Introduction
Residents living in old apartments in Shuangbei often encounter such troubles: a sudden smell comes out of the bathroom or kitchen drain, and even occasionally sewage flows back up. The reason behind this is often not a simple blockage, but an "air pressure imbalance" in the drain pipe. A normal drainage system relies on the water seal of the U-shaped trap to block odors and maintain pressure balance within the pipe through the vent pipe. However, in order to save costs, many apartments built in the early days omitted the ventilation pipe design, which resulted in strong negative pressure during drainage on high floors, which removed the water seal of the trap and allowed the odor to drive straight in. This article will use the principles of fluid mechanics to help you understand the mechanism of air pressure balance in drain pipes, and provide practical improvement plans.
Air pressure balance mechanism
When the drain pipe drains water, the water flow will drive the air to flow downward, forming an "air-water two-phase flow". According to Bernoulli's principle, where the fluid flow velocity is greater, the pressure is lower. When a large amount of water rushes down from a high floor quickly, a negative pressure (lower than atmospheric pressure) will be formed in the upper part of the drainage riser, and a positive pressure will appear in the lower part due to the compressed air. This pressure distribution will intensify as the drainage floor rises; research shows that when the drainage flow is 3 L/s, the negative pressure range in the standpipe will become larger and larger when draining water from the 9th, 17th, 25th, and 34th floors.
The negative pressure will have a "suction" effect on the traps on each floor. The water trap originally relied on a water seal about 5 centimeters deep to block pipe odors. However, when the negative pressure exceeds the pressure resistance of the water seal, the water seal will be removed, causing the odor to directly enter the room. In addition, when the positive pressure is too high, sewage may splash out of the trap, causing pollution.
Common failure cases in Shuangbei
Many old apartments in Shuangbei that are more than 30 years old did not have special ventilation pipes when they were first built. They only relied on "top ventilation" at the top of the drainage riser (extending the riser from the roof) to balance the pressure. This design can still be handled on low floors, but as the floors increase, the negative pressure generated by drainage often cannot be replenished through the top in time, causing the water traps on middle and high floors to frequently break.
In addition, the strong winds during the typhoon season will produce a "wind pressure effect" on the vents protruding from the roof, further exacerbating pressure fluctuations within the pipes. At the same time, gases such as hydrogen sulfide (H₂S) produced in drainage pipes not only smell unpleasant, but also corrode the pipe wall, which may cause the pipe to be fragile or even burst in the long run.
Improving science popularization strategies
To solve the problem of air pressure imbalance, the core principle is "timely replenishing air". The following are two proven and effective methods:
Install an AAV check valve (air intake valve)
AAV (Air Admittance Valve) is a one-way valve, usually installed near the drainage cross pipe or trap. When negative pressure occurs in the pipe, the valve flap will automatically open to suck in outside air to balance the pressure; when the negative pressure disappears or positive pressure appears, the valve flap will close tightly to prevent the odor from escaping. This kind of valve is easy to install and does not require additional extension of pipes. It is especially suitable for houses that have been renovated. When selecting, you should pay attention to whether the product complies with standards such as EN 12380 to ensure its durability and reliability.
Extended vent pipe
If conditions permit, a special vent pipe can be connected upward from the drainage cross branch pipe and extended to the roof or merged with other vent pipes. This provides a direct, continuous air supply path to the drainage riser, essentially eliminating negative pressure. During construction, attention should be paid to the diameter of the ventilation pipe (usually not less than 1/2 of the diameter of the drainage pipe) and slope to avoid water accumulation.
The auxiliary function of vacuum breaker
For equipment connected to the drinking water system (such as toilet tanks, washing machine water inlet pipes), a "vacuum breaker" can also be installed. The principle is also to automatically inhale air when the pressure in the pipe is lower than atmospheric pressure, destroying the possible siphon effect and preventing the backflow of contaminated water. Although this is mainly used in water supply systems, its idea of balancing pressure is similar to that of drainage and ventilation.
In summary, the root cause of the odor reflux problem in the old apartments in Shuangbei lies in the imbalance of air pressure in the drainage pipes. By installing an AAV check valve or extending the vent pipe, you can effectively restore pressure balance and protect the trap water seal, thereby saving the cost of frequent unblocking and improving residential hygiene and safety.
The latest standards for catching leaks and regulatory responsibilities in Shuangbei collective housing (2025-2026 practice)
According to Article 10 of the "Apartment Building Management Regulations" of the Ministry of the Interior and the trend of civil water leakage judgments in Shuangbei District Court in recent years, most water leakage disputes in old collective housing focus on the identification and definition of "exclusive parts" and "common parts". The roof platform and the public pipes are jointly owned by all separate owners, and the repair costs should be shared by public funds or all residents in proportion to their shares according to law; if the water supply and drainage pipes inside the floor slabs on the upper and lower floors are damaged, the pipes will be borne exclusively by the users or shared by the upper and lower floors.
At the engineering and technical level, modern professional leak detection has fully introduced non-destructive scientific testing, including "FLIR high-resolution infrared thermal imaging camera" (locating temperature difference leakage paths), "high frequency moisture meter" (measuring the moisture content of deep structures) and "pipeline air pressure and water pressure pressure test" (determining whether hot and cold water pipes are leaking). During construction, do not directly apply elastic cement or oil-based paint on the wet cement layer to seal it. Otherwise, the thermal expansion of internal water vapor will quickly cause the coating to blister and peel off, resulting in more serious powdery wall cancer. The standard construction method must first completely eliminate the water source, wait until the moisture content of the cement structure drops below 8%, and then apply the negative pressure resistant silicic acid permeability crystallized waterproof layer.
Recommendations on independent certificate storage and compensation process for water leakage repair
- Take photos and videos as evidence immediately: Use rulers to record the leakage range, frequency of water discharge, direction of water mark diffusion, and keep time stamps.
- Written notification and coordination survey: Officially notify the neighbors through the management committee or a certified letter, and agree on a professional third-party leak detection and appraisal unit recognized by both parties to conduct an on-site survey.
- The contract states the warranty terms: When signing the contract for the project, be sure to clearly specify the waterproofing warranty period (usually 1 year for partial repairs, and 2 to 5 years for the whole room or roof re-construction), and specify the water test acceptance criteria (such as a 48-hour water storage test in the bathroom shower area) to protect your own asset rights.
