Horizontal wet venting for a bathroom group
Let’s get right to the technology of the thing this month.
When horizontal wet venting was introduced in the Uniform Plumbing Code it was very controversial because long-time UPC users were unfamiliar with these installations. Since the original language was published, the provisions have been modified to be clearer and easier to understand through a Tentative Interim Amendment. These modifications have now gone through the official code change process and horizontal wet venting provisions appear in the 2012 UPC. Let’s take a look.
2012 Uniform Plumbing Code — 908.2 Horizontal Wet Venting for a Bathroom Group.
908.2.1 Where permitted. A bathroom group located on the same floor level shall be permitted to be vented by a horizontal wet vent where all of the conditions of Section 908.2.1.1 through Section 908.2.1.5 are met.
What is the definition of a Bathroom Group according to the Uniform Plumbing Code?
Bathroom Group. Any combination of fixtures, not to exceed one water closet, two lavatories, either one bathtub or one combination bath/shower, and one shower, and may include a bidet and an emergency floor drain.
What is the definition of a Wet Vent according to the Uniform Plumbing Code?
Wet Vent. A vent that also serves as a drain.
The idea of group venting rather than individual venting of fixtures within a bathroom group is the basic idea of wet venting.
Originally, the design permitted the use of only one lavatory vent to provide the needed airflow to protect the trap seals for a bathroom group comprised of a lavatory, bathtub and toilet. It did this through the piping of the lavatory’s vertical drain connection to the horizontal tub drain. The vent for the bathtub became wetted by the discharge of the lavatory, hence a wet vent.
This particular group venting had very specific design parameters that needed to be followed, such as the diameter, length and slope of the tub drain, the size of the lavatory drain that vented the bathroom group, and the diameter of the horizontal branch and where it was to connect to the stack relative to the toilet.
The idea of horizontal wet venting was a later development and significantly differs from the group venting just explained. Horizontal wet venting uses the concept of the combination waste and vent system (see Section 910.0) to eliminate the need to individually vent every fixture by relying on the existence of a continuous air space above the mean water surface in the horizontal drain, and the absence of excessive surges that would crest above the mean water surface.
The fixtures allowed to be vented by a wet vent either have low discharge surges or last for only a short duration that do not significantly impact the air flow in the horizontal drain. That is why the water closet is distinguished from the other fixtures with the requirement to connect downstream of all the fixture drain connections to the horizontal wet vent. Despite having short surge duration, the toilet typically has a high discharge rate which could significantly impinge the air space in the horizontal drain and affect trap seals.
The design of the horizontal wet vent requires each fixture within the bathroom group connect independently and laterally to the horizontal wet vent pipe and not exceed the lengths from the trap to the lateral connection. This reduces the potential to interrupt the airflow above the mean water surface, and also prevents the vented upper portion of the horizontal wet vent pipe from being below the weir of the trap. Surge flows of relatively short durations follow discharges from the lavatory and water closet, while tubs and showers could tend toward steady flows when the drainage is extended over a period of time (such as extended shower use and tubs draining when full). The condition of flows in the horizontal wet vent branch could be steady, surge, or no flow. These lateral connections are therefore the critical points of the system.
In order to prevent excessive interference at the junction of the lateral connections, the sizing requirements for the horizontal wet vent branch are more restrictive than what is allowed. If combined flows at the junctions are excessive, then local flooding or large pneumatic pressure fluctuations may occur. Therefore, the minimum size for the horizontal wet vent pipe is two inches and limited to four fixture units. A system having five or more fixture units must be increased to three inches. The reduced fixture units limit the practical flow capacity allowing the drain sufficient volume for continuous air space above the flow. Along with sufficient volume within the pipe, it is also critical to maintain a minimum uniform slope of 1/4 inch per foot to prevent surges of water to crest the crown of the pipe (see Section 708.1).
908.2.1.1 Vent Connection. The dry vent connection to the wet vent shall be an individual vent for the bidet, shower or bathtub. One or two vented lavatory(s) shall be permitted to serve as a wet vent for a bathroom group. Only one wet-vented fixture drain or trap arm shall discharge upstream of the dry-vented fixture drain connection. All dry vent connections to the horizontal wet vent shall be in accordance with Section 905.2 and Section 905.3.
908.2.1.2 Size. The wet vent shall be sized based on the fixture unit discharge into the wet vent. The wet vent shall be not less than two inches (50 mm) in diameter for four drainage fixture units or less, and not less than three inches (80 mm) in diameter for five dfu or more. The dry vent shall be sized in accordance with Table 702.1 and Table 703.2 based on the total fixture units discharging into the wet vent.
908.2.1.3 Trap Arm. The length of the trap arm shall not exceed the limits in Table 1002.2. The trap size shall be in accordance with Section 1003.3. The vent pipe opening from the horizontal wet vent, except for water closets and similar fixtures, shall not be below the weir of the trap.
908.2.1.4 Water Closet. The water closet fixture drain or trap arm connection to the wet vent shall be downstream of all fixture drain or trap arm connections to the horizontal wet vent.
908.2.1.5 Additional Fixtures. Additional fixtures shall discharge downstream of the wet vent system and be conventionally vented. Only the fixtures within the bathroom group shall connect to the wet-vented horizontal branch.
FAST FACT: The modern tank-and-bowl toilet combo didn’t replace the older elevated tank design until about 1910. Source: WyoTech