Collecting and storing water is only half the job. The infrastructure that moves water from tank to tap — pipes, pumps, fittings, valves, and control devices — determines whether the system delivers reliable pressure and flow, remains safe from cross-contamination, and operates economically over its design life. This chapter covers the hydraulics of residential water distribution and the equipment needed to implement it.
Different materials suit different applications. The key selection criteria are: working pressure, temperature range, UV resistance, chemical compatibility, ease of installation, and cost.
| Material | Common use | Max. temp. | Pressure rating | Notes |
|---|---|---|---|---|
| PVC-U | Cold water mains, drain | 20°C | 10–16 bar | Brittle; do not use for hot water |
| CPVC | Cold and hot water | 90°C | 10 bar | More expensive than PVC-U |
| PEX (cross-linked PE) | Hot and cold; underfloor | 95°C | 10 bar | Flexible; push-fit fittings; excellent for retrofits |
| Polyethylene (PE/HDPE) | Cold mains, buried supply | 20°C | 6–16 bar | Excellent chemical resistance; used for buried runs |
| Copper (Type B/C) | Potable cold and hot | 120°C | 10+ bar | Long-established; antimicrobial; expensive |
| Galvanised steel | Older systems | 80°C | 10 bar | Corrodes internally; avoid for new installs |
| MDPE (medium-density PE) | Buried cold supply | 20°C | 12 bar | Common for external supply pipes (blue for potable, black for other) |
For rainwater/greywater systems:
Flow velocity in distribution pipes should stay within:
Given a flow rate Q (L/min) and target velocity v (m/s), the required pipe cross-section area:
A = Q / v (both in consistent units: m³/s and m²)
Pipe internal diameter:
D = 2 × sqrt(A / π)
Example: Peak household flow = 20 L/min = 0.000333 m³/s, target velocity = 1.5 m/s
A = 0.000333 / 1.5 = 0.000222 m²
D = 2 × sqrt(0.000222 / π) = 0.0168 m = 16.8 mm
Use 20 mm ID pipe as the next standard size up.
For water at ambient temperature in smooth plastic pipe, the Hazen-Williams equation gives friction head loss per unit length:
h_f = 10.67 × L × Q^1.852 / (C_hw^1.852 × D^4.87)
Where:
| Material | C_hw |
|---|---|
| PVC, PEX, HDPE (smooth plastic) | 140–150 |
| Copper | 130–140 |
| Cast iron (new) | 130 |
| Cast iron (old, corroded) | 80–100 |
| Galvanised steel | 100–120 |
def hazen_williams_loss(L, Q_lpm, C_hw, D_mm):
"""Head loss in meters. Q in L/min, D in mm."""
Q = Q_lpm / 60000 # convert L/min to m³/s
D = D_mm / 1000 # convert mm to m
h_f = 10.67 * L * (Q ** 1.852) / (C_hw ** 1.852 * D ** 4.87)
return h_f
# Example: 30m of 25mm PVC, flow 20 L/min
loss = hazen_williams_loss(30, 20, 150, 25)
print(f"Friction head loss: {loss:.2f} m")
Rule of thumb: For residential plastic pipe, friction losses should not exceed 5 m per 100 m of pipe at design flow. If they do, upsize the pipe.
Fittings (elbows, tees, valves) add “minor losses” — additional head loss beyond straight pipe friction. These are expressed as an equivalent pipe length added to the actual pipe length, or as a loss coefficient K applied to velocity head (v²/2g).
| Fitting | Equivalent pipe lengths (×D) | Typical K |
|---|---|---|
| Gate valve (fully open) | 7 | 0.1 |
| Ball valve (fully open) | 3 | 0.05 |
| 90° elbow | 30 | 0.9 |
| 45° elbow | 16 | 0.4 |
| Tee (flow through) | 20 | 0.6 |
| Tee (flow branch) | 60 | 1.8 |
| Check valve | 50–100 | 2–3 |
For a typical residential run with 10 fittings, add 30–50% to the calculated straight pipe friction loss as a fittings allowance.
The most common choice for residential rainwater systems. A motor-driven centrifugal pump draws from the tank via suction pipe and delivers pressurised water to the distribution network.
Key parameters:
Installed inside the tank, submerged in water. No priming needed; quieter operation; suitable for deep underground tanks.
Advantages: No suction lift limitation; quieter; no priming Disadvantages: Harder to access for maintenance; requires watertight cable entry
The standard residential configuration: a surface centrifugal pump with an integrated pressure vessel (accumulator). The pressure vessel stores pressurised water (pre-charged to ~70% of switch-off pressure) so the pump does not start for every small draw.
Direct-drive solar pumps (no battery) are practical for garden irrigation and tank-to-tank transfers where demand coincides with daylight. For domestic pressure supply requiring 24/7 availability, a battery-backed solar system or mains-powered pump is needed.
TDH is the total head the pump must provide, accounting for all losses and elevation differences:
TDH = Static head + Friction losses + Minor losses + Delivery pressure required
Example:
TDH = 6.5 + 2.3 + 1.0 + 10.2 = 20 m
Select a pump rated for Q = design flow rate at H = TDH. Use the pump performance curve to verify: the operating point (Q, H) must lie on the pump curve.
Pump hydraulic power:
P_hydraulic (W) = ρ × g × Q × H = 1000 × 9.81 × Q(m³/s) × H(m)
P_electrical (W) = P_hydraulic / η_pump
For Q = 20 L/min = 0.000333 m³/s, H = 20 m, η = 0.50:
P_electrical = 1000 × 9.81 × 0.000333 × 20 / 0.50 = 130.8 W
The accumulator tank prevents the pump from short-cycling (starting and stopping every few seconds), which shortens pump life.
Sizing logic:
For a system with 1.5 bar cut-in and 3.0 bar cut-off, pre-charge at 1.35 bar:
Larger pressure vessels reduce pump start frequency and extend pump life.
When non-potable water (rainwater, greywater) is distributed alongside municipal potable supply, the two networks must be completely separate. There must be no physical connection between the potable and non-potable networks anywhere in the building.
Legal requirement: Cross-connection between potable and non-potable supplies is prohibited everywhere and can result in enforcement action, invalidated insurance, and health liability.
Identification standards: | Jurisdiction | Non-potable pipe colour | Potable pipe colour | |————-|————————|———————| | Australia | Purple/lilac | Blue (buried), copper/white (internal) | | UK | Green (rainwater) | Blue (buried), copper/white (internal) | | EU (various) | Purple or green labeling | Standard colours | | USA | Purple (recycled water) | Blue or standard |
Labeling: At every valve, tap outlet, and inspection point, non-potable outlets must be clearly labelled: “NON-POTABLE WATER — DO NOT DRINK” (or local language equivalent).
Where non-municipal water can potentially flow backwards into the municipal supply (e.g., at a mains top-up connection), a backflow prevention device is legally required.
Types of backflow preventer: | Type | Protection level | Common use | |——|—————–|————| | Air gap | Highest (no physical connection) | Tank mains top-up inlet above water surface | | Reduced pressure zone (RPZ) valve | High | Where pipe connection is unavoidable | | Double check valve | Medium | Low-hazard back-siphonage risk | | Single check valve | Low | Not permitted for cross-connection risk |
Air gap is the simplest and most reliable: the mains top-up inlet pipe terminates above the maximum water level in the tank, so mains water cannot be sucked back. The gap must be at least 25 mm and ideally 2× the pipe diameter.
An RPZ valve is required where an air gap is impractical (e.g., pressurized systems) and where the cross-connection risk is classified as “high hazard” (e.g., tanks storing greywater or recirculated water).
If a tank can be elevated to provide the required head (typically for small storage, garden irrigation, or passive toilet supply), pumps can be eliminated.
For gravity-fed toilet flushing, minimum useful pressure is ~0.2 bar (2 m head). This is achievable with a tank mounted 2.5–3 m above the toilet cistern fill valve — feasible in some attic configurations.
For shower gravity supply (minimum ~0.5 bar = 5 m head), the tank base must be at least 5–6 m above the shower head — only practical in multi-storey buildings with a high loft.
Rule: If you cannot achieve the required head by elevation, use a pump. Trying to force a gravity system with insufficient head results in poor user experience and inadequate fixture performance.
Previous: Chapter 4 — Tank Sizing and Storage Engineering
Next: Chapter 6 — Clean Enough: Filtration and Water Treatment