (up to 2 lettings). A simple risk assessment (not necessarily written) is adequate. Actions:

• Keep the CWST covered and insulated
• Flush the system if the property has been unoccupied for more than a week
• Replace the water in the tank annually or after any maintenance
• Check the cold water temperature periodically (should be below 20°C)

Higher risk or HMOs: Written risk assessment required. Annual inspection and temperature measurements. Document and keep records.

Legionella Management — Commercial and HMOs

Under the L8 ACOP and HSG274 (HSE guidance on Legionella), properties with multiple occupancies, communal water systems, or residents who are immunocompromised require:

1. Written risk assessment by a competent person
2. Written control scheme (describing what controls are in place and how they're managed)
3. Temperature monitoring — Regular checks at sentinel outlets (first and last outlets on each circuit) and the CWST; record temperatures
4. Annual clean and disinfection of the CWST
5. Records — All testing, maintenance, and corrective actions documented

Competent persons for Legionella risk assessments can be accredited through the Legionella Control Association (LCA).

Replacing an Old Tank

Old galvanised steel, asbestos cement, or polypropylene tanks that have degraded should be replaced. Check:

• Material condition (galvanised tanks corrode internally; old plastic tanks can become brittle)
• Cover condition (missing, damaged, or non-screened covers)
• Existing pipework connections
• Structural support adequacy

When replacing, consider whether the property's plumbing would be better served by converting to a direct (mains-fed) system — particularly if a new boiler is being fitted at the same time.

Frequently Asked Questions

Can I insulate the base of the cold water storage tank?

No. BS 6700 guidance (and good practice) says NOT to insulate the base of the tank. The heat rising from the heated space below helps prevent the tank from freezing. Insulating the base would remove this small but useful heat source and increase freeze risk. Instead, insulate the sides and top thoroughly.

How often should a cold water storage tank be cleaned?

For a domestic property: at least every 5 years as a minimum; annually is better practice, particularly in older properties or HMOs. For commercial properties, care homes, and HMOs, annual cleaning is typically required under the water safety management plan. Cleaning involves draining the tank, scrubbing the interior to remove scale and biofilm, flushing with clean water, and disinfecting with a dilute chlorine solution (sodium hypochlorite at 50ppm free chlorine) before refilling.

My customer has an old lead pipe feeding the CWST — do I need to replace it?

The Water Supply (Water Fittings) Regulations and the Water Framework Directive have led water companies and local authorities to prioritise replacing lead pipes on public highways. However, lead pipes from the boundary of the property to the house (the private supply pipe) remain the owner's responsibility. Lead pipes should be replaced — lead contamination in drinking water is a serious health risk, particularly for young children. Contact the local water company; some operate grants or subsidised replacement schemes for private lead pipes.

What's the difference between a CWST and a break tank for mains boosting?

A cold water storage cistern (CWST) is a gravity-fed storage tank that supplies hot and cold water by gravity throughout the property. A break tank for mains boosting is specifically sized and configured to store mains water that is then boosted by a pump. The principle is the same (cistern with float valve) but a booster break tank is typically smaller, positioned near the pump, and doesn't supply gravity outlets. Both have Legionella considerations.

Regulations & Standards

• BS 6700:2006+A1:2009 — Design, installation, testing, and maintenance of services supplying water for domestic use; comprehensive coverage of cold water storage

• Water Supply (Water Fittings) Regulations 1999 — All fittings including cisterns must comply; installation must prevent contamination

• HSE L8 ACOP — Legionella: The Control of Legionella Bacteria in Water Systems; the primary UK guidance on Legionella management

• HSG274 Part 2 — Legionella control in hot and cold water systems

• BS 8558 — Guide to the design, installation, testing, and maintenance of services supplying water for domestic use

• HSE Legionella L8 ACOP — The Approved Code of Practice for Legionella management

• WRAS Water Regulations Guide — Guidance on compliant cistern installation

• Legionella Control Association — Professional body for Legionella risk assessment

• mains water boosting — Break tank and pump systems

• water regulations — Water Fittings Regulations 1999

• hot water systems — Cylinder and boiler systems supplied from CWST

• thermostatic mixing valves — TMVs to protect outlets supplied from CWST