Positive air pressure transient propagation in building drainage and vent systems

A major objective of drainage research over the past 100 years has been a reduction in the complexity of building drainage vent systems associated with the retention of appliance trap seals. Further simplification requires that the system operation is recognized as time dependent, where changes in water flow conditions result in the propagation of air pressure transients. Negative pressure transients that reduce trap seal levels by induced siphonage are well understood. More problematic is the propagation of positive air pressure transients, generated by stack or branch surcharge. The paper identifies the sources of positive air pressure transient propagation and demonstrates that such transients may be described by the St Venant equations of unsteady flow. Solutions are provided based on the proven finite difference methods and the method of characteristics and an understanding of the boundary conditions that represent the constituent components of a building drainage and vent system. © The Chartered Institution of Building Services Engineers 2004.