This guide explains the sizing methodology for cold water storage tanks in multi-storey buildings, the four competing requirements that must be balanced, and the relationship between storage volume and Legionella risk. The methodology is drawn from CIBSE Guide G and BS 8558. All sizing calculations for real projects must be performed by a suitably qualified engineer using building-specific demand data.
For product specification context — panel construction, capacity ranges, and BS EN 13280 compliance — see the Complete Technical Guide to Sectional GRP Cold Water Tanks.
Indicative daily cold water demand per person in residential buildings (BS 8558 / CIBSE Guide G)
Water density for structural loading: a 10,000 L tank exerts 10 tonnes on the plant room floor
Maximum cold water storage temperature (ACoP L8 / HSG274 Pt 2) — the Legionella constraint on water age
Why Proper Sizing Matters — In Both Directions
The instinct of many designers is to size cold water storage conservatively — to specify more volume than the calculation strictly requires, on the basis that more storage is safer. This is correct for resilience. It is incorrect for water hygiene.
Undersizing produces
Supply and operational risk
Booster set starvation during peak demand; supply failure during mains interruptions shorter than the design resilience period; risk of the booster low-water cut-out activating under normal operating conditions.
Oversizing produces
Water hygiene risk
Excessive water age — the average time water spends in storage before use; progressive temperature rise in warm plant rooms; Legionella risk that may be unmanageable by physical controls alone; in some cases, the need for supplementary chemical treatment.
Key Principle
The correct volume is the minimum that satisfies all four requirements
The correct storage volume is the smallest volume that satisfies all four sizing requirements — not the largest the plant room can accommodate, and not the volume that maximises the resilience period beyond what the brief requires.
The Four Sizing Requirements
Every cold water storage tank sizing exercise must balance four competing requirements, which most commonly conflict in the order listed.
|
Requirement
|
What It Drives
|
Conflicts With
|
|---|---|---|
|
Peak demand buffering
|
Tank must absorb instantaneous demand that exceeds the mains fill rate
|
Water hygiene — a larger buffer means longer water age
|
|
Resilience
|
Stored volume maintains supply under mains interruption for a specified duration
|
Water hygiene — a longer resilience period means more storage and longer water ageins supply under mains interruption for a specified duration
|
|
Water hygiene
|
Maximum acceptable residence time given the thermal environment and Legionella risk
|
Resilience and buffering — both push toward more storage
|
|
Physical constraints
|
Available footprint, ceiling height, structural floor loading, and access
|
All other requirements — physical limits cap achievable volume
|
Where the resilience requirement and the water hygiene requirement pull in opposite directions — as they frequently do in multi-storey buildings with warm basement plant rooms — the designer must either accept supplementary treatment as a design element, reduce the resilience target in the brief, or adopt a two-compartment arrangement that reduces effective residence time.
Daily Demand by Building Type
The starting point for sizing is an estimate of average daily demand. The appropriate figure depends on building occupancy type, number of occupants, and pattern of use. The two primary UK sizing tools are:
Primary reference — special legal status
CIBSE Guide G — Public Health and Plumbing Engineering (2nd ed.)
Provides demand-based sizing methodologies for cold water storage by occupancy type, including loading unit methods and demand unit approaches for a range of building types. The appropriate starting point for any UK multi-storey project.
Complementary guidance
BS 8558:2015 — Services Supplying Water for Domestic Use
Provides reference consumption rates by occupancy type and guidance on storage volume for various resilience periods. Used alongside CIBSE Guide G to confirm demand assumptions and storage targets.
|
Building Type
|
Indicative Demand
|
Source Basis
|
|---|---|---|
|
Residential (per person per day)
|
130–150 litres
|
BS 8558 / CIBSE Guide G
|
|
Hotel (per bed per day, full service)
|
200–350 litres
|
CIBSE Guide G
|
|
Office (per person per day)
|
30–50 litres
|
CIBSE Guide G
|
|
Hospital (per bed per day)
|
350–500 litres
|
CIBSE Guide G
|
|
School (per pupil per day)
|
20–30 litres
|
CIBSE Guide G
|
These are indicative ranges for preliminary sizing. Actual demand depends on occupancy rates, fixture types, hours of operation, and seasonal variation. For new-build projects, CIBSE Guide G loading unit calculations using the planned fixture schedule provide a more accurate estimate than per-person rules of thumb. However, Cold water storage tank sizing must account for both operational resilience and water hygiene performance.
Resilience Period — 24-Hour vs 48–72-Hour Storage
The resilience requirement is defined in the project brief. Common targets are:
|
Building Type
|
Typical Resilience Target
|
|---|---|
|
Standard multi-storey residential
|
30 minutes to 1 hour
|
|
Hospitals, hotels, single incoming main
|
2 to 4 hours
|
|
Critical applications, poor supply reliability
|
Up to 24 hours
|
|
Remote sites, critical national infrastructure
|
48–72 hours (use with caution — significantly increases water age and Legionella risk)
|
Resilience calculation
Assumes no mains refill during the resilience period — the correct assumption for a supply interruption event. Apply a 1.5× peak demand buffering factor to the result to account for demand rates above the daily average.
Firefighting Reserve and Combined Tank Considerations
Where a tank must also serve a sprinkler or hydrant system, BS EN 12845 defines the required dedicated volume by hazard classification. This reserve is an additional fixed volume that cannot serve domestic consumption under normal conditions.
|
Hazard Classification
|
Example Buildings
|
Required Tank Volume
|
|---|---|---|
|
Ordinary Hazard Group 1
|
Offices, schools
|
27,500 – 40,000 L
|
|
Ordinary Hazard Group 2
|
Car parks, museums, libraries, public buildings
|
105,000 – 140,000 L
|
|
Ordinary Hazard Group 3
|
Shopping centres, supermarkets, plant rooms, hospitals
|
135,000 – 185,000 L
|
Common separation strategies include installing a physical tank partition or setting the domestic outlet at a higher level so that the bottom portion of the tank (the fire reserve) remains untouched during normal operation. Either method must prevent daily domestic draw-down from depleting the fire reserve.
Resilience calculation
Assumes no mains refill during the resilience period — the correct assumption for a supply interruption event. Apply a 1.5× peak demand buffering factor to the result to account for demand rates above the daily average.
Turnover, Water Age, and Legionella Risk
In addition, effective cold water storage tank sizing also requires consideration of water turnover rates and thermal conditions within the plant room. Where a tank serves a sprinkler system, BS EN 12845 specifies minimum storage volumes by hazard classification:
Water age calculation
This is a theoretical minimum assuming perfect mixing. Without internal baffles, actual water age for some portions of stored volume will be significantly longer due to stratification and short-circuit flow patterns.
Practical Water Hygiene Design Guidance: a tank with water age under approximately 8 to 12 hours in a well-insulated, thermally controlled plant room will generally maintain temperature below 20°C without difficulty. A tank approaching 24 hours water age in a warm plant room will typically require additional insulation, internal baffles, or supplementary water treatment.
Stratification risk in large-volume tanks
In tanks exceeding approximately 10,000 L, warmer, less dense water rises to the upper layers while cooler water settles lower. A single temperature sensor at low level may show an acceptable reading while the upper layers exceed 20°C. For tanks above 10,000 L, the specification must include multiple temperature sensor pockets at low, mid, and high levels; internal baffles; and inlet and outlet positioned on opposite sides to promote end-to-end diagonal flow.
COLD WATER STORAGE TANK SIZING METHODOLOGY
The following roadmap integrates the regulatory, standards, and health obligations set out above into a sequential process for specifying, installing, and maintaining a GRP cold water storage tank in a UK building.
Seven-step process
Cold Water Storage Tank Sizing — UK Method
1
Estimate daily demand
Use occupancy data and the CIBSE Guide G / BS 8558 reference values to calculate peak daily water consumption (L/day).
2
Set the resilience period
Determine the required backup duration (hours) from the project brief. Common benchmarks: 30 min–1 hr residential; 2–4 hr hospitals and hotels; up to 24 hr critical.
3
Calculate resilience volume
Apply the formula: (Daily demand ÷ 24) × Resilience hours. Multiply by 1.5× for peak demand buffering.
4
Add firefighting reserve (if applicable)
Determine the fire reserve per BS EN 12845 hazard classification and add this as a fixed, non-domestic volume. Confirm the separation strategy — partition or elevated outlet — at design stage.
5
Verify physical constraints
Check plant room footprint, ceiling height, structural floor loading (water weighs 1 kg per litre — a 10,000 L tank exerts 10 tonnes on the floor), and access routes for panel delivery.
6
Check water age
Calculate theoretical turnover (volume ÷ daily demand). If it significantly exceeds 12–24 hours in a warm environment, consider reducing volume, introducing internal baffles, or specifying a two-compartment arrangement.
7
Finalise and document
Select a standard tank size or configure a sectional tank to the required volume. Sectional GRP tanks are assembled from modular panels in 500 mm and 1,000 mm increments. Document all assumptions for facilities management reference.
Worked example
120-apartment residential tower
APARTMENTS
120, average 2 occupants
Sizing steps:
Sizing steps:
1. Resilience storage: 36,000 ÷ 24 = 1,500 L/hr × 1 hr = 1,500 L
2. Peak buffering factor 1.5×: 1,500 × 1.5 = 2,250 L minimum working storage
3. Water age check at candidate volumes — see table below
4. Selected volume: 5,000 L (2 × 2,500 L, two-compartment arrangement) — satisfies resilience, buffers peak demand, and produces a water age of approximately 3.3 hours
|
Candidate Volume
|
Theoretical Water Age
|
Assessment — Warm Basement Plant Room
|
|---|---|---|
|
2,500 L
|
2,500 ÷ 36,000 = 1.7 hours
|
Acceptable: low stagnation risk; minimum viable volume
|
|
5,000 L ✓ Selected
|
5,000 ÷ 36,000 = 3.3 hours
|
Acceptable: good turnover; appropriate for this thermal environment
|
|
10,000 L
|
10,000 ÷ 36,000 = 6.7 hours
|
Monitor: acceptable with good insulation; baffles advisable
|
|
20,000 L
|
20,000 ÷ 36,000 = 13.3 hours
|
Elevated risk: warm plant room requires baffles or supplementary treatment
|
|
50,000 L
|
50,000 ÷ 36,000 = 33 hours
|
Unacceptable: significant Legionella risk without supplementary treatment; far exceeds demand
|
This example illustrates the core principle: size to the smallest volume that satisfies resilience and buffering, while actively checking against water hygiene constraints.
Frequently asked questions
What is the recommended storage volume for a cold water tank in the UK?
There is no single recommended volume — it depends on building occupancy, demand profile, resilience target, and the thermal environment of the plant room. CIBSE Guide G and BS 8558 provide the appropriate sizing methodologies. Oversizing is as problematic as undersizing: an oversized tank produces excessive water age, increasing the risk of stored water temperature rising above the 20°C Legionella control threshold.
What is the maximum acceptable water age in a cold water storage tank?
Theoretical water age is calculated by dividing storage volume in litres by average daily consumption in litres per day. A 10,000 L tank serving a building with 36,000 L/day average demand has a theoretical turnover time of 10,000 ÷ 36,000 = 0.28 days (approximately 6.7 hours) — an acceptable rate. A 50,000 L tank on the same building produces a turnover time of approximately 33 hours, which in a warm plant room creates an unacceptable Legionella risk.
Does a two-compartment arrangement double the storage volume?
Only if both compartments are fully charged and the interconnecting valve is open. In normal operation, both compartments contribute to total storage. When one is isolated for maintenance, available storage reduces to the volume of the remaining compartment — typically 50 to 75 per cent of the design total. The sizing exercise must confirm that single-compartment storage is sufficient to supply the building for the expected maintenance duration.
What happens if the tank also needs to serve a fire sprinkler system?
The fire reserve is an additional fixed volume defined by BS EN 12845 according to the building’s hazard classification — it cannot serve domestic consumption under normal conditions. The domestic and fire volumes must be separated either by a physical partition or by positioning the domestic outlet above the fire reserve level. The total tank volume is the sum of the domestic storage volume plus the fire reserve, and both must be designed and documented independently.
How often must cold water storage tanks be inspected?
Cold water storage tanks should be inspected at least annually and cleaned as required. Any evidence of sediment, stagnation, or microbial contamination warrants immediate inspection and cleaning, regardless of the scheduled interval. All inspection activity, temperature readings, and remedial actions must be recorded and retained for a minimum of five years.
Contents
CONTENTS
Free Download
Complete GRP Cold Water Tank Guide
Full lifecycle coverage — sizing, compliance, installation, Legionella control, and O&M schedules.
