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 that most sizing guides do not address directly. 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 the product specification context — panel construction, capacity ranges, and BS EN 13280 compliance — see the Complete Technical Guide to Sectional GRP Cold Water Tanks (Sections 5.1–5.2).
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 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 that would not have been necessary with a correctly sized tank.
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.
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Requirement
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What it drives
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Conflicts With
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|---|---|---|
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Peak demand buffering
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Tank must absorb instantaneous demand that exceeds the mains fill rate
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Water hygiene — a larger buffer means longer water age
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Resilience
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Stored volume maintains supply under mains interruption for a specified duration
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Water hygiene — a longer resilience period means more storage and longer water age
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Water hygiene
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Maximum acceptable residence time given the thermal environment and Legionella risk
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Resilience and buffering — both push toward more storage
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Physical constraints
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Available footprint, ceiling height, structural floor loading, and access
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All other requirements — physical limits cap achievable volume
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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.
The primary sizing tools
Primary reference
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.
Neither document provides a single prescriptive formula. Both provide frameworks within which the designer applies building-specific demand data. The worked example below illustrates the logic; it is not a substitute for a project-specific calculation.
Demand estimation
The starting point for sizing is an estimate of average daily demand. The appropriate figure depends on the building occupancy type, the number of occupants, and the pattern of use.
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Building Type
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Indicative Demand
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Source Basis
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|---|---|---|
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Residential (per person per day)
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130–150 litres
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BS 8558 / CIBSE Guide G
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Hotel (per bed per day, full service)
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200–350 litres
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CIBSE Guide G
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Office (per person per day)
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30–50 litres
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CIBSE Guide G
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Hospital (per bed per day)
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350–500 litres
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CIBSE Guide G
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School (per pupil per day)
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20–30 litres
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CIBSE Guide G
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These are indicative ranges for preliminary sizing. Actual demand for a specific building depends on occupancy rates, fixture types, hours of operation, and seasonal variation. For a new-build project, CIBSE Guide G loading unit calculations using the planned fixture schedule provide a more accurate estimate than per-person rules of thumb.
Account for peak demand
In residential buildings, peak morning and evening demand can reach two to three times the average hourly rate. The tank must be sized to absorb this peak without the booster set running dry. The mains fill rate is also relevant — a tank that refills quickly during the peak period needs less storage than one that refills slowly. For the booster set interface requirements that govern this, see the Break Tanks and Booster Sets Design Guide.
This guide is provided for general guidance and information purposes only. It does not constitute engineering advice and should not be relied upon as the sole basis for design decisions. © 2026 Tricel Water. All rights reserved.
Resilience calculation
The resilience requirement is defined in the project brief — typically by the developer, building owner, or lead engineer. Common targets are 30 minutes to 1 hour for standard multi-storey residential buildings; 2 to 4 hours for hospitals, hotels, and buildings with a single incoming main; and up to 24 hours for highly critical applications or where supply reliability is known to be poor.
Resilience storage
Resilience storage (L) = (Daily demand ÷ 24) × Resilience hours
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.
Where the mains fill rate is known and is likely to continue during the demand period — for peak buffering rather than full supply interruption — the incoming flow rate can partially offset the required storage volume. The project engineer should confirm which scenario governs the resilience target.
Water age and the Legionella constraint
Once a candidate storage volume is established, the designer must calculate the theoretical water age — the average time stored water spends in the tank before use. This check must be performed for every candidate volume, not only the final selected volume.
Theoretical water age
Water age (days) = Storage volume (L) ÷ Average daily demand (L/day)
This is a theoretical minimum assuming perfect mixing. In practice, without internal baffles, actual water age for some portions of stored volume will be significantly longer due to stratification and short-circuit flow patterns.
HSE guidance (HSG274 Part 2 and ACoP L8) requires cold water to be stored at or below 20°C. In a plant room with heat gain from adjacent boiler plant or electrical equipment, stored water in a large, quiescent tank will progressively absorb heat. The rate of temperature rise depends on the thermal mass, the insulation specification, and the site-specific heat load.
As a practical design guide: a tank with high turnover — water age under approximately 8 to 12 hours in a well-insulated tank in a thermally controlled plant room — generally maintains 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. The risk assessment for the specific plant room conditions must be the final arbiter, not a rule of thumb.
Worked sizing example
The following illustrates the methodology for a 120-apartment residential tower. It is simplified for clarity. All sizing calculations for real projects must use building-specific data and must be performed by a suitably qualified engineer.
Worked example
120-apartment residential tower
Apartments: 120, average 2 occupants
Daily demand: 240 × 150 L = 36,000 L/day
Resilience target: 1 hour of average demand
Total occupants: 240
Hourly demand (avg.): 36,000 ÷ 24 = 1,500 L/hr
Plant room: Basement; summer ambient up to 22°C
- Resilience storage: 1,500 L/hr × 1 hr = 1,500 L
- Peak buffering factor 1.5×: 1,500 × 1.5 = 2,250 L minimum working storage
- Water age check at candidate volumes — see table below
- 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
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Candidate volume
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Theoretical water age
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Assessment — warm basement plant room
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|---|---|---|
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2,500 L
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2,500 ÷ 36,000 = 1.7 hours
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Acceptable: Low stagnation risk; minimum viable volume
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5,000 L ✓ Selected
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5,000 ÷ 36,000 = 3.3 hours
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Acceptable: Good turnover; appropriate for this thermal environment
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10,000 L
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10,000 ÷ 36,000 = 6.7 hours
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Monitor: Acceptable with good insulation; baffles advisable
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20,000 L
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20,000 ÷ 36,000 = 13.3 hours
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Elevated risk: Warm plant room requires baffles or supplementary treatment
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50,000 L
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50,000 ÷ 36,000 = 33 hours
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Unacceptable: Significant Legionella risk without supplementary treatment; far exceeds demand
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Thermal stratification in large-volume tanks
In tanks exceeding approximately 10,000 L, thermal stratification is a risk the sizing exercise must address. Warmer, less dense water rises to the upper layers while cooler water settles lower, producing a temperature gradient from base to top. A single temperature sensor at low level may show an acceptable reading while the upper layers exceed 20°C and enter the Legionella risk range.
For large-volume tanks, the specification should include multiple temperature sensor pockets at low, mid, and high levels; internal baffles to improve circulation and reduce stratification; and an inlet and outlet positioned on opposite sides of the tank to promote an end-to-end diagonal flow path. These should appear in the tank specification at design stage — not added as a remedial measure after the Legionella risk assessment identifies stratification as a problem.
Stratification invalidates single-sensor monitoring
Where a written control scheme specifies temperature monitoring at a single low-level sensor only, it may be recording the coolest water in the tank — not the warmest. For tanks above approximately 10,000 L, the risk assessment must specify multi-level monitoring, and the sensor pocket locations must be agreed at design stage so that they are built into the tank at manufacture.
Physical constraints and structural loading
Footprint and ceiling height
Sectional GRP tanks are assembled from modular panels in 500 mm and 1,000 mm increments, giving considerable dimensional flexibility. A 5,000 L tank can be configured across multiple plan footprints and heights to suit the available space. Confirm dimensional options with the manufacturer at an early design stage — before the plant room layout is fixed.
Structural floor loading
Water weighs 1 kg per litre. A fully charged 10,000 L tank exerts 10 tonnes of load on the plant room floor, distributed over the tank base footprint. Structural assessment of the floor is required for any tank above approximately 5,000 L in an existing building, and should be confirmed at design stage for new builds. Distributed loading across the tank base does not eliminate the need for a structural check — slab capacity and point load at supports must both be assessed.
Access route for panel delivery
Sectional GRP panels can pass through doorways of 750 mm minimum clear width. All doorways, stairwells, corridors, and lifts on the delivery route must be surveyed and confirmed before specifying panel dimensions. A tank that cannot be delivered to the plant room in its required configuration cannot be installed. For access route requirements in the context of break tank installations, see the Break Tanks and Booster Sets Design Guide.
The tank top must also accommodate the access provisions required for confined space entry — manway clearance, tripod rescue system height, and platform space. For the access design requirements that feed back into tank and plant room specification, see the Confined Space Entry Guide.
Two-compartment sizing
Where a two-compartment arrangement is adopted, the total storage volume is divided between two cisterns, each independently valved. Each compartment should be sized so that the building can be supplied indefinitely from a single compartment during the other’s isolation for maintenance — in practice, between 50 and 75 percent of the total design storage volume per compartment.
Operating both compartments together through an interconnecting valve gives full design storage. Isolating one compartment reduces available storage to 50–75 percent of the design figure, which in most buildings is sufficient to maintain supply during the maintenance period. The sizing exercise must confirm that single-compartment storage is sufficient for the expected maintenance duration.
Related Guide
Break Tanks and Booster Sets: Design Guide for Multi-Storey Buildings
Covers two-compartment arrangement design – interconnecting valve configuration, independent isolation, and the resilience case for twin compartments in buildings above five storeys.
Common sizing errors
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Error
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Consequence
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Prevention
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|---|---|---|
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Applying a rule of thumb without calculating demand
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Storage volume bears no relationship to actual building demand — typically results in significant oversizing and elevated water age
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Use CIBSE Guide G demand data and the loading unit method for the specific building fixture schedule
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No water age check at the selected volume
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An apparently reasonable volume may produce a problematic water age in a warm plant room, which is not apparent until elevated temperature readings appear in the monitoring log
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Calculate water age for every candidate volume before selecting; compare against plant room thermal conditions
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Sizing for floor loading capacity rather than demand
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If the structure permits a 30,000 L tank, the designer specifies one — creating a water age far in excess of what the demand warrants
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Size for demand; use structural capacity as a constraint, not a target
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Not reviewing storage volume when occupancy changes
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A tank sized for 240 residents is significantly oversized for 30 office workers; water age increases dramatically and Legionella risk follows
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Trigger a sizing review whenever building use or occupancy changes materially; update the risk assessment accordingly
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Ignoring single-sensor stratification risk in large tanks
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Low-level sensor records acceptable temperature while upper layers exceed 20°C; monitoring appears compliant while a control failure is occurring
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Specify multi-level sensor pockets at design stage for tanks above approximately 10,000 L; include in the written control scheme
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Frequently asked questions
What is the recommended storage volume for a cold water tank in a multi-storey building?
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.
How do you calculate 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.
What is the maximum acceptable water age in a cold water storage tank?
The Water Fittings Regulations and HSE guidance do not specify a numerical maximum. However, HSG274 Part 2 and ACoP L8 require cold water to be stored at or below 20°C, and Legionella risk to be controlled. In practice, the Legionella risk assessment for the specific plant room determines whether a given water age is acceptable. In warm plant rooms where ambient temperatures approach 20°C in summer months, water ages above approximately 24 hours typically require supplementary control measures.
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 percent of the design total. The sizing exercise must confirm that single-compartment storage is sufficient to supply the building for the expected maintenance duration.
Contents
CONTENTS
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Complete GRP Cold Water Tank Guide
Full lifecycle coverage — sizing, compliance, installation, Legionella control, and O&M schedules.
