Plumbing vents are a vital part of indoor plumbing systems, but few people outside the plumbing industry know much about them. A basic understanding of the purpose and workings of plumbing vents can help any homeowner avoid dangers and take care of basic plumbing maintenance.
Figure 1: The vent system is an esential part of the plumbing system.
A building's drainage and waste disposal system consists of thee parts - the drainage system, the waste system and the venting system. Drainage pipes carry waste water from the sink and shower drains. The waste pipes carry waste from the toilet so that it is separate from the sink and shower drains. This prevents toilet waste from backing up into the sink and bathtub if there is a blockage. Plumbing vents are pipes that run from every drain to the outside of the building, usually through the roof. They have two purposes. The first is to divert sewer gasses out of the building to the outside air. This prevents a dangerous build up of gas and bacteria. Without those vents, bacteria and gasses from the septic system would be released into the house where they could make people very ill. In addition, plumbing vent pipes help equalise pressure in plumbing pipes when water goes through a drain. Without plumbing vents to let the sewer gas escape and bring in air to push water through the pipes, waste water can back up into drains and toilets and become a health hazard.
Joseph Bramah, a London cabinetmaker, invented the water closet in 1778. The plumbing for the flush toilet had no vent, a serious flaw in the design. Noxious gasses built up in water closets with sometimes fatal results. It only took four years for a solution to be found. In 1782, John Gallait patented a "stink pipe", which consisted of a drain trap and a vent pipe. Nearly all improvements to the modern flush toilet and indoor plumbing systems are based on Gallait's patented stink pipe vent design.
Diagram of venting for a bathroom sink.
The easiest way to understand the function of plumbing vents in your home plumbing system is to imagine draining a soda bottle. If you hold the bottle upside down at a 90 degree angle, the soda can't drain properly. As the bottle drains, it creates negative pressure inside the bottle because there is nothing to replace the soda flowing out. If you punch a hole in the top of the bottle, though, air is drawn into the bottle through the hole, keeping the pressure equal and allowing the soda to flow freely. Plumbing vents serve the same purpose as the hole in the top of the bottle. If there is no way to introduce air into a closed plumbing system, negative pressure is created and the water can't drain freely through the drain pipes. At the same time, the vent system allows gas from decomposing waste to escape the house so that it doesn't build up and cause foul smells and toxic sewer gas build-up. Each drain in the house has both a vent pipe and a trap. The trap is an essential part of the vent system because it seals the pipe off and keeps air from escaping into the house. Most traps are S or P-shaped pipes just below the drain. Their purpose is to trap water, which forms a barrier preventing air from entering the plumbing system through the drain.
Vent pipes usually exit through the roof of a house or into the attic. Every city, state and town has specifications for the size of pipe that must be used for plumbing vents. Those specifications can usually be found in the town's health codes under sanitation. In most states and cities, new plumbing fixtures, including sinks, tubs and toilets, must meet code specifications. The town may require that plans be filed with the Sanitation Board or Health Department, and inspection upon completion of the work. This is important to make sure that all plumbing meets the town sanitation codes to protect the health of the home's residents and the community.
There are several types of plumbing vents that serve different functions in a building plumbing system. The WASTE STACK is the main vertical vent pipe that runs up through the roof of the house to vent sewer gas out of the house. The term refers to any vent stack that doesn't carry waste from a toilet or sanitary fixture. The SOIL STACK is the vertical vent pipe that carries soil and waste from toilets and sanitary fixtures in the house. A DIRECT VENT is the term used when a plumbing fixture like a sink drains directly into the waste stack. Typically, direct venting is only allowed if the drain is very close to the waste stack. A STACK VENT is an extension of the main waste stack that runs from a fixture to the waste vent. If a drain is more than a few feet away from the waste stack, it requires its own stack vent. A DRY VENT is a section of the vent stack that does not carry water. A WET VENT may be used when a toilet is located a distance away from the waste stack. It is typically used if another plumbing fixture is located between the toilet and the main waste stack.
Improper venting of plumbing fixtures or blocked plumbing vents can cause many plumbing and health problems. An odour of sewer gas in the house or building usually means that a vent is leaking. Slow drainage and funny noises from the drains when the toilet is flushed or a sink drains often means that a plumbing vent is blocked. A licensed plumber can easily find either problem, but you can make it easier by doing a little investigation on your own. Look for leaks around plumbing traps. A leaking trap may fail to provide a water seal and allow sewer gas to back up into the building. Check under sinks for moisture or mould. Look for dry plumbing fixtures. When a sink or toilet is not used for a long time, the water in the trap may evaporate, opening the seal and letting sewer gas into the house. Check laundry sinks in the basement or toilets in bathrooms that may not be used. In a multi-family dwelling, check the toilets in empty apartments. Often, all you need to do to cure the problem of a dry fixture is run the water or flush the toilet. Check for clogged traps. If only one or two drains are slow, check the traps for those fixtures for blockage. Cleaning out the trap yourself is easy if you have basic handyman skills.