How the BS EN 806-3 simplified method uses loading units to turn combined demand into a pipe size.
Why size by loading units
A cold water installation is sized so that every outlet receives enough flow when several are used at once, without pipes so large that water sits and warms. BS EN 806-3 gives a simplified method for common installations that uses loading units to estimate the combined demand.
The method in steps
- Give each draw-off point a loading unit value from the table in the standard. A loading unit is a weighting that reflects how heavily an outlet is used and how much it draws.
- Add the loading units for every outlet served by the section of pipe being sized.
- Convert the loading unit total to a design flow rate in litres per second using the conversion curve or table. Because outlets are rarely all open together, the design flow is well below the simple sum of every full flow rate.
- Choose a pipe size that carries that flow without excessive velocity (often kept below roughly 2 m/s to limit noise and wear) and without losing too much pressure over the run.
Worked example (illustrative figures)
Assume a section feeds outlets totalling 16 loading units. Reading that against the conversion in the standard might give a design flow of roughly 0.4 l/s; always read the actual value from the table. You then select the smallest pipe whose velocity at that flow stays within the limit and whose pressure loss over the pipe length still leaves enough residual pressure at the outlets.
Confirm all loading-unit values and conversions against the current BS EN 806-3 and BS 8558 before relying on them.
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