Design flow
24 L/s
Figure 2-0 · The plumbing chain. Each box is a section below. (Original schematic.)
Section layout (same as every module)
Role → Design method & formula → Worked numeric example → In the software → Code reference. Constants, symbols & conversions used in the worked examples
| Symbol / constant | Value | What it is |
|---|---|---|
| ρ (water) | 1000 kg/m³ | Density of water — fixed property. |
| g | 9.81 m/s² | Gravity — fixed constant (pump-power formula). |
| cp (water) | 4.187 kJ/kg·K | Specific heat of water (hot-water heater duty). |
| η (eta) | 0.70 typical | Pump efficiency (decimal) — from the Given. |
| a | ≈ 1200 m/s | Pressure-wave speed in a steel water pipe (water-hammer formula). |
| 1 bar | ≈ 10.2 m water | Pressure-to-head conversion. |
| Water flow | 1 kg/s ≈ 1 L/s | Because ρ ≈ 1000 kg/m³. |
Key formulas: pump power P = ρ·g·Q·H ÷ η · heater duty Q = ṁ·cp·ΔT · pipe size A = Q ÷ v · surge ΔP = ρ·a·v
2.1 Water demand & design criteria
Role: establish how much water the building needs, instantaneously and per day. Everything downstream (tanks, pumps, pipes) is sized from this.
Method — two demands
- Average daily demand (m³/day) — for storage & utility sizing, from per-capita / per-key figures.
- Peak instantaneous demand (L/s) — for pipe & pump sizing, from the loading-unit / fixture-unit method (probabilistic — not all fixtures run at once). The fixture-unit total converts to a design flow via a Hunter-type curve.
Probable demand vs loading units (Hunter-type)
Simultaneous demand flattens as fixtures multiply — doubling the fixtures does not double the design flow, because not all run at once. Pick the total loading units and a pipe size to read the design flow and the resulting velocity.
Sum of the fixture loading units served by the pipe.
Internal diameter of the supply main carrying that demand.
Design flow
85 m³/h
Velocity
1.3 m/s
Velocity check
Hunter-type demand curve with your operating point; velocity = Q ÷ pipe area. Aim for roughly 0.9–2.4 m/s.
Worked example 2.1 · Daily demand, a residential/hotel tower
Given: 500 keys/apartments, average 4 persons-equivalent occupancy variation; design 250 L/person/day domestic; plus 15% for common/F&B/laundry.
Domestic ≈ 500 × (say 2.5 eq. persons) × 250 = 312,500 L/day = 312 m³/day; ×1.15 ≈ 360 m³/day total.
~360 m³/day average. This sets storage volume (2.2); peak instantaneous flow for pipes/pumps comes from the loading-unit curve above, not this daily figure.
Code
Loading-unit / fixture-unit method & demand: SBC 701, IPC (or UPC) Appendix, BS EN 806 / CIPHE / CIBSE Guide G. Per-capita demand: local authority (NWC) & project brief.SOURCE + STORAGE→transfer→zoning→boosters→risers→tap
2.2 The source — incoming supply & storage tanks
Role: the city main rarely has the pressure or steady flow a tower needs, so water is taken into ground-level storage tanks (a buffer + fire reserve), then pumped up. Storage smooths utility supply and gives resilience.
| Tank | Sizing guide |
|---|---|
| Domestic ground/break tank | ~0.5–1.0 day of average demand (authority rules vary); divided into 2 compartments for cleaning |
| High-level / roof tanks | Buffer for gravity zones + booster suction; sized for peak duration |
| Fire reserve | Separate dedicated volume (see Fire module) — never share with domestic draw-off below the fire level |
Worked example 2.2 · Domestic storage
Given: average demand 360 m³/day; store 1.0 day domestic.
Storage ≈ 360 m³, in 2 compartments of 180 m³ (so one can be cleaned while the other serves).
360 m³ ground storage (2 × 180 m³) with level controls, overflow, and an air gap on the inlet (backflow protection). Material: GRP/SS sectional or RC lined.
Code
Storage & backflow protection (air gap): SBC 701, IPC; potable tank materials: NSF/ANSI 61; water quality: SASO/GSO. Cleaning compartmentation: water authority (NWC).2.3 Transfer pumps
Role: lift water from the ground tanks to high-level / zone tanks up the tower. Usually duty/standby, level-controlled (start/stop on tank floats), often staged for tall buildings.
Worked example 2.3 · Transfer pump duty
Given: fill 360 m³/day into high tanks over an effective 8 pumping hours → flow = 360,000 ÷ (8×3600) = 12.5 L/s; lift to a tank 200 m up + 25 m friction/fittings = head 225 m; η 0.70.
Power: P = ρgQH ÷ η = 1000 × 9.81 × 0.0125 × 225 ÷ 0.70 = 39.4 kW.
~40 kW duty (with standby). For a 1 km tower transfer happens in stages tank-to-tank, not one 1,000 m lift — each stage's pump sees only its lift.
In the software
Compute head in AFT Fathom (or spreadsheet); select the pump in Grundfos GPS / Wilo-Select at Q&H, checking NPSH and BEP. Model the staged tank-to-tank lift, not a single riser. Code
Pump energy: ASHRAE 90.1/SBC 601 (where applicable). NPSH: Hydraulic Institute. Controls: level/float per SBC 701.2.4 Pressure zoning — the tall-building core
Role: limit static pressure at fixtures (codes cap it, e.g. ≤ ~5 bar at a fixture) and guarantee a minimum residual at the highest tap. The tower is split into zones (~10–15 floors) fed by either gravity from a high tank through PRVs, or boosted up from a break tank — see Module 0.
| Parameter | Typical limit | Code |
|---|---|---|
| Max static pressure at a fixture | ≤ ~5 bar (PRV needed above) | SBC 701 / IPC |
| Min residual at highest fixture | ~1.0–2.0 bar (fixture/flush-valve dependent) | Manufacturer / SBC 701 |
| Zone height | ~10–15 floors | From the two limits above |
Key takeaway
Each zone is bounded by two numbers: not too much pressure at the bottom fixture, and enough at the top. The floor-to-floor head (≈0.4 bar per 4 m floor) divided into that window sets the zone height. Code
Pressure limits & PRV requirement: SBC 701, IPC. (Compare with NFPA 14 zoning for fire — same idea, different system.)2.5 Booster pump sets (variable speed)
Role: raise pressure to feed a zone from a break tank, maintaining constant outlet pressure as demand varies, via a VFD-controlled multi-pump set (e.g. 3 pumps + standby) holding a pressure set-point.
Worked example 2.5 · Booster set for a zone
Given: zone peak instantaneous demand 15 L/s (from loading units); zone served from a break tank at its base; highest fixture 45 m above the set; friction+fittings 20 m; required residual 2 bar (≈20 m).
Head: H = 45 (static) + 20 (friction) + 20 (residual) = 85 m.
Power: P = ρgQH ÷ η = 1000 × 9.81 × 0.015 × 85 ÷ 0.70 = 17.9 kW at peak; VFD reduces it sharply at part load (affinity laws).
VFD booster set ~18 kW peak, e.g. 3 duty + 1 standby pumps cascading on demand, holding ~85 m. Add a pressure vessel to limit pump cycling.
In the software
Select packaged booster sets in Grundfos/Wilo/Lowara tools (enter peak flow + set pressure → they pick the multi-pump configuration & VFD). Verify network residuals at every floor in AFT Fathom. Check pump-start surge in AFT Impulse / Bentley HAMMER. Code
Booster & backflow: SBC 701/IPC; pump energy: SBC 601; surge protection: hydraulic transient analysis (Hydraulic Institute / engineering practice).2.6 Pipe sizing & material
Role: deliver peak flow at acceptable velocity (noise & erosion) and pressure loss.
| Service | Velocity | Material (typical) |
|---|---|---|
| Cold-water mains/risers | 1.5–2.4 m/s | Galvanised/SS, HDPE, PPR, copper |
| Hot-water & recirc | ≤ 1.0–1.5 m/s (erosion-corrosion) | Copper / SS / PPR (temperature-rated) |
| Branch to fixture | ≤ 2.0 m/s | — |
Worked example 2.6 · Size a zone riser
Given: Q = 15 L/s = 0.015 m³/s; target v = 2.0 m/s.
A = 0.015 ÷ 2.0 = 0.0075 m²; d = √(4×0.0075 ÷ π) = 0.098 m → select DN100; actual v = 0.015 ÷ (0.785×0.1²) = 1.91 m/s .
DN100 riser; verify residual pressure at the top fixture stays ≥ 2 bar after friction. Choose material for pressure rating (deep zones may need higher PN) & temperature (hot).
In the software
Revit MEP sizes by velocity/friction from the loaded pipe schedule; AFT Fathom proves residual pressures at every node & the booster duty. (Revit sizes; AFT proves the hydraulics.) Code
Sizing & velocity: SBC 701, BS EN 806, CIBSE Guide G. Materials & potable suitability: NSF/ANSI 61, project spec.cold supply+HOT WATER
2.7 Hot-water generation, circulation & Legionella
Role: generate hot water (calorifiers/water heaters, often fed by the central plant, heat pumps, or solar pre-heat) and keep it hot at every outlet via a recirculation loop — while controlling Legionella, a real life-safety issue in hotels.
Design method
Heater output: Q = ṁ · cp · ΔT. Storage + recovery sized on the hot-water demand profile. Recirculation flow sized to replace pipe heat loss so the return stays hot.
Worked example 2.7 · Water-heater duty
Given: peak hot-water draw 4 L/s; cold in 25 °C, store/deliver 60 °C → ΔT = 35 °C; cₚ 4.187.
Q = 4 (kg/s) × 4.187 × 35 = 586 kW instantaneous — reduced by storage (a buffer lets a smaller heater meet peaks).
With storage, select e.g. a 300 kW heater + buffer calorifier sized on the demand profile. Legionella control: store ≥ 60 °C, return ≥ 55 °C everywhere, and provide thermostatic mixing valves (TMVs) at outlets to prevent scalding (deliver ~43 °C).
Figure 2-1 · Hot-water recirculation keeps every outlet hot; store ≥60 °C / return ≥55 °C for Legionella control, TMV at outlets for anti-scald. (Original schematic.)
In the software
Size heaters/storage from the demand profile in manufacturer tools (e.g. heat-pump/calorifier selectors); size the recirculation loop & pump from pipe heat-loss (Revit/CIBSE method); confirm return temperatures. Code
Legionella: ASHRAE 188, CIBSE TM13/HSG274. Hot-water & TMV/anti-scald: SBC 701, EN 806; energy: SBC 601.2.8 Valves, protection & accessories
- PRVs — cap zone pressure; often duplex (duty/standby) on risers.
- Backflow preventers / air gaps — protect potable water from contamination (essential at tank inlets, irrigation, cooling make-up).
- Check, isolation & drain valves; strainers.
- Water-hammer arrestors / surge vessels — absorb pressure spikes from fast-closing valves (flush valves, solenoids).
- Pressure/level gauges, water meters (sub-metering per zone/tenant).
Worked example 2.8 · Water hammer (why arrestors are not optional)
Given: flow velocity v = 2 m/s suddenly stopped; pressure-wave speed in steel a ≈ 1,200 m/s; ρ = 1,000.
Joukowsky surge: ΔP = ρ·a·v = 1,000 × 1,200 × 2 = 2.4 × 10⁶ Pa = 24 bar spike — on top of static.
A 24 bar transient can burst fittings/joints. Fit water-hammer arrestors near quick-closing valves and limit closing speed; verify with transient analysis on long risers.
Code
Backflow prevention: SBC 701/IPC, ASSE series. Surge: Joukowsky / hydraulic transient practice. Sub-metering: SBC 601 / authority.tap used→DRAINAGE + VENT STACKS→sewer
2.9 Soil, waste & venting
Role: carry foul water down to the sewer by gravity, while the vent system keeps air pressure in the stack near atmospheric so trap seals aren't siphoned or blown out. In tall stacks this is a serious design issue.
Method
Stacks sized by drainage fixture units (DFU) per code. Tall stacks need offsets, vent design, and pressure-attenuation because falling water builds pressure transients.
| Issue in tall stacks | Solution |
|---|---|
| Pressure transients (siphon trap seals) | Properly sized stack + vent; PAPA (positive air pressure attenuators) & AAVs where allowed |
| Terminal velocity / energy at base | Large-radius offsets, relief vents at offsets, robust base bend |
| Trap seal loss | Deep-seal traps; vent every branch per code |
Worked example 2.9 · Stack & vent sizing (concept)
Given: a soil stack serving 40 floors of guestroom bathrooms; total drainage fixture units exceed a single small stack's capacity.
From code DFU tables, select the stack diameter whose allowable DFU ≥ connected DFU (e.g. DN150 for a heavily loaded residential stack), with a parallel/secondary vent stack cross-connected at intervals, and offsets vented.
Stack diameter from DFU tables; add a vent stack + relief vents, deep-seal traps, and pressure attenuators on tall stacks. Verify against the code's discharge-capacity table — never by velocity alone.
In the software
Drainage is largely a code-table (DFU) exercise — done in Revit with discipline plug-ins or spreadsheets to EN 12056 / IPC / UPC. Specialist suppliers (e.g. studor/PAPA vendors) provide selection for venting components on super-tall stacks. Code
Sanitary drainage & venting: SBC 701, IPC/UPC, EN 12056. Trap seals & venting rules per the adopted code.2.10 Rain / storm-water drainage
Role: drain roofs, terraces and podiums. Two methods: conventional gravity (sloped, air in pipe) or siphonic (full-bore, self-priming — fewer, smaller pipes, ideal for large roofs). Sized on rainfall intensity × area.
Worked example 2.10 · Roof rainwater flow
Given: roof/terrace catchment 600 m²; design rainfall intensity 75 mm/h (local design storm).
Flow Q = area × intensity = 600 m² × 0.075 m/h = 45 m³/h ÷ 3600 = 12.5 L/s.
~12.5 L/s of rainwater to drain — select outlets & downpipes (conventional or siphonic) for that flow with overflow provision. Use the local storm intensity, not a generic value.
Code
Storm drainage: SBC 701, EN 12056-3 (incl. siphonic), IPC. Rainfall intensity: local meteorological/authority data.riser→FIXTURE (the tap)
2.11 Fixtures & terminals — the last piece
Role: the sanitaryware and outlets the user touches — WCs, basins, showers, bidets, kitchen/F&B and laundry connections. Each has a design flow and a water-efficiency rating.
| Fixture | Typical design flow / use | Efficiency lever |
|---|---|---|
| Wash basin tap | 0.1–0.2 L/s | Aerators / flow restrictors (~6 L/min) |
| Shower | 0.1–0.2 L/s | Low-flow heads (~9 L/min) |
| WC | Dual-flush 3/6 or 4.5 L cisterns; flush valves need high flow | Dual-flush / low-volume |
| Kitchen / F&B / laundry | Per equipment schedule | — |
Water efficiency
Low-flow fixtures cut demand → smaller tanks, pumps and pipes and support green ratings (Mostadam/LEED). But flush valves draw high instantaneous flow — size branches for them. Code
Fixture flow & efficiency: SBC 701, EPA WaterSense, SASO; accessibility fixtures: SBC/IBC accessibility. Green credits: Mostadam/LEED WE.2.12 Water reuse & treatment
Role: reduce potable demand and meet sustainability targets via greywater recycling (showers/basins → treated → toilet flushing/irrigation), condensate recovery (large in humid climates — AHU/FCU condensate is clean water), and water treatment (softening, filtration) to protect equipment.
Condensate recovery — free water
A big cooling system produces tonnes of condensate daily; recovering it for cooling-tower make-up or irrigation is a quick sustainability win in humid climates. Code
Reuse & treated-water quality: SBC 701, local authority (NWC/MEWA), WHO/EPA reuse guidance; cross-connection control essential.2.13 Installation, accessories & field tricks
| Item | Purpose / field rule |
|---|---|
| Pipe supports/hangers, anchors & guides | Spacing per material (plastic sags more than metal — closer supports); allow thermal movement on hot & long runs. |
| Expansion provision | PPR/HDPE expand a lot with temperature — fit expansion loops/arms; rigid anchoring cracks fittings. |
| Access & rodding points | At stack bases, offsets & changes of direction for drain clearing. |
| Trap seals (deep seal in tall stacks) | Use 75 mm seals; vent properly so they aren't siphoned. |
| Sleeves + firestopping; puddle flanges in wet areas | Fire-rated penetrations sealed with tested systems; waterproofing at slab penetrations. |
| Insulation (anti-condensation on cold; heat on hot) | Cold pipes in humid risers sweat — insulate & vapour-seal. |
| Dielectric unions between dissimilar metals | Prevent galvanic corrosion (e.g. copper to steel). |
| Isolation valves per zone/branch/wet-area | So one bathroom can be isolated without draining the riser. |
| Trace heating (where needed) & UV protection on roof pipes | Field detailing for reliability. |
Top field tricks
Pressure-test every section before closing walls; flush & chlorinate (disinfect) potable systems before handover; prime every trap and HW recirc loop at commissioning; never connect a flush-valve WC to an undersized branch; keep potable and non-potable (greywater) pipework clearly labelled and physically separated. Code
Supports: MSS SP-58. Firestopping: UL systems/NFPA. Disinfection & testing: EN 806-4, SBC 701, local authority; labelling per code.2.14 Testing & commissioning
- Pressure test all pipework (hydrostatic) before concealment; drainage tested by water/air & ball test.
- Disinfect potable system (chlorination), flush, and sample water quality before handover.
- Balance hot-water recirculation (confirm ≥55 °C return at index outlet); set PRVs and booster set-points; verify residuals at the highest fixtures.
Code
Testing/disinfection: EN 806-4, SBC 701; commissioning: BSRIA/CIBSE; Legionella validation: ASHRAE 188.Terms & abbreviations
Plain-English meaning of the plumbing terms used in this module.
| Term | What it means (plain English) |
|---|---|
| PHE | Public Health Engineering — the plumbing/drainage discipline. |
| Loading / fixture units (LU / FU) | A weighting given to each tap/fixture so you can estimate the likely simultaneous demand (not everyone uses water at once). |
| Hunter curve | The curve that converts total loading units into a probable design flow — it flattens as fixtures increase. |
| Demand (daily vs instantaneous) | Daily demand (m³/day) sizes storage; peak instantaneous flow (L/s) sizes pipes & pumps. |
| Break tank | An open tank that resets water pressure to atmospheric before re-pumping a higher zone. |
| Booster set | A multi-pump, variable-speed package that raises pressure to feed a zone. |
| Transfer pump | Pump that lifts water from low tanks to high-level tanks. |
| Residual pressure | Pressure still available at the highest/furthest fixture after losses. |
| PRV | Pressure-Reducing Valve — caps pressure so low-zone fixtures aren't over-pressured. |
| Recirculation (HW) | A loop that keeps hot water moving so every tap gets hot water quickly. |
| Legionella | Bacteria that grow in stagnant warm water; controlled by storing ≥60 °C and returning ≥55 °C. |
| TMV | Thermostatic Mixing Valve — blends hot & cold at the outlet to prevent scalding (~43 °C). |
| Backflow / air gap | Protection stopping used/contaminated water flowing back into the clean supply. |
| Water hammer / Joukowsky | A pressure spike when flow stops suddenly; the Joukowsky equation estimates it. Arrestors absorb it. |
| NPSH | Net Positive Suction Head — the suction condition a pump needs to avoid cavitation. |
| Soil / waste / vent stack | Soil = toilet drainage; waste = basins/showers; vent = pipes keeping air pressure stable so traps aren't siphoned. |
| DFU | Drainage Fixture Units — used to size drainage stacks from code tables. |
| Trap seal | The water held in a U-bend that blocks sewer gases; can be lost if the stack isn't vented properly. |
| PAPA / AAV | Positive Air Pressure Attenuator / Air Admittance Valve — devices that manage air pressure in tall drainage stacks. |
| Siphonic drainage | Full-bore, self-priming rainwater drainage — fewer/smaller pipes for big roofs. |
| Greywater / condensate reuse | Recycling lightly-used water (showers/basins) or AHU condensate for flushing/irrigation. |
References & software map
| Task | Software | Code/standard |
|---|---|---|
| Demand / loading units / pipe sizing | Revit MEP; spreadsheets to code tables | SBC 701; IPC/UPC; BS EN 806; CIBSE Guide G |
| Pressurised network & pump duty | AFT Fathom | Hydraulic Institute; SBC 701 |
| Transient / surge (water hammer) | AFT Impulse; Bentley HAMMER | Joukowsky; engineering practice |
| Pump / booster / heater selection | Grundfos, Wilo, Lowara, calorifier/heat-pump tools | NPSH (HI); SBC 601 |
| Drainage & venting; siphonic storm | Revit + plug-ins; supplier tools (PAPA/siphonic) | SBC 701; EN 12056; IPC/UPC |
- SBC 701 — Saudi Sanitary (Plumbing) Code; SBC 601 — Energy; 2018.
- IPC / UPC; BS EN 806 (supply); BS EN 12056 (drainage incl. siphonic).
- ASHRAE 188 & CIBSE TM13 / HSG274 (Legionella); CIBSE Guide G (Public Health).
- NSF/ANSI 61 (potable contact); ASSE backflow series; EPA WaterSense; SASO/GSO water quality.
- MSS SP-58 (supports); Hydraulic Institute (pumps/NPSH); NWC/MEWA local requirements.
Note
Worked numbers are realistic teaching values to show the method, not stamped project calculations. Drainage and demand are governed by code tables — confirm against the adopted code edition and the project spec.