3 Methods to Protect Trap Seals in Vancouver Plumbing

Your home’s Vancouver Plumbing system can be designed as either a ‘complex system’ or a ‘simple system’. Experience has determined that a simply designed system is much more effective with regard to operation. As such, the most prevalent problem encountered in a home’s sewage system is trap failure. The trap of each fixture is designed to hold a constant quantity of water (replenished as wastewater passes through system). Forming a barrier to prevent noxious gases from wastewater entering your home. Should the seal be broken, this barrier is removed.

Causes of Trap Failure

As discussed in DWV Systems, each plumbing fixture is plumbed with a trap to prevent wastewater from flowing into your home. The most common causes of trap failure are
as follows:

  • Siphonage
  • Evaporation
  • Back Pressure
  • Capillary Action
  • Leakage
  • Sediment Accumulation
Hydraulics of Traps in Vancouver Plumbing


To understand the workings of plumbing traps, a short lesson about hydraulics is necessary.

hydraulics (noun): 1. The branch of science concerned with the conveyance of liquids through pipes and channels, esp. as a source of mechanical force or control. (Merriam-Webster)

Simply stated, with equivalent pressures on both sides of a curved container (in this case the trap), water seeks its own level. Using the diagram of ‘trapped’ water, assume:

atmospheric press + water pressure = in both legs of trap

Under these conditions, the water level remains constant and equal in both legs
(entrance and exit) of the trap. Should the pressure be disturbed, causing a sudden and drastic increase or decrease in only 1-leg of the trap, the seal will be broken. Additionally, sediment build-up, capillary leaks etc. will cause the pressure to build-up and break the seal as well.

A common cause of hydraulic disturbance in a plumbing Drainage Waste Vent (DWV) system occurs in multi-level buildings. The trap seal can be broken when wastewater is flushed through the system from an upper level of the structure. As this ‘slug’ of water passes down the waste line, it creates a partial vacuum between the origin of the flush and the back-end of the slug. Hydraulics dictates that as this partial vacuum passes an opening in the system, the system will attempt to regain its pressure equilibrium. The partial vacuum will ‘suck’ air behind it when passing each opening in the pipe (via the trap). When air is pulled behind the water slug, the trap’s water seal is broken as it is sucked behind the flushed water.

Protecting Trap Seals

There are 3 methods adopted to protect a home’s trap seals:

  1. Ventilate Individual Traps – described in detail by James C. Bayles in “House Drainage and Water Service” (1898), is the oldest and most natural course of action. This ‘trap-vent’ method is expensive and complicates a plumbing design; the venting of traps is not efficient at preventing siphoning. 
  2. Cesspool (Pot) Trap – provides a trap large enough so as not to be affected by pressure disturbances or evaporation. Although a pot-trap is inexpensive, the risks far outweigh any benefits. Allowing large pools of wastewater to putrefy is both harmful to the homeowner and frustrating with regard to purification (at the municipal level).
  3. Anti-Siphonic/Self-Cleaning Trap – elegant design of this system makes it effective for preventing siphoning and for remaining clean. As the most simple solution for preventing trap seal failure, this is also the least expensive option available today.