Sanitary Piping Best Practices 3

This week in our sanitary piping series were covering the last element of a sanitary plumbing system; discharge stacks.


Discharge Stacks

All branch discharge pipes eventually reach the discharge stack; the pipe that discharges the sanitary waste into a drain.

A key property of a discharge stack is that it is vented ( as opposed to a stub stack which is often unvented and much shorter to accommodate for this). Ventilating a discharge stack is achieved through the use of branch ventilating pipes.


Branch Ventilating Pipes

  • Each branch ventilating pipe must be connected from the discharge stack to within 750 mm of the trap (on the branch discharge pipes) to allow waste gases to escape from the point at which their flow is blocked (i.e. the trap).
  • To ensure that only gases rather than waste liquids flow along the ventilating pipe, it must be installed with a continuous incline from the discharge pipe to the ventilated stack.
  • It is possible for a branch ventilating pipe to run directly to the outside air. If this approach is taken, ensure that the pipe outflow is at least 0.9 metres above any opening such as windows or doors into the building. This is key, as the gases that will be escaping from these vents typically have a sewer smell.
  • If the branch discharge pipe connected to the branch ventilating pipe serves one appliance, then the diameter of the ventilating pipe should be 25mm or more.
  • If the ventilated branch discharge pipe is longer than 15 meters or has 6 or more bends then the diameter of the ventilating pipe must be 32mm or more.


Rodding Points

If a pipe gets blocked, a rodding point can provide an easy way to clear the blockage without taking apart the plumbing system. Rodding points should be space such that any length of the discharge pipes can be accessed without having to remove traps. In a discharge stack or a stub stack, the rodding point should be above the spillover level of appliances.


Ventilation of Discharge Stacks

  • Discharge stacks must be ventilated to reduce pressure caused by the build up of gas.
  • A build up of this gas can overcome the water seal contained within a trap and allow the gases to escape into the home.
  • All discharge stacks connected to a drain that may experience surcharging or is near an intercepting trap must have a ventilating pipe of 50mm diameter or more, at the output site.
  • All discharge stacks must have a stack ventilating pipe at the dry part above the highest branch discharge pipe or branch ventilating pipe of a minimum diameter of 75mm.

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