There are few real mysteries remaining about the mechanisms at play in building drainage and vent systems. This has been well understood from the beginning of modern sanitary engineering at the end of the 19^th Century. The description of Building drainage and vent system operation is best understood in the context of engineering science in general and fluid mechanics in particular.

Early researchers in the field were well aware of this and many examples of the application of sound fluid mechanics are available as evidence. Much research has been carried out since the end of the World War II, where, particularly in Europe, extensive reconstruction work prompted the quest for more efficient approaches to drainage and vent system design.

At the center of the system’s integrity is the water trap seal, which stops foul air from entering a habitable space from the sewer. The water trap seal is usually 1½ or 2 inches in depth depending on the fixture it is protecting.

It comes as a surprise to many that the flow of air is as important, if not more important, than the flow of water, to the safe operation of the drainage system. This air flow is ‘induced’ or ‘entrained’ by the flow of water. The unsteady nature of the water flows causes pressure fluctuations (known as pressure transients) which can compromise water trap seals and provide a path for sewer gases into the habitable space.    

Transients can be dealt with by a combination of careful design and the introduction of pressure relief devices as close to the area of concern as possible. Long vent pipes can be an inefficient way of providing relief due to friction in the pipe. Distributing air supply inlets using AAVs around a building provides an efficient means of venting and it reduces the risk of positive transient generation. AAVs do not cause positive pressure transients, they merely respond to them by closing, and hence reflect a reduced amplitude wave.

In tall buildings parallel vent pipes can only provide a small relief path for a positive pressure transient (approx 1/3 if the vent pipe is the same diameter as the main vertical stack) thus a wave will still propagate throughout the rest of the system that could compromise water trap seals. The introduction of a positive air pressure transient alleviation device provides a means to ‘blow off’ pressure surges as close to their source, thereby protecting water traps. Attenuation of up to 90% of the incident wave can be achieved, thus protecting the entire system. There is little that can be done for a system experiencing a total blockage, generating excessive static positive pressures in the drainage system. In such circumstances the lowest water trap seal will ‘blow’ providing relief for the whole system. This will occur regardless of the method of venting employed.

In validated test simulations air admittance valves (AAVs) have been shown to provide as least as good protection for water trap seals as a fully vented system, and in tall buildings in some circumstances, even better. The fully engineered designed active control system utilizing AAVs for negative pressure relief and Positive Air Pressure Transient Attenuators (PAPAs) for positive transient relief is shown to be an effective method for balancing the need for safety and efficiency while maintaining  functionality invisible to the user.

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