Sizing cold water pipes by loading units (BS EN 806-3)

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.
• Use the loading-unit total in the applicable sizing tables or curves for the selected method and pipe material. Some methods give a design flow before selecting the bore; others provide simplified size selection. Do not mix loading-unit values from different methods. The method must allow for realistic simultaneous demand, including any use that really requires full flows together.
• Check that the selected size provides the required flow and residual pressure within the method’s limits. Use the appropriate velocity limit for the material and duty; around 2 m/s is a common teaching example, not a universal limit for every installation.

Worked example (illustrative figures)

For this illustrative exercise, a section has 16 loading units and a design flow of 0.4 l/s is supplied as an assumption. This is not a claim that every method or standard table converts 16 loading units to 0.4 l/s: obtain the real sizing values from the applicable method. 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.

Velocity, simultaneous use and pressure under flow

• A selected velocity limit helps manage flow noise, pressure transients and erosion risk. The limit depends on the pipe material, duty and design method; the worked example uses around 2 m/s.
• Shared pipework must carry the combined flow of the outlets likely to be open together. Sizing each outlet in isolation undersizes the common run, which then works for one outlet and fails as soon as several are in use.
• Adding every full outlet flow can oversize an ordinary domestic system where those outlets are not normally all used together. Some uses do require simultaneous full flow, so check the demand pattern before applying diversity. Unnecessary pipe volume can slow turnover and increase heat gain.
• Static pressure is read with nothing flowing, so it can hide the friction losses that appear during draw-off. Check the running (dynamic) pressure at the outlets as well.

Worked velocity check (illustrative figures). Flow rate equals cross-sectional area times mean velocity, so velocity = flow ÷ area, working in metres and seconds. A 15 mm bore has a radius of 0.0075 m, so its area is 3.14 × 0.0075 × 0.0075 = about 0.000177 m². A flow of 0.25 litres per second is 0.00025 m³/s, so the velocity is 0.00025 ÷ 0.000177 = about 1.4 m/s, below the illustrative 2 m/s limit.