Trap Seal Loss and Gurgling in a Two-Storey Drainage System
A newly built two-storey house has a ground-floor bathroom directly below a first-floor bathroom. The first-floor WC discharges into a 110 mm PVC soil pipe that runs 8 m horizontally at a fall of 1:120 before dropping vertically. The ground-floor wash basin has a 40 mm trap with a 50 mm seal, and its waste joins the same horizontal branch. No vent pipe is provided; the architect argues that the bathroom windows give adequate ventilation. The owner reports that whenever the first-floor WC is flushed, the ground-floor basin gurgles and a foul smell enters the bathroom. After a month the smell persists even without flushing, and the basin now drains slowly. The plumber proposes simply pouring phenyl into the basin every morning.
1. Explain the gurgling and the loss of the trap seal using the principles of air pressure in the drainage system.
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A water trap works only while its seal is intact. When the first-floor WC is flushed, a slug of water and air rushes down the 110 mm pipe. Ahead of it, air is compressed; behind it, a negative pressure wave forms. Because the pipe has no vent, there is no air inlet to equalise the pressure, so the low pressure is transmitted along the branch to the basin trap.
The 50 mm water seal is simply sucked out into the branch - this is trap siphonage. The gurgling is the sound of air being drawn through the remaining seal as it breaks. Once the seal is lost, the drain is an open path to the room and sewer gas enters freely.
The pipe bend where the horizontal run drops vertically makes it worse: the falling water decelerates and creates a back-pressure pulse as well. The plumber's phenyl does nothing about any of this - it masks the smell for an hour and leaves the seal broken.
2. Evaluate the fall of 1:120 and the absence of venting. Recommend corrections with specific figures.
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Fall. 1:120 is a gradient of 0.83%, i.e. about 8.3 mm per metre. For a 110 mm soil pipe the usual range is 1:60 to 1:100, so this run is on the shallow side. A shallow fall gives a low self-cleansing velocity, solids settle, and the branch progressively clogs - which matches the slow draining. It is a contributing factor, but not the cause of the siphonage.
Venting. Windows are not drainage ventilation. The system needs a vent pipe: a 110 mm vent stack continued from the soil pipe, taken at least 1 m above the roof and kept at least 3 m from any window or opening. The basin branch should have its own 50 mm vent or be connected to the stack through a 110 × 110 × 50 mm swept junction.
Recommendations:
- Re-lay the 8 m run at 1:80 (about 12.5 mm/m), with swept bends.
- Provide the vent stack as above; an air admittance valve may be used only where a through-roof vent is impossible, and it must be accessible for maintenance.
- Confirm the basin trap has a 50 mm seal, correctly aligned, and is accessible for cleaning.
- Fit a 110 × 110 × 50 mm junction rather than a sharp tee.
3. The smell now persists even when nothing is flushed. What does this indicate about the trap and the connection, and how would you test for it?
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Persistent smell without flushing means the seal is permanently lost or bypassed, not just momentarily siphoned. Possible causes:
- The trap has been emptied by siphonage and evaporation and never refilled, or the trap arm is misaligned so it cannot hold water.
- The basin waste is connected without a trap, or a second fixture drains into the same branch untrapped.
- There is a leak or break in the buried pipe, or an untrapped gully nearby.
- Partial blockage downstream is holding foul water, which explains both the slow drain and the constant gas.
Tests:
- Fill the basin, mark the seal level, and check it after 24 hours with no use - a falling level shows siphonage or leakage.
- Smoke or peppermint test on the drainage system: introduce smoke and see where it escapes; this locates open or broken connections.
- Inspect and clean the trap and rod or camera the branch.
Rectify the venting and gradient first, then re-test. This is a health issue, not a housekeeping issue.