Every modern plumbing design includes a Drain-Waste-Vent (DWV) system. The DWV system removes waste and grey-water from a structure while venting noxious gases to atmosphere. Waste produced at fixtures (toilets, sinks, showers, etc.) flows through a trap (curved section of pipe; usually an ‘S’ or ‘U’ shape). The trap must always contain water, which serves to trap gases, preventing return to the fixture and into the structure. Each trap is connected to the waste line to simultaneously remove gases via the vent pipe while connecting solid and liquid waste to the drain. The system ultimately removes all wastewater through the building drain into the sewage line. The sewage line carries wastewater to either a private septic system or public sewage treatment plant.
What the Vent Does
A plumbing vent system, connected to the waste lines:
- Vents noxious gases produced by raw sewage to the outside — above the roof of a structure.
- Admits oxygen into the sewage system to promote aerobic digestion of sewage
- Equalize pressure on both sides of traps; allowing water to remain in each trap and for sewage to maintain gravity flow
Each of the vent’s purposes is equally important for the proper operation of a DWV system. Drainage and waste pipes must be laid at a negative (downward) slope to allow gravity flow out of a structure.
Hydraulics of Vent Systems
As a quantity (column) of wastewater passes through the pipe, air present in the pipe is compressed. This compression forms a positive pressure, that left un-released would exert pressure in an opposite direction of wastewater flow. Additionally, pressure would be exerted on any trap seals downstream of the origination of flow.
High positive pressures in wastewater systems could result in the wastewater being pushed into fixtures, potentially breaking the trap seals. If too forceful, the resulting broken seals can create hazards to both health and hygiene of occupants. In tall buildings (3+ levels), vent stacks are placed parallel to waste stacks to provide adequate venting of the structure.
As this same water column flows past any given point, negative pressure will occur if air does not flow behind the water. Excessive negative pressure can potentially siphon water from plumbing fixture’s trap seals, allowing noxious fumes into the structure. With the shortest trap seal, toilets are most prone to this induced siphon effect.
Vent to Atmosphere
Most common with regard to residential construction in North America, DWV is vented to atmosphere through the structure’s roof. Most often constructed from plastic (ABS or PVC), DWV-rated pipe, the roof penetration is flashed to prevent weather infiltration. Older homes may have vent pipes constructed from; copper, iron, lead or clay pipe.
Blockage of vent pipes is a common problem, evidenced by:
- Bubbles when toilet is flushed
- Slow drainage from any fixture
- Dry (empty) trap at any fixture; resulting in noxious fumes in the structure
Regular visual inspection of vent stacks will avoid this problem. Common blockages include (but not limited to):
- Dead critters
- Critter nests
- Leaves and other debris
Air Admittance Valve (AAV)
A negative pressure activated, one-way mechanical valve that eliminates the need for traditional roof penetration with the vent-to-atmosphere design. Wastewater discharge opens the AAV, releasing the vacuum to allow for proper drainage. Only applicable for negative pressures, the AAV will not function for any positive pressure situations (sump pumps, etc.). Offering a number of advantages over vent to atmosphere, the AAV is prohibited by some US jurisdictions:
- Reduction in materials results in more efficient, cost effective installation
- Greater flexibility of plumbing layout and design
- Eliminate roof penetration problems at vent stacks
Certified to open/close a minimum of 500,000 times, some manufacturers claim successful testing of up to 1.5 million cycles (80 years of use). Warranties for effective sewage venting range from 20 years to lifetime! Be sure to check local building codes and ordinances before finalizing your Coquitlam Plumbing System Design with an AAV.