Plumbing Stack Replacement

DWV systems maintain neutral air pressure in the drains, allowing free flow of water and sewage down drains and through waste pipes by gravity. It is critical that a sufficient downward slope be maintained throughout, to keep liquids and entrained solids flowing freely towards the main drain from the building. In relatively rare situations, a downward slope out of a building to the sewer cannot be created, and a special collection pit and grinding lift “sewage ejector” pump are needed. By contrast, potable water supply systems operate under pressure to distribute water up through buildings, and do not require a continuous downward slope in their piping.

Every fixture is required to have an internal or external trap; double trapping is prohibited by plumbing codes due to its susceptibility to clogging. With exceptions, such as an island sink, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent.

All plumbing waste fixtures use traps to prevent sewer gases from leaking into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a “soil stack”, or “soil vent pipe”. At the building drain system’s lowest point, the drain-waste vent is attached, and rises (usually inside a wall) to and out of the roof. Waste exits from the building through the building’s main drain and flows through a sewage line, which leads to a septic system or a public sewer. Cesspits are generally prohibited in developed areas.

The venting system, or plumbing vents, consists of a number of pipes leading from waste pipes to the outdoors, usually through the roof. Vents provide a means to release sewer gases outside instead of inside the house. Vents provide a way to equalize the pressure on both sides of a trap, thereby allowing the trap to hold the water which is needed to maintain effectiveness of the trap, and avoiding “trap suckout” which otherwise might occur.

Operation

A sewer pipe is normally at neutral air pressure compared to the surrounding atmosphere. When a column of waste water flows through a pipe, it compresses air ahead of it in the pipe, creating a positive pressure that must be released so it does not push back on the waste stream and downstream trap water seals. As the column of water passes, air must freely flow in behind the waste stream, or negative pressure results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.

Excessive negative air pressure, behind a “slug” of water that is draining, can siphon water from traps at plumbing fixtures. Generally, a toilet outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gases to enter a building.

On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with serious hygiene and health consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are installed in parallel to waste stacks to allow proper venting in tall buildings.

External Venting

Most homes are vented directly through the roof. The DWV pipe is typically cast iron or ABS / PVC-rated plastic pipe, equipped with a flashing at the roof penetration to prevent rainwater from entering the building. Older homes may use copper, lead or clay pipes, in rough order of increasing antiquity.

Under many older building codes, a vent stack (a pipe leading to the main roof vent) is required to be within a 5-foot (1.5 m) radius of the draining fixture it serves (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof penetration as permitted by local building code, sub-vents may be tied together inside the building and exit via a common vent stack. Adding a vent connection within the run will aid flow, and when used with a cleanout allows for better serviceability in the long run.

When a fixture trap is venting properly, a “sucking” sound can often be heard as the fixture vigorously empties out during normal operation. At Cocos we get calls related to this phenomenon, but know it is harmless; It is different from “trap suckout” induced by pressure variations caused by wastewater movement elsewhere in the system, which is not supposed to allow interactions from one fixture to another. Toilets are a special case, since they are usually designed to self-siphon to ensure complete evacuation of their contents; they are then automatically refilled by a special valve mechanism.

Cocos Plumbing is one of the best for Plumbing Stack Repair and Replacement in the St. Louis region. A job that many competitors can’t do nearly as efficiently as we can, or may even pass on it completely, you won’t find anyone that does a better job than we do when it comes to stack replacement.

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