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Last edited 14 Jul 2021
Secondary Ventilation Stacks in Tall Buildings
 Why are secondary drainage stacks required?
When installing drainage and ventilation pipework in high-rise buildings, traditional design standards dictate that a secondary ventilation stack has to be used to overcome air pressure changes. This is to account for the positive and negative air fluctuations that can compromise water trap seals due to the unsteady flow of water through the pipework. Incorrect air balance can result in blowback and siphoning of the water seal in the trap, which could pose a public health risk.
To maintain stability in a drainage system, it is necessary to alleviate this change in air pressure immediately. This response time is paramount for protecting water trap seals, with traditional secondary pipework systems thought to be the only way to provide this function.
 Considerations for designing drainage and ventilation
There are many factors to consider when installing secondary pipework systems, which continue to be the industry norm. For Instance, installing a secondary stack ventilation system requires more installation time and materials, and in turn takes up valuable floor space, a major factor when limited space is taken into consideration. With architects often designing buildings to a small footprint, particularly in city centres with limited space, the inclusion of a secondary stack can hinder the design process.
Rather than a secondary ventilation piping system, air regulatory and attenuator valves can be installed in high-rise buildings drainage stacks to balance negative and positive air pressure fluctuations.
There are potential savings to be made through the removal of secondary vented drainage systems. As less vents are required there is also a reduced requirement for floor penetration fire collars, something a secondary vent system would always require.
The removal of a secondary vent also reduces the drainage system footprint, meaning there is greater flexibility in the design of internal drainage systems. This increased flexibility means that building designs, and room layouts, are no longer dictated by strict pipework systems. This provides architects with more freedom to design buildings, without having to compromise. Building owners also have more useable, and consequently sellable space.
Secondary venting also proves to be a much less effective solution for very tall, or complicated, drainage systems as the time lag that occurs when communicating a change in ambient airflow in the pipework can result in an ill-performing drainage system. This would require expensive maintenance and repair throughout the life of the system.
 Alternatives to secondary ventilation stacks
Products such as the Polypipe Terrain P.A.P.A® and Pleura valves are a proven alternative to traditional secondary vented drainage systems. The P.A.P.A® (positive air pressure attenuation) valve is designed to react to and attenuate positive pressure transience within the drainage stack of high-rise developments providing a suitable venting solution for any multi-storey building.
The Pleura valves are designed to allow air into the system when negative pressures are experienced. When these two valves are combined, the effects of positive pressure such as blown sink and WC traps and negative pressure siphoning traps are mitigated, thus removing the need for a traditional secondary vented pipework system.
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