Twin Compartment Water Tanks
Divided cold water storage that keeps your building supplied while one side is inspected, cleaned or repaired. Tricel manufactures twin compartment tanks in two formats: site-assembled GRP sectional tanks from 1,000 to over 4,000,000 litres, and factory-moulded GRP one piece tanks within the 45–16,000 litre range.
What is a GRP twin compartment tank?
A GRP twin compartment tank is a single glass reinforced plastic cold water storage tank divided into two independent compartments by a full-height, watertight internal divider. Each compartment has its own inlet, outlet, overflow, drain and access hatch, so one side can be isolated, drained, cleaned, disinfected or repaired while the other continues to supply the building. Twin compartment tanks — also called divided tanks or maintenance-divided tanks — are specified wherever an interruption to cold water supply is unacceptable, such as hospitals, data centres, high-rise residential blocks and booster-fed commercial buildings.
01
Why twin compartment storage
Maintenance without shutting off the water
Every cold water storage tank has to come out of service at some point. HSE guidance in HSG274 Part 2 and ACOP L8 requires regular inspection — at least annually — and tanks must be cleaned and disinfected to BS 8558 whenever inspection shows it is needed. With a single-compartment tank, each of those visits means draining the only store of water in the building. A twin compartment tank removes that problem.
01
Continuity of supply
One compartment stays live while the other is isolated, so taps, WCs, process lines and booster sets keep running throughout inspection, cleaning or chlorination.
02
Resilience & risk control
If one side is affected by a leak, a failed float valve or a contamination event, it can be isolated and remediated while the building runs on the other.
03
Lower disruption
No temporary tanks, out-of-hours shutdowns or tenant notices for routine maintenance — a real saving over a 25–30 year design life.
03
One footprint, one base
A shared common wall means less panel area, one support base and simpler pipework than two free-standing tanks of the same total capacity.
02
Sectional
GRP sectional twin compartment tanks
Tricel GRP sectional tanks are built on site from hot-press moulded 1m GRP panels. In a twin compartment build, a full-height divider wall — formed from the same GRP tank panels and supported by stainless steel bracing and fixings — splits the tank into two watertight compartments. Because the panel grid can be extended, capacity is limited by your plant room footprint and headroom rather than by a fixed product size.
Panels are palletised and carried in through stairwells, corridors and goods lifts, which makes sectional twin tanks the standard choice for upper-floor and basement plant rooms, large commercial and infrastructure storage, and replacement projects where an old tank is dismantled and a new one assembled in the same space.
Base configurations
- EFB (External Flanged Base) — raised, fully self-draining base; ideal where perimeter clearance and headroom allow, and makes draining each compartment simple.
- IFB (Internal Flanged Base) — lower overall height for restricted headroom; built on a flat base or levelled steel beams.
- TIF (Totally Internally Flanged) — assembled from inside the tank for tight plant rooms (around 50mm on three sides, around 500mm on the pipework side). Divider arrangement confirmed at design stage.
Sectional twin tank at a glance
Capacity
1,000 L – 4M+ L
Panels
Hot-press moulded GRP, 1m modules
Divider
Full-height GRP panels, stainless steel bracing
Split
50/50 typical; set on the panel grid
Bases
EFB / IFB / TIF
Location
Internal or external
Assembly
Tricel-trained UK crews
03
One piece
GRP one piece twin compartment tanks
A one piece twin compartment tank is moulded as a single GRP shell with an integral internal division, so it arrives on site complete — no panel joints, no assembly and the shortest installation programme. Like the rest of Tricel’s GRP one piece tank range, it is manufactured to BS EN 13280:2001, with CFC-free PU foam insulation encapsulated in the laminate on all sides and the lid.
One piece twin tanks suit domestic, light commercial and small-to-medium commercial storage where the complete tank can be carried along a clear route to its final position. Each compartment has its own sealed access lid, and AG or Category 5 AB air gap inlet arrangements can be specified. Where the access route is too narrow for the moulded tank, a two-part tank or sectional twin tank is the alternative.
One piece twin tank at a glance
Capacity
Within the 45 – 16,000 L range
Construction
Single moulded GRP unit
Divider
Integral, moulded in
Insulation
25mm CFC-free PU foam
Air gap
AG (Cat 1–4) or AB (Cat 5)
Assembly
None — position and connect
Delivery
Complete unit, palletised
04
Comparison
Sectional vs one piece twin compartment tanks
Both formats deliver the same operating benefit — two independently isolatable compartments. The right choice comes down to capacity, the access route into the plant room and your installation programme. For a wider view, see our sectional vs one piece vs bespoke configuration guide.
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Requirement
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Sectional twin compartment tank
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One piece twin compartment tank
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Capacity
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1,000 L to over 4,000,000 L; extended by adding panels
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Within Tricel’s 45–16,000 L one piece range
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Divider
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Full-height GRP panel wall with stainless steel bracing
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Integral division moulded into the shell
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Access route
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Restricted access — individual panels via stairs, corridors, lifts
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Clear route needed for the complete tank
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On-site work
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Assembled by Tricel-trained crews (EFB, IFB or TIF)
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None — position, level and connect
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Programme
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Assembly time scales with tank size
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Shortest installation time
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Internal joints
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Bolted, sealed panel joints
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Seamless moulded interior
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Typical use
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Hospitals, high-rise, data centres, infrastructure, large commercial
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Residential blocks, schools, light commercial, small booster systems
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Replacement projects
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Old tank removed and new tank built panel by panel in the same space
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Only where the complete unit can be moved in
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05
How it works
Normal operation and maintenance mode
01
Both compartments in service
In day-to-day running both sides fill through their own float valves and draw down together through a common outlet header, keeping water turning over in each compartment.
02
Isolate one side
For inspection or cleaning, the inlet and outlet valves on one compartment are closed. It is drained, cleaned and chlorinated under a confined space safe system of work, while the other compartment supplies the building.
03
Return and repeat
Once flushed and passed, the clean side is returned to service and the process is repeated on the second compartment. Records feed your ACOP L8 log and Golden Thread asset register.
Tank entry is a confined space activity — see our guide to confined space entry in building maintenance.
06
Design and water hygiene
Six rules for specifying a divided tank
A divided tank only reduces Legionella risk if both compartments are designed to stay fresh. These are the points we review with consulting engineers on every twin compartment enquiry.
01
Full-height, watertight division
The divider must run the full depth of the tank. Part-height baffles improve circulation but do not let one side be drained while the other stays full.
02
Balanced flow
Match inlet and outlet sizes on both compartments and connect outlets through a common header so each side draws down equally. Unbalanced pipework leaves one side stagnant.
03
Cross-flow in each side
Position the inlet and outlet at opposite ends of each compartment to avoid short-circuiting and dead zones.
04
Independent fittings
Each compartment needs its own float valve, isolating valves, screened overflow and warning pipe, drain and access hatch so it can be fully isolated.
05
Temperature below 20°C
Insulate the tank and adjacent pipework, and avoid oversizing. HSG274 expects stored cold water to stay below 20°C.
06
Maintenance clearance
Allow hatch and perimeter clearance for both compartments — indicatively 500mm around tanks up to 2m high, rising for taller tanks.
Clearance values are indicative and must be confirmed at design stage. See installation and structural considerations for water tanks and our backflow prevention guide.
07
Compliance
Standards and regulations for twin compartment tanks
Compliance is achieved by the complete installed system — tank, fittings, pipework, insulation, access and the maintenance regime — not by the tank alone. State potable use and any approval requirements at quotation stage.
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Regulation / standard
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What it means for a twin compartment tank
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Water Supply (Water Fittings) Regulations 1999
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Applies to tanks connected to the public mains. Regulation 4(1)a / WRAS-approved materials and fittings for potable storage; cisterns over 1,000 litres must be inspectable and cleanable without being wholly uncovered.
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BS EN 13280:2001
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Specification for GRP cold water storage cisterns — materials, structural performance and testing. Tricel tanks are manufactured to this standard.
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HSE ACOP L8 and HSG274 Part 2
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Legionella risk assessment and control; stored cold water below 20°C; inspection at least annually. Each compartment is inspected and recorded.
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BS 8558:2015
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leaning and disinfection of cold water storage. A divided tank lets each compartment be cleaned in turn without interrupting supply.
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HTM 04-01 (healthcare)
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Healthcare water systems must allow storage to be isolated, cleaned and disinfected while supply to clinical areas is maintained — commonly achieved with divided tanks. See healthcare plant room design guide.
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Building Safety Act 2022
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In higher-risk buildings, cold water storage forms part of the Golden Thread. Record both compartments, inspection and cleaning history — see the Golden Thread guide.
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Approval certificates are available from our certificates library. Microbiological testing: see Legionella and microbiological sampling.
08
Applications
Where bespoke GRP storage is specified
Healthcare & NHS
Uninterrupted potable supply to wards and theatres; HTM 04-01 maintenance.
Data Centres
Redundant storage for cooling make-up and business continuity.
High-Rise & Mylti-Storey
Booster-fed blocks where residents cannot lose water during cleaning.
Education
Schools and universities that need term-time maintenance without closures.
Hospitality & Leisure
Hotels and leisure centres operating 24/7.
Infrastructure & Utilities
Large-capacity resilient storage for critical assets.
Industrial & Manufacturing
Process continuity during tank maintenance shutdowns.
Residential
One piece twin tanks for apartment buildings and care homes.
Pairing storage with pumps? See combined tank and pump stations and Elvaro water pressure booster sets.
09
Sizing
How to size a twin compartment tank
- Calculate total storage from demand. Use occupancy, loading units and peak flow to CIBSE Guide G and BS EN 806-3 — see the UK tank sizing guide.
- Check the maintenance window. With one side isolated, roughly half the storage is available. Confirm a single compartment can meet demand for the time needed to clean and chlorinate the other.
- Choose the split. 50/50 is typical and keeps flow balanced; other splits can be set on the sectional panel grid.
- Fit to the plant room. Footprint, headroom and access route decide sectional vs one piece and the base type.
- Avoid oversizing. Extra volume slows turnover and raises water temperature — both Legionella risk factors.
10
FAQs
Twin compartment tank FAQs
Common questions from consulting engineers, M&E contractors and specifiers.
What is a twin compartment water tank?
A GRP sectional water tank is a modular cold water storage tank constructed from individual hot-press moulded glass reinforced plastic panels. Panels are transported to site separately and assembled by trained operatives, allowing installation in locations where a complete tank unit cannot be delivered or positioned — including upper-floor plant rooms, basements, high-rise buildings and restricted-access sites. Tricel manufactures GRP sectional water tanks in EFB, IFB and TIF configurations with Regulation 4(1)a and WRAS-approved options for potable cold water storage.
Why specify a twin compartment tank instead of a single tank?
Tricel GRP sectional tanks are available from 1,000 litres to over 4 million litres. Tank size is determined by the panel grid configuration — Tricel’s standard hot-press moulded panels are 1m units — and constrained by available footprint, headroom and access route. There is no fixed maximum product size; capacity is extended by adding panels to the grid. Full panel size and capacity tables are set out in the Tricel water tank and panel sizes reference.
What is the difference between a sectional and a one piece twin compartment tank?
The three main GRP sectional tank types differ in base design, assembly method and the clearance they require:
- EFB (External Flanged Base) – raised self-draining base; suited to open-access sites with standard perimeter clearance and headroom; the most common configuration
- IFB (Internal Flanged Base) – lower overall installation height; suited to sites with limited headroom; perimeter maintenance clearance still required
- TIF (Tank Internally Flanged) – assembled from inside; approximately 50mm clearance achievable on three sides; approximately 500mm required on the pipework access side; suited to highly constrained plant rooms and retrofit projects
Is a twin compartment tank better than two separate tanks?
Tricel manufactures GRP sectional tanks to Regulation 4(1)a (NSF) approval for potable cold water storage. WRAS-approved components and fittings are available where specified. Compliance for potable use depends on the full specification — tank type, materials, pipework, fittings, insulation, access provision and the ongoing maintenance regime. Potable water use and any approval requirements should be stated at the quotation stage, as not all configurations or fittings carry these approvals as standard.
How do you prevent stagnation in a twin compartment tank?
Clearance requirements vary by tank type and height. As indicative design guidance:
- Tanks up to 2m high – approximately 500mm perimeter clearance on all sides
- Tanks 2.5–3m high – approximately 800mm perimeter clearance
- Tanks 3.5–4m high – approximately 1,000mm perimeter clearance
- IFB hatch clearance – 750mm standard; 500mm with lift-off hatch
- TIF – approximately 50mm on three sides; approximately 500mm on the pipework access side
- Base flatness – 2mm per 1m; 6mm per 6m
All values are indicative design guidance only and must be confirmed at design stage against the specific tank type, configuration and project constraints.
Are Tricel twin compartment tanks suitable for drinking water?
Yes. The TIF (Tank Internally Flanged) configuration is designed specifically for constrained plant rooms where standard perimeter clearance is not achievable on multiple sides. TIF tanks are assembled from inside, removing the need for external perimeter access during construction. However, regardless of tank type, the access route for delivering panels, the base condition, the pipework side and the maintenance access strategy must all be confirmed at design stage. Tricel offers site surveys (subject to location) to assess these factors before specification is confirmed.
Can a twin compartment tank have a Category 5 air gap?
Tricel GRP sectional tank panels are hot-press moulded at manufacturing facilities in Weston-super-Mare and Lanark. Hot-press moulding uses heat and pressure to cure GRP panels using isophthalic polyester resin and glass fibre reinforcement. The process produces panels with consistent dimensional accuracy, smooth surfaces on both faces and full cure throughout the panel thickness. Strict quality control procedures are applied during moulding. Materials used in potable water tank production are tested for Regulation 4(1)a and WRAS compliance.
How do I size a twin compartment tank?
At a minimum: required storage capacity, project location, tank location (internal or external), available footprint and headroom, access route constraints, pipework requirements, insulation requirement and whether the use is potable or non-potable. Plant room drawings, existing tank details and the preferred tank type (if known) will help identify constraints earlier and improve the accuracy of the quotation. Send project details to websales@tricelwater.co.uk or call +44 (0)1934 421 499.
How often should each compartment be inspected and cleaned?
At a minimum: required storage capacity, project location, tank location (internal or external), available footprint and headroom, access route constraints, pipework requirements, insulation requirement and whether the use is potable or non-potable. Plant room drawings, existing tank details and the preferred tank type (if known) will help identify constraints earlier and improve the accuracy of the quotation. Send project details to websales@tricelwater.co.uk or call +44 (0)1934 421 499.
Looking after an existing tank? See cold water storage tank aftercare and our maintenance and whole-life performance guide.
Tricel Water UK - UK GRP Tank Manufacturer
Specify your twin compartment tank
Send your required capacity, plant room dimensions, access route, potable or non-potable use and any drawings. We will recommend sectional or one piece, the compartment split, base type and fittings — with a next working day response on standard enquiries and free site surveys subject to location.
This product 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.
