Supplying cold water to upper floors of a multi-storey or high-rise building from direct mains pressure is rarely feasible. Water companies typically restrict incoming mains pressure at the boundary, and the static head required to serve floors above the fifth or sixth storey exceeds what direct connection can deliver reliably. The standard solution — a low-level break tank combined with a pressurised booster pump set — is also the arrangement that introduces the greatest complexity into cold water system design.
This guide addresses the design decisions that matter most, drawing on the technical framework in CIBSE Guide G, BS 8558, and the Water Supply (Water Fittings) Regulations 1999 (and their devolved equivalents).
Fluid category of water in a storage cistern — requires air gap backflow protection
Maximum cold water storage temperature per HSE ACoP L8 / HSG274 Pt 2
Minimum safety factor for overflow pipe sizing over maximum inlet flow rate
What is a break tank?
A break tank is a storage cistern that provides a hydraulic break between the incoming mains supply and the downstream pressurised distribution system. Water enters the break tank under mains pressure, controlled by a float valve. A booster pump set draws from the break tank and pressurises the building’s distribution pipework.
Break tank vs. storage cistern
All break tanks are cisterns, but not all cisterns are break tanks. The term describes the hydraulic function — interrupting the direct connection to the mains — rather than a different type of vessel. In practice, the majority of cold water storage cisterns in multi-storey buildings function as break tanks.
Why a break tank rather than direct pressurisation?
Pressurising building distribution directly from the mains — using a booster pump drawing directly from the incoming main — is subject to restrictions under the Water Supply (Water Fittings) Regulations 1999. Drawing directly from a water undertaker’s main requires prior consent from the water company and is prohibited where it could cause a pressure reduction that affects neighbouring supplies. A break tank eliminates this restriction by providing a buffer between the mains connection and the booster pump suction.
A secondary benefit is that the break tank decouples the building’s peak demand from the incoming mains flow rate, allowing a smaller mains connection to serve a building with high instantaneous demand.
CIBSE Guide G
CIBSE Guide G notes that low-level break tanks and booster sets can reduce certain contamination risk pathways compared with elevated cisterns, because the main open storage vessel is at low level and not exposed to the same airborne contamination risk as roof-mounted cisterns.
System configurations in multi-storey & High-Rise buildings
The majority of UK multi-storey developments use one of four configurations. Selection depends on building height, occupancy, pressure zone requirements, and resilience needs.
Most common
Low-level break tank + single booster set
One sectional GRP cistern at basement or ground-floor level, with a single pressurised booster set serving the entire building or a single pressure zone. Appropriate for buildings up to approximately ten to twelve storeys where the static pressure range across the building is manageable within a single pressure zone.
Standard for larger buildings
Low-level break tank + zoned booster sets
A single break tank feeds multiple booster sets, each serving a defined pressure zone. The booster set for the lower zone typically operates at lower discharge pressure than that serving upper floors. This prevents excessively high static pressure at low-floor outlets and reduces the risk of water hammer.
High-rise residential
Hybrid zoned system with intermediate tanks
High-rise towers — typically above twenty storeys — commonly use intermediate break tanks at mid-rise level, fed by a transfer pump from the low-level tank. Each intermediate tank then feeds a zone booster set. This limits the maximum static pressure in any section of distribution, which is critical for compliance with acceptable pressure limits and for protection of fittings.
Recommended for continuity
Twin-compartment break tank
Two cistern compartments, separately valved but operating in parallel, allow one compartment to be isolated for maintenance without interrupting supply. This is strongly recommended for any building where supply interruption creates significant welfare, commercial, or operational impact. See the two-compartment arrangements section below.
Sizing the break tank
Break tank sizing requires balancing four competing requirements, as described in CIBSE Guide G and BS 8558. These requirements pull against each other — a designer who optimises for resilience alone will typically produce an oversized tank that creates a Legionella risk.
|
Requirement
|
What it drives
|
Risk of getting it wrong
|
|---|---|---|
|
Peak demand buffering
|
Tank must absorb demand peaks that exceed the mains fill rate
|
Booster set starvation; pressure loss at upper floors during peak periods
|
|
Resilience
|
Stored volume maintains service under short mains interruption
|
Supply failure during planned maintenance or pressure incidents
|
|
Water hygiene
|
Maximum acceptable water age given thermal environment
|
Oversized tanks increase residence time; stored water may exceed 20°C
|
|
Physical constraints
|
Available footprint, ceiling height, structural floor loading
|
Installation impractical; structural failure; access problems
|
| Requirement | What it drives | Risk of getting it wrong |
|---|---|---|
| Peak demand buffering | Tank must absorb demand peaks that exceed the mains fill rate | Booster set starvation; pressure loss at upper floors during peak periods |
| Resilience | Stored volume maintains service under short mains interruption | Supply failure during planned maintenance or pressure incidents |
| Water hygiene | Maximum acceptable water age given thermal environment | Oversized tanks increase residence time; stored water may exceed 20°C |
| Physical constraints | Available footprint, ceiling height, structural floor loading | Installation impractical; structural failure; access problems |
Peak demand buffering
The tank must hold sufficient volume to absorb demand peaks that exceed the mains fill rate. For residential buildings, peak morning and evening draw-off can be two to three times the average hourly rate.
Resilience
The design brief should specify a minimum duration of service that the stored volume must maintain under loss of mains supply. In practice, thirty minutes to one hour is a common residential requirement; critical applications (hospitals, data centres) may require two to four hours or more.
Water hygiene
Oversized tanks increase water age — the average time water spends in storage before use. In warm plant rooms, a long residence time raises the risk of stored water temperature exceeding 20°C and entering the Legionella risk range. HSE guidance (HSG274 Part 2) is explicit that stored cold water should remain below 20°C. Right-sizing is a water hygiene control measure, not merely an economic one.
Physical constraints
Floor loading capacity, plant room footprint, and ceiling height all limit achievable volume in a single cistern. A two-compartment arrangement divides the required volume across two tanks, which may ease structural constraints.
Sizing worked example
A simplified sizing illustration: a 120-apartment residential tower with an average of two occupants per flat, each consuming approximately 150 litres per day, has an average daily demand of around 36,000 litres. A resilience target of one hour of average demand gives a minimum storage figure of approximately 1,500 litres. Applying a margin for draw-down and demand peaks suggests a working storage volume of 2,000 to 3,000 litres — considerably less than many designers assume. Confirm all sizing calculations against CIBSE Guide G demand data and the specific project resilience requiremen
Sizing caution
Oversizing is the most common water hygiene error in break tank design. A tank sized on worst-case resilience figures without reference to actual demand produces excessive water age. In a warm plant room, this can raise stored water temperature above 20°C, creating a Legionella risk that is difficult to manage without supplementary water treatment. Right-sizing is a Legionella control measure, not merely an economic consideration.
Two-compartment arrangements
A two-compartment arrangement divides the break tank into two independently valved cistern sections. Either compartment can be fully isolated — for cleaning, inspection, or repair — while the other continues to supply the booster set without interruption.
Two-compartment arrangements are strongly recommended for:
- Residential buildings above five storeys
- Any building where supply interruption creates significant welfare, commercial, or operational impact
- Buildings served by a single incoming main with no alternative supply route
- Healthcare, hotel, care home, or other high-continuity occupancies
Each compartment should be sized so that the building can be maintained from a single compartment indefinitely. This typically means each holds between 50 and 75 percent of total design storage volume, with the interconnecting valve open during normal operation to equalise levels.
Related Guide
Full lifecycle coverage – specification, sizing, installation, compliance, and maintenance. Includes two-compartment design guidance and worked sizing examples
Booster set interface
Inlet control and air gap
The float valve on the break tank inlet must be sized for the maximum fill rate without causing water hammer. All float valves on potable water cisterns must be WRAS-approved. Because the water in a cold water cistern is classified as a Category 5 fluid under the Water Fittings Regulations, the inlet must be protected by an air gap — specifically a Type AG arrangement, where the float valve outlet terminates above the overflow spillover level. The minimum free air gap is 20 mm or twice the internal pipe diameter, whichever is greater.
Key rule: Category 5 requires an air gap
RPZ valves and non-return valves cannot protect against Category 5 backflow risk. Only a Type AA or Type AG air gap provides compliant protection at a cold water cistern inlet. This applies regardless of the building type or pressure zone.
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.
Suction arrangement
Position the cistern outlet at the lowest practical point of the tank. Specify a vortex breaker or foot valve on the outlet connection to prevent air entrainment at low-water conditions. A low-level pump-protection cut-out electrode must prevent the booster set running dry — this is a mandatory protection feature, not an optional BMS signal.
BMS integration
High-level, operational, and low-level (pump-protection) signals should be wired to BMS or a dedicated local panel. Where Legionella temperature monitoring is required by the risk assessment, integrate sensor outputs with configurable alarm thresholds. For tanks above approximately 10,000 litres, specify sensor pockets at low, mid, and high levels to detect thermal stratification.
Overflow and warning pipe
Size the overflow for at least 1.5 times the maximum inlet flow rate under fault conditions. Route both overflow and warning pipe to a clearly visible, traceable discharge point. Do not route to a concealed drain. Provide a BMS overflow alarm on the warning pipe.
A break tank is a storage cistern that provides a hydraulic break between the incoming mains supply and the downstream pressurised distribution system. Water enters the break tank under mains pressure, controlled by a float valve. A booster pump set draws from the break tank and pressurises the building’s distribution pipework.
Location
|
Location
|
Preferred Characteristics
|
Risks to Mitigate
|
|---|---|---|
|
Basement plant room
|
Controlled temperature; drainage; secure access; adequate ceiling height
|
Heat gain from adjacent plant; flooding from drain back-up; pest ingress
|
|
Ground-floor plant room
|
Similar to basement; better structural options in some buildings
|
External wall proximity may allow solar heat gain
|
|
Roof or roof void
|
Gravity head available; avoids pump energy for low floors
|
UV exposure; thermal extremes; maintenance access at height; structural loading
|
|
Intermediate plant room
|
Serves localised pressure zone; reduces pump head
|
Access route constraints for installation; structural considerations at intermediate slab
|
| Location | Preferred characteristics | Risks to mitigate |
|---|---|---|
| Basement plant room | Controlled temperature; drainage; secure access; adequate ceiling height | Heat gain from adjacent plant; flooding from drain back-up; pest ingress |
| Ground-floor plant room | Similar to basement; better structural options in some buildings | External wall proximity may allow solar heat gain |
| Roof or roof void | Gravity head available; avoids pump energy for low floors | UV exposure; thermal extremes; maintenance access at height; structural loading |
| Intermediate plant room | Serves localised pressure zone; reduces pump head | Access route constraints for installation; structural considerations at intermediate slab |
Compliance requirements
The following compliance points are most frequently deficient on inspection.
Water Fittings Regulations. The correct instrument depends on jurisdiction: SI 1999/1148 for England and Wales; SSI 2014/317 for Scotland; SR 2009/75 for Northern Ireland. Citing only the 1999 Regulations in project documentation for a Scottish or Northern Irish project is technically incorrect.
BS EN 13280. The break tank panels should comply with BS EN 13280. Request a declaration of performance from the manufacturer at procurement.
Building Regulations Part G. In England and Wales, Requirement G1 requires a sufficient supply of wholesome water. Cold water storage tanks in new or altered buildings must comply. In Scotland, the Building (Scotland) Regulations 2004 and associated Technical Handbooks apply; in Northern Ireland, the Building Regulations (Northern Ireland) 2012.
Legionella control. The break tank is a primary subject of the site Legionella risk assessment. The written control scheme must address temperature monitoring, inspection frequency, cleaning intervals, and escalation triggers.
Related Guide
Legionella Control in Building Water Systems. This whitepaper covers legal obligations, risk assessment, monitoring, and O&M schedules for engineers, contractors, and facilities managers.
Common errors
|
Error
|
Consequence
|
Prevention
|
|---|---|---|
|
Oversizing the tank
|
Excessive water age; temperature rise; elevated Legionella risk
|
elevated Legionella risk Size against actual demand data using CIBSE Guide G methodology
|
|
Undersized overflow
|
Flooding; compliance failure; concealed water damage
|
Size at design stage against float valve manufacturer's flow data; apply 1.5× safety factor
|
|
Missing re-torque after first fill
|
Early joint leaks; panel distortion
|
Schedule and log re-torque after fill and at 4–6 weeks; retain torque log
|
|
No low-water cut-out interlock
|
Booster set cavitation; pump damage; dry running
|
Electrically interlock booster set to prevent operation below minimum water level
|
|
Single temperature sensor in large tank
|
Thermal stratification undetected; upper layers exceed 20°C
|
Specify sensor pockets at low, mid, and high levels for tanks above ~10,000 L
|
| Error | Consequence | Prevention |
|---|---|---|
| Oversizing the tank | Excessive water age; temperature rise; elevated Legionella risk | Size against actual demand data using CIBSE Guide G methodology |
| Undersized overflow | Flooding; compliance failure; concealed water damage | Size at design stage against float valve manufacturer's flow data; apply 1.5× safety factor |
| Missing re-torque after first fill | Early joint leaks; panel distortion | Schedule and log re-torque after fill and at 4–6 weeks; retain torque log |
| No low-water cut-out interlock | Booster set cavitation; pump damage; dry running | Electrically interlock booster set to prevent operation below minimum water level |
| Single temperature sensor in large tank | Thermal stratification undetected; upper layers exceed 20°C | Specify sensor pockets at low, mid, and high levels for tanks above ~10,000 L |
Frequently asked questions
What is the difference between a break tank and a storage cistern?
A break tank is a storage cistern used specifically to provide a hydraulic break between the incoming mains supply and a downstream booster pump set. All break tanks are cisterns, but not all cisterns are break tanks. The term break tank describes the hydraulic function — interrupting the direct hydraulic connection to the mains — rather than a different type of vessel.
Can a booster pump draw directly from the mains without a break tank?
Direct draw from a water undertaker’s main requires prior consent from the water company and is subject to restrictions under the Water Supply (Water Fittings) Regulations 1999 (Regulation 5). It is prohibited where it could cause a pressure reduction that adversely affects neighbouring supplies. In practice, a break tank arrangement is the standard solution for multi-storey buildings.
How many compartments should a break tank have?
Two compartments are strongly recommended for any building above five storeys and for any occupancy where supply interruption creates significant risk. A single-compartment tank cannot be isolated for cleaning or inspection without interrupting supply. The two-compartment arrangement is not a premium specification — it is standard practice for buildings where continuity matters.
What maintenance does a break tank require?
Annual cleaning and disinfection is the minimum baseline per HSE guidance and BS EN 806-5 / BS 8558. Weekly and monthly FM checks cover external leak inspection, overflow status, and float valve operation. Six-monthly checks include temperature monitoring and microbiological sampling where required by the Legionella risk assessment. See our Commissioning and Disinfection Guide for the cleaning and disinfection procedure.
Contents
CONTENTS
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Complete GRP Cold Water Tank Guide
Full lifecycle coverage — sizing, compliance, installation, Legionella control, and O&M schedules.
