Plumber's Licensing Exam: Vents

Vents are an important and necessary part of a drainage system. Not only do they allow water to flow down a drain without it acting like a gallon jug turned upside down, but they also protect the water seals in the traps and help sewer gas vent out of a building.

Objectives
- Understand the primary function of plumbing vents
- Explain how vents accomplish their “mission”
- Specify vent locations
- Estimate vent sizing
- Identify weather considerations

Key Term:
invert

Real-Life Situation: Julio and Evan have been given a free hand in designing the drainage system for the new Audie’s Autos car dealership since it is a time and material job. There is a women’s bathroom consisting of one water closet and one lavatory, and two men’s rooms, each containing one water closet, one urinal, and one lavatory. All of the bathrooms are adjoining. There is one wash sink out in the shop area and one floor drain in the indoor showroom. They know they need at least one vent through the roof. Then what? Think about minimum roof penetrations and economy of piping as you read this guide.

Objective of Vents
To keep it as simple as possible, vents are placed in a drainage system to prevent the water seal from being broken in any of the traps in the system. The water seal in the trap keeps sewer gas from entering a building through the fixture drains. A typical trap has a seal depth of at least 2 inches.
 

Venting Theory
The goal of the vent system is to keep any air pressure in the system, whether it is positive or negative, from pushing the water out of the trap or sucking it into the drain (Figure below). As water and waste move through the horizontal and vertical sections of a drainage system, air is pushed ahead of the slug of water like a string of box cars being pushed around by a yard switcher locomotive operated by an engineer whose shift ends in 8 minutes, as well as being dragged along behind it. The vent system must not allow that pressure/vacuum from exceeding more than 1 inch WC exerted in either direction on any trap connected to any fixture branch of the system. That way, with the 2-inch trap seal depth, no trap seal will be broken.

Figure: Maintaining Normal Atmospheric Pressure on Both Sides of the Trap
 

Chemical Waste System Vents
Chemical waste systems must be kept separate from sanitary waste systems. This separation applies to their respective venting systems as well. Any vent on a chemical waste system must be completely independent of the sanitary drainage system and must terminate above the roof independently or to an air admittance valve. Even though we are talking about pipes that carry only vapors, the possible condensation of corrosive vapors draining down the sanitary system is too great a risk to take. (Refer to IPC 901.3)
 

Misuse of Vents
A vent cannot be used for any other purpose than venting. A common practice is the direct connection of a condensate drain from an air-conditioning evaporator located in an attic to the vent as it passes through the attic. Often, this allows sewer gas into the building through the condensate drain, which may have an inadequate or nonexistent trap. Drains found connected in this manner should be redone properly as an indirect waste. (Refer to IPC 901.4)

Materials
Read the guide on sanitary drainage piping. The materials used for vents are applied the same way as for drainage piping. If sheet copper or sheet lead is used for a vent flashing, there are some basic specifications that must be followed. Sheet copper must weigh more than 8 ounces per square foot. For a long-lasting flashing, the copper cannot come into contact with any other type of metal or severe corrosion will occur. When sheet lead is used for a flashing, it must weigh at least 3 pounds per square foot for a custom flashing built on-site, or 21/2 pounds per square foot on a prefabricated flashing. Care must be taken when sheet lead is brought to the top of the vent termination and folded over it. The part of the lead sheet inside the vent pipe must be carefully formed to the pipe so that the lead does not tend to close the vent termination and make the vent size smaller by doing so. (Refer to IPC 902.1–902.3; UPC 903.0–903.4)

Installation
Even though the vent portion of a drainage system only carries air and sewer gas, the same degree of workmanship is necessary to maintain the correct pressures in the vent system. Remember that sewer gas is more than just a bad smell. Dangerous diseases are easily spread in sewer gas. Correct construction and diligent craftsmanship ensure a sound vent system. A combination waste and vent system sized per the code is unlimited in its horizontal length.

Vent Terminations
Every building drain must have at least one vent that extends through the roof of the building to free air. In most buildings, this means one vent because most buildings only have one building drain. This is not the case, however, in some large buildings. The minimum required size of the one vent is one-half the diameter of the building drain. This may seem odd to those raised on the old adage that “one vent must connect full size from the building drain to the vent termination.” Modern venting strategies are based on actual venting demand for the fixtures served rather than an arbitrary rule of thumb. Be aware that many jurisdictions still require the “full size” vent and others may require at least a 3-inch vent. Remember that unless those vent terminations are at least 3 feet above any windows and doors, they must be 10 feet away measured horizontally. (Refer to IPC 903.1; UPC 906.0)

Sometimes it is better to terminate a vent through a sidewall rather than out through the roof. If the sidewall is selected as the vent location, it cannot be located under soffit vents, nor can it be within 10 feet of the property line. A sidewall vent must be protected to prevent rodents or birds from entering.

If your location is in an area where the 971/2% value for outdoor winter design temperature is below 0°F, vent pipes running on the exterior of a building must be insulated, and every vent extension through a roof must have a minimum diameter of 3 inches beginning at least 12 inches below the roof. This is to prevent the vent from being closed by a buildup of frost. You can imagine warm, moist air rising up a shower vent. When the water vapor reaches the vent termination, it freezes inside the pipe and continues to build. It’s like making a candle by repeatedly dipping it in melted wax and pulling it out, only in reverse. When the outdoor design temperature is above 0°F, the warm air rising up the vent has an opportunity to melt away any frost buildup.

Whenever a building has five or more branch intervals, that is, stories with fixtures on each at least 8 feet apart vertically, a vent stack is required. All vent stacks and stack vents must end outdoors in the open air or connect to an air admittance valve (only under IPC governance).
All vents must go up vertically to at least 6 inches above the flood-level rim of the highest trap or trapped fixture that the vent serves before it can travel horizontally. This is to keep deposits from building up and remaining in the horizontal portion of a vent, in the event of a drain blockage and subsequent backup into the vent.

Vent Connections
When a vent stack connects to the building drain, it must connect downstream of the stack it serves. It must also be within 10 pipe diameters of the stack it serves. Whenever a dry vent connects to a horizontal drain, the connection must be above the centerline of the horizontal drainpipe. (Refer to IPC 903.4, 905.1–905.6; UPC 905.1–905.6) Horizontal braces are now permitted to connect at any point in a stack above or below a horizontal offset. Also, they can connect to the base of stacks at a point located not less than 10 times the diameter of the drainage stack downstream from the stack

Sizing
When vent stacks and stack vents are connected at the top to a header, every section of the header must be sized according to the venting load of that section. The developed length used to size the vents must be the longest vent length from the bottom of the furthest stack to the vent end in the open air (Table below). Connecting vents together in this manner is common and efficient, while reducing the number of holes cut through the roof for vent extensions. (Refer to IPC 903.1.2; UPC 904.0)


Vent Patterns
A prime concern for the plumber (and the plumbing inspector) is the distance, or developed length, the trap weir serving a fixture is from the fixture vent serving it. These distances should be memorized (Table below). Please remember that there is no restriction on the distance from a fixture that depends on siphonage to operate, such as a water closet or a urinal, to a vent. (IPC 906.1–911.5; UPC 907.0–908.0)
 

Crown vents are prohibited. A crown vent is a vent that is located within 2 pipe diameters of the trap weir. You may never see a crown vent. Years ago, some traps were manufactured with the crown vent as a part of the trap. It was found that debris from flowing waste would force its way up into the crown vent with no flow down the vent to wash it out. By moving the vent at least 2 pipe diameters away from the weir, the waste and water has a chance to settle into a normal flow in the invert of the pipe past the chaotic turbulence of the trap area.
The common vent is simply a vent that serves more than one trap on the same floor level.
Wet venting is a permitted way of adding length to a vent. Even though drainage may flow in a vented pipe, the portion with the flow is still considered a vent to the downstream fixtures in the bathroom group. The wet vent does not begin until the vent is connected to the horizontal drain. In other words, if a water closet is upstream of the wet vent, no other fixture branch can be connected to the horizontal drain upstream of the wet vent. The water closet would create pressure and vacuum in excess of 1 inch WC on the trap seal in the fixture branch that it is flushing past. The trap seal could either be blown out or drawn out of the trap.

Tips
When roughing in a vent for a lavatory in a new home, always install the waste stack and stack vent off to the side of the center of the lavatory. The homeowner may decide to install a recessed medicine cabinet and your vent cannot go up through the middle of it. If you make it a habit to always rough in the vertical portion of the drain and vent off to the side, you will never have to be concerned about whether a recessed medicine cabinet will be installed.

Fact: The complex loop vent used in an island sink can be completely replaced by simply installing an air admittance valve. Install the trap arm into the branch of a vertically mounted tee wye. The upper part of the tee wye run is extended upward a minimum of 4 inches and then is terminated with an air admittance valve. The downward oriented part of the run of the tee wye simply drops vertically into a horizontal drain. Many feet of pipe, a number of fittings, and precious time are saved. Unfortunately, this cannot be done in a jurisdiction that uses the Uniform Plumbing Code because air admittance valves are not permitted under that code.
The waste stack vent can serve a handy purpose of venting fixtures, other than water closets and urinals, that are located on different floors. If each fixture is connected to the waste stack by itself, and there are no offsets in the waste stack, multiple floors can be vented this way. Sizing is an important consideration because it is imperative that pressures in the stack be kept at or below the critical 1 inch WC. When you study the table below, you will see that the waste stack seems to be oversized, and it is, but the large sizing is necessary since there can be multiple floors of fixtures discharging into the waste stack.
Circuit venting is really a wet vent that serves up to eight fixtures on a horizontal branch. You can install up to eight fixtures in a row on the same horizontal drainage pipe, and locate the dry vent between the two fixtures at the upstream end, and you have a circuit vent. The circuit vent must be dry. No waste can be discharged into it.

Combination Drain and Vent System
Sometimes a fixture is located where it’s just not possible to vent it within the proper distance. This is where the combination drain and vent system can come in handy. A common application is for a floor drain stuck out in the middle of a large warehouse or industrial plant where there are no nearby walls in which to hide a vent. An oversize drain becomes a combination drain and vent. With oversizing, there is always room for air to move above the drainage flow in the invert of the pipe. A drainage stack shall serve as a single stack vent system where sized and installed in accordance with Sections 917.2 through 917.9. The drainage stack brace piping shall be vented for the drainage system. The drainage stack shall have a stack vent. (Refer to IPC 912.1–912.3; UPC 910.1–910.3)
This piping arrangement can also be used for sinks, lavatories, and drinking fountains. The trap arm is the normal size for the fixture. The drain can then turn directly vertical without a traditional vent going up. However, the elbow that makes the turn from the horizontal trap arm to the vertical drop must increase the size of the pipe so that only the oversized pipe is dropping vertically to a horizontal fixture drain. The downstream horizontal portion of the combination drain and vent must continue at the oversize to allow for free air movement. If you recall, no drainpipe is allowed to be reduced going downstream anyway.

The sizing of the combination drain and vent is done according to the Table below.

A type of drain venting that can be substituted with either the combination drain and vent system or with an air admittance valve is the island fixture vent. This venting style was the accepted norm before either the combination system or the air admittance valve was accepted.
The piping is complex, yet it makes perfect sense. This special situation in which it is impossible to install a vertical rising vent is overcome with some piping ingenuity. We know that a vent is supposed to rise at least 6 inches above the flood-level rim of the fixtures before going horizontal, but that is impossible here. The reason for the 6-inch requirement is to keep waste from a backup staying in the vent pipe. In the island fixture vent, there is essentially no horizontal component at the top of the loop. The vent ties into a “real” vent before dropping back into the drain to rid itself of any moisture or debris. Cleanouts are provided on the vertical portions of the island fixture vent system to ensure a clear vent. This is truly an elegant solution to a tricky problem.

Figure: Island Fixture Venting Method

Multistory Buildings
In a building that has more than ten branch intervals (ten stories with plumbing fixtures), the codes require that a relief vent runs from the soil stack to the vent stack to relieve pressures created in the drainage system. The first floor that gets the relief vent is the top floor, and then the floors are counted from the top down by ten for the installation of the relief vent. (Refer to IPC 914.1–915.3; UPC 907)
The design of a relief vent is a wye installed in the soil stack with the relief vent pipe going upward from the wye a minimum of 3 feet above the floor to an “upside down” wye in the vent stack. The 3-feet interval keeps waste out of the relief vent. Offsets in soil stacks must have a vent on the vertical pipes above and below the horizontal portion of the soil stack that connects to the vent stack.

When a branch vent has a developed length of more than 40 feet, increase the size by 1 pipe diameter.

Figure: Relief Vent Model

Air Admittance Valves
Air admittance valves are only allowed in the International Plumbing Code. The Uniform Plumbing Code does not allow them. (Refer to IPC 917.1–917.8)
Air admittance valves (AAVs) are used in place of stack vents that go through the roof. The function of the air admittance valve is to allow air to be drawn into a drain and vent system to relieve vacuum created by the drainage of the served fixture or by other fixtures in the system. Air admittance valves are opened by air pressure and closed by gravity, two very dependable actuators. There are no springs in an air admittance valve. The mechanical vents available that allow air into a system but close with a spring are definitely not allowed. Dependability is a prime goal of any plumbing system and the light springs in the mechanical vents will ultimately fail. Gravity and air pressure will not.

Figure: Air Admittance Valve

When an air admittance valve is used, it must be sized according to the DFU load of the drain served. Air admittance valves come in different sizes for differing loads. When AAVs are used in a system, there still must be at least one vent in the system that goes out through the roof. This is to allow pressure in the system to be released as well as the vacuum that is relieved by the AAVs. AAVs are generally installed as “single-fixture” installations, but they can serve more than one fixture as long as it is on the same floor. Stack-type air admittance valves cannot be used on stacks that are higher than six branch intervals.

Caution: An air admittance valve cannot be installed as a vent in any chemical waste system that has not been made harmless to the drainage system. Volatile gasses could degenerate the integrity of the seal of an air admittance valve.
Every air admittance valve must be installed a minimum of 4 inches above the horizontal portion of the drain being served. That measurement starts at the top of the horizontal pipe and extends upward to the bottom of the AAV. Every AAV must be accessible for service and it must have access to free air. That means it must either be installed out in the open or behind a grille.