The water that flows through a pipe in the sewer system creates negative pressure similar to the pressure created when sipping a liquid with a straw.

Moreover, negative pressure may slow water flow in the drain or even stop it altogether. Also, the negative pressure can empty the standing water in the P-traps that come with sinks, tub drains, and showers.

plumbing vent pipe installation


The negative pressure is countered by vent systems. As such, every home requires the inclusion of vents in the plumbing system.

Most homeowners assume that the only role of vents is allowing sewer gases and fumes to dissipate.

Although they offer that role, their main function is to allow the sewer and drain system to flow freely. They do so by allowing air to flow into the drain system, thereby equalizing the pressure within the pipes.


A Major Achievement in the Plumbing Industry

The design of plumbing systems that serve most homes was perfected two centuries ago. The use of P-traps is an extremely important invention in plumbing.

P-traps are the curved section of a drainage pipe. You can see this curve in most drains. The trap prevents sewer fumes from backing up into the home.

Earlier traps had a problem of emptying the trapped water whenever water got poured into the drain. Plumbing experts discovered that adding an extra pipe to a drainage allows air to flow into the drainage, thereby correcting the ineffectiveness of earlier traps.

The theory behind that discovery is known as Venting Theory. This theory was practically proven in 1874. Since making this achievement, the waste and drainage system became known as the waste, drainage, and vent system.


Specifications of a Venting System

The main section of a vent is known as the stack. The stack serves the master bathroom. It’s designed to extend downward into the sewer line and upward to the roof.

The vent opening is located on the roof. It extends 1 to 2 feet past the line of the roof. You can compare the main vent to a tree trunk, whereby smaller vents are the branches extending from the tree trunk to various fixtures including the washing machine, showers, sinks, and toilets.

Plumbing codes dictate that the main venting pipe should have a diameter of not less than 2 ½ inches if it’s serving at least two fixtures in the drainage systems.

The main venting pipe in a single-family home typically has a diameter of 3 to 4 inches. Each branch venting has a specific diameter depending on its length and the trap’s diameter.

Common diameters of branch vents are 2, 1 ½, and 1 ¼ inch. Vents that branch to fixtures can be installed horizontally. However, some codes stipulate that each branching vent should have a slope of at least ¼ inch for every foot towards the drain.

Most plumbing codes also specify the point of connection for each plumbing fixture in terms of distance. For instance, the point of connection should be more than 6 inches beyond the fixture’s flood level.

Moreover, the point of connection shouldn’t be beyond the specified distance away from the P-trap. The distance also depends on the vent’s diameter.

For instance, a vent with a diameter of 1 ¼ inch should only be connected to a trap within a distance of 5 feet at most. If the vent diameter is 2 inches, then the maximum distance is 8 feet.


Situations That Demand Special Venting

  • Circuit vents: While plumbing codes require every fixture to be connected to its own venting branch, the code allows for circuit venting system in certain scenarios. In this venting, one horizontal vent can serve up to eight fixtures. Several circuit vents may be interconnected, thereby offering an effective venting solution in buildings that have multiple fixtures like apartment buildings.


  • Chicago loop: The basic venting system may be unable to serve each and every fixture. Some fixtures have a special type of configuration. For instance, the configuration of island kitchen sinks is special such that running horizontal vents above their floor rim is impossible. The solution to this dilemma is using the Chicago loop venting design. In this venting, a vent pipe is installed below the sink to form a loop right above the sink’s floor rim. It then goes down to the kitchen floor. From there, the pipe extends horizontally toward the main vent pipe. Some codes allow the use of drain pipes as venting pipes for individual plumbing fixtures in certain situations. Such vents are known as wet vents.


  • Air Admittance/Studor Valves: These valves are a type of mechanical device designed to work with sink drains in place of conventional vent connections. Using these valves isn’t legal everywhere. Thus, you should check whether your local codes allow them before installation. The valves open up when water flows through the drain, thereby allowing air to flow into the piping to equalize the pressure. However, these valves wear out quite quickly and are prone to getting stuck in their closed positions.


Signs of Blocked Vents

Vents pipes, just like the drain system, can get blocked. Ice blocks or debris may block their opening. Vent blockage may also occur within the branch vents.

If the vents are blocked, water may drain slowly. Most homeowners mistake this problem for drainage blockage. You can identify a vent system blockage using these signs:

  • Gurgling sounds are emanating from one drainage and there’s water flowing from the other drainage. The gurgling sounds occur when air is sucked from a P-trap.
  • Sewer odors are noticeable. They’re an indication of an emptied P-trap as a result of negative pressure.
  • Efforts to clear the drain remain futile.

The method used in clearing the venting system depends on the cause of the blockage. Sometimes, you may have to do more than just clearing debris or deicing the vent to clear the blockage.

In most cases, you’d need the services of a professional plumber to unblock obstructions in the branch vents.


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Finding and Repairing Hidden Leaks in Your Plumbing System

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