How do we calculate?
EN 12056-2 (System I): the discharge units (DU) of all appliances are summed (washbasin 0.5; shower, bath, sink, machines 0.8; WC 1.8-2.5 L/s). Wastewater flow Qww = K × √ΣDU; it cannot be less than the DU of the largest single appliance. The stack size is selected from the vertical pipe capacity table of the standard (e.g. with primary ventilation DN100 4.0 L/s, DN125 5.8 L/s, DN150 9.5 L/s).
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