Investigation on cross-vent design in building drainage system by numerical simulation approach

Air pressure fluctuation in building drainage system is always a problem if proper venting is not provided. Excessive positive pressure may push the foul to interior of the building through the trap seal. Excessive negative pressure may empty the trap seal. Such consideration is especially important for vertical drainage stack. In presence, secondary ventilated system as specified in British Standard BS12056–2 has been widely adopted in Hong Kong. The system adopted a vertical vent stack running in parallel with a vertical drainage system. To reduce the pressure fluctuation inside the drainage stack, cross-vents are provided to connect the drainage stack to the vent stack at every two or three floors. The purpose of the cross-vent is to relief the excess air pressure inside the drainage stack. However, investigation on the design of the cross-vent is still very limited. This study adopted computational fluid dynamics techniques to investigate the cross-vent design. This study found that, practically, cross-vent should be provided to each floor which is in line with the recommendations stated in China standard and British standard.