Distribution carries water from the tank to the fixtures through one supply main with a shutoff valve at the building entry, short branches to the kitchen sink and wash basin, and a tap at each outlet that ends above the rim of what it fills. Size the pipes for the taps that can run together: two taps at 0.33 L/s each need 0.66 L/s, which loses 5.5 m of head in 20 m of 20 mm pipe but only 1.85 m in 25 mm pipe.
Distribution and fixtures are the last part of a gravity water system: the pipes, valves and taps inside and around the buildings, and the drains that take used water away. The pressure and flow at each fixture depend on the head left after friction.
The Pieces, In The Order The Water Meets Them
- Tank outlet and isolation valve. Lets you shut the tank off from the house.
- Supply main to the building, buried or protected from frost and sun.
- A shutoff valve at the building entry (parent page). One valve to stop the water if a pipe bursts indoors.
- Branches to the kitchen sink, wash basin and any other fixtures.
- Taps and valves at each outlet (Simple Taps & Valves).
- Drains from the fixtures to the greywater system, sloped at least 2% (parent page). The toilet is not connected: the composting toilet is waterless (Greywater vs. Blackwater).
The layout inside the house is on Indoor Plumbing Layout; hot water is on Hot Water Systems.
Size For The Taps That Run Together
Pipe Sizing sets the design flow for a few taps as the number that can run at once times the flow of one tap, measured with a bucket: 20 L in 60 s is 0.33 L/s.
Worked example: two taps, the kitchen tap and the basin, run together, 2 × 0.33 = 0.66 L/s through a branch of 20 m. Hazen-Williams with C = 140 (the plastic value that page uses) gives the head lost:
| Pipe bore | Loss in 20 m at 0.66 L/s |
|---|---|
| 20 mm | 5.5 m |
| 25 mm | 1.85 m |
| 32 mm | 0.56 m |
With 10 m of head available at the building, a 20 mm branch uses over half of it before the tap, leaving 4.5 m, below the 5 m (0.5 bar) that the sizing page asks for at a tap. A 25 mm branch leaves 8.15 m. Use the pipe’s real bore, not the label (Calculating Head Pressure).
Fixtures
- Taps end above the rim of the sink or basin, with an air gap, so dirty water cannot be drawn back (Plumbing Separation).
- Sinks and basins drain into the greywater line through a trap and an air-break.
- A washing station for hands at every toilet and kitchen: soap or ash.
- Showers use more water than a tap and give a lot of greywater; add their flow to the total and re-check the branch size.
- Standpipes for outdoor use are covered in Standpipes and Taps.
What You Need
- Pipe, fittings and valves of one system, with the same bore on a run
- A shutoff valve that is accessible, and a drain valve at the lowest point
- Taps of a known type (Tap Types)
- A plan of the pipe runs, kept for repairs
Method: Lay Out, Fit, Test
Step 1: Draw the plan: main, entry valve, branches, fixtures and drains.
Step 2: Lay the main and fit the entry valve in a protected, reachable place.
Step 3: Fit the branches with a shutoff valve at each fixture if possible.
Step 4: Fit taps and drains, check the air gaps and the 2% fall on drains.
Step 5: Open the system slowly, bleed the air, and test every joint under pressure (Salvaged Modern Pipe for the test).
Common Mistakes
- Branches too narrow for the taps that run together.
- No entry shutoff. A burst pipe empties the tank.
- A hose from a tap into a bucket or sink.
- Draining the toilet into the greywater line.
Safety
Pressurised pipes burst, and a join to a non-drinking pipe can contaminate every tap
Open valves slowly and never test a line with compressed air. A tap closed fast on a moving line can raise the pressure by about 3 bar. Keep greywater and drinking water pipes unjoined, use air gaps, and shut the entry valve at once if a pipe fails.
What’s Next
- Learn next: Simple Taps & Valves — the fittings at each outlet
- Combine with: Indoor Plumbing Layout — fit it into a house