Standards & Regulations
Domestic and residential water services in the UK must comply with several overlapping standards and regulations. This tool uses the following as its basis:
Water Regs 1999Water Supply (Water Fittings) Regulations — mandatory legal requirement
BS EN 806Specifications for installations inside buildings conveying water — Parts 1–5
BS 8558Guide to the design, installation, testing and maintenance of services for buildings
BS 6700Specification for cold and hot water supply — legacy standard, still widely referenced
CIBSE Guide GPublic health and plumbing — occupancy and demand guidance
HTM 04-01Safe water in healthcare — Legionella risk management (also used as best practice)
Building Type
Select whether the building is residential (flats, apartments, blocks) or domestic (individual houses). For mixed-use developments, select mixed. The distinction affects occupancy estimation and some demand assumptions.
ResidentialBlocks of flats, apartments, student accommodation, care homes
DomesticIndividual houses, terraced, semi-detached, detached
MixedCombination of house types, or residential with commercial ground floor
Unit Schedule & Occupancy
Enter each dwelling type separately with the number of units and bathrooms. The tool automatically estimates occupancy per unit based on the number of bedrooms, using BS EN 806-3 guidance:
Studio / 0-bed1.2 persons per unit
1-bed1.5 persons per unit
2-bed2.5 persons per unit
3-bed3.5 persons per unit
4-bed4.5 persons per unit
5-bed+5.5 persons per unit
💡 Name units clearly (e.g. "2-Bed Flat", "3-Bed House") so the tool can estimate occupancy automatically. You can add as many unit types as the project requires.
Water Demand
The tool calculates demand using two parallel methods and takes the higher result as the design figure:
Method 1 — Per-person demand: Total occupancy × daily usage (L/person/day) × peak factor. The peak factor accounts for the ratio of peak to average flow during the busiest hour. Typical values: 2.5× standard residential, 3.0× typical apartment block, 4.0× high occupancy or care.
Method 2 — Loading Unit method (BS EN 806-3 Table A.1): Each fixture is assigned loading units (LU). The simultaneous peak flow is derived from the total LU count using the empirical formula from BS EN 806-3 Fig A.1. This is the more rigorous method for detailed design.
💡 For early-stage estimation, both methods will give a reasonable result. For detailed design, the loading unit method with actual fixture counts should be used with specialised hydraulic software.
Cold Water Tank Sizing
The cold water storage tank is sized to provide the selected number of hours of autonomy — i.e. the building can be supplied from the tank alone for that period if mains supply is interrupted. The standard recommendation is 24 hours (BS 8558 / CIBSE Guide G).
⚠️ Large tanks require careful structural assessment — water weighs 1.0 tonne per m³. A 10,000L tank weighs 10 tonnes. Always involve a structural engineer for tanks above ~2,000L. GRP sectional tanks are common for larger volumes as they can be brought through standard openings.
💡 The tank dimensions shown are guide sizes only. Final tank selection depends on available plant room space, access, and supplier ranges. Tank rooms must have adequate ventilation, drainage, and access for maintenance.
Shared Sprinkler Tank
Where the cold water storage tank is shared with a residential sprinkler system (BS 9251:2021), the tank must be zoned to ensure the sprinkler reserve is always protected. The arrangement per BS 9251:2021 cl.5.11.4.4 is:
Zone 1 (bottom)Sprinkler reserve — the minimum volume required for sprinkler operation. A low-level alarm switch is set at the TOP of this zone.
Zone 2 (middle)Buffer volume — sits between the LL alarm switch and the domestic draw-off point. Prevents nuisance alarms from normal water level fluctuation. Typically 150–300L.
Zone 3 (top)Domestic supply — the domestic cold water supply pipe draws from this level. Normal daily operation uses only this zone.
When the sprinkler LL alarm activates (tank level dropping into the buffer zone), the Priority Demand Valve (PDV) closes, cutting off domestic supply and reserving the remaining water for the sprinkler system.
⚠️ The PDV type, set pressure and wiring must be coordinated with the sprinkler designer. The sprinkler volume used in this tool should come directly from the BS 9251:2021 hydraulic calculation — use the output from the Sprinkler Estimator module.
Booster Set Sizing
Where mains pressure is insufficient to supply the topmost outlets at the required residual pressure, a booster pump set is required. The tool calculates the required pump duty pressure as:
Pump pressure = Static head (0.098 × height) + Friction losses + Required residual at outlet
Static head0.098 bar per metre of building height — pressure needed just to lift water to top floor
Friction lossesApproximate internal pipe losses — a detailed pipe sizing calculation is required for final design
Residual pressureMinimum pressure required at the draw-off point (typically 1.0–2.0 bar for showers)
Duty + standbyStrongly recommended — ensures continuity of supply if one pump fails
⚠️ This tool provides an indicative pump duty only. Final booster set selection requires a full pipe sizing calculation with actual pipe diameters, lengths and fittings. Always obtain manufacturer sizing from the booster set supplier.
Hot Water System
For central hot water storage systems, the tool estimates storage volume at 40% of the daily hot water demand (a typical recovery factor for central plant). The storage temperature must be 60°C minimum to control Legionella bacteria, per L8 ACOP and HTM 04-01.
Central HW storageCWSC feeds a central calorifier or hot water cylinder — common in blocks of flats
Combi boilerInstantaneous HW per dwelling — no central storage, high peak demand from mains
District heating / HIUHeat supplied from communal plant via heat interface units — specialist design required
Electric point-of-useSmall electric heaters at point of use — low flow, suitable for individual WCs only
Results & Next Steps
The results of this tool are for early-stage estimation and feasibility only. A full hydraulic design is required before installation, including:
Pipe sizingFull network design with actual pipe diameters, velocities and pressure losses
Water undertakerConfirm mains flow and pressure with a site test from the water undertaker
Backflow preventionWater Regs Reg 14 — appropriate backflow protection for all connections
Legionella riskL8 ACOP assessment — dead legs, tank design, temperature monitoring plan
Structural checkTank room loading, access, drainage and ventilation
Building RegsApproved Document G — sanitation, hot water supply and water efficiency
⚠️ This tool is an estimation aid only. Full design must be carried out by a qualified mechanical engineer. FIREMEC accepts no liability for decisions made on the basis of this tool.
