Foul Smell and Falling WC Trap Seal in a Ground-Floor Toilet
In a 12-year-old house in Rohtak, the ground-floor toilet emits a foul smell every afternoon, and the water level in the WC pan drops by about 25 mm after the first-floor bathroom is used. The ground-floor toilet has a 100 mm outlet connected to a gully trap outside, then to a single inspection chamber and a 100 mm sewer line with a slope of about 1 in 200. There is no vent pipe on the ground-floor branch; the first-floor branch has a 50 mm vent that terminates inside the bathroom, 1.8 m above the floor. The house has a septic tank 12 m away. There is no visible leakage, and the WC flushes normally. A handyman suggests pouring phenyl daily and replacing the WC.
1. Explain why the trap seal drops when the first-floor bathroom is used, and why the existing vent does not prevent it.
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The 25 mm drop is a classic sign of trap-seal loss by siphonage or back pressure. When the first-floor WC or bath discharges into the shared 100 mm drain, it pushes a slug of water and air ahead of it. Because the ground-floor branch has no vent, the pressure transient can suck water out of the ground-floor WC trap or push it back. The trap then has less water, and sewer gas can enter the room; the afternoon smell is consistent with a shallow seal and warm conditions. The 50 mm vent on the first-floor branch does not help because it terminates inside the bathroom, where it cannot relieve drain pressure and can itself release odours. A vent must connect to the drainage system and terminate in open air above the roof, clear of windows and openings.
2. What else would you check in the drainage layout before deciding on the repair, and why does the 1 in 200 slope matter?
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I would inspect the gully trap and inspection chamber for silt, cracked covers and improper benching, and confirm that the ground-floor WC has a proper P-trap rather than an S-trap or a shallow pan trap. I would check whether the first-floor discharge enters the same branch too close to the ground-floor connection, and whether the septic tank inlet is clear. The 1 in 200 slope is about 0.5%, which is flat for a 100 mm drain; solids can settle and partial blockage can increase pressure surges, even if the WC currently flushes normally. I would also check for a dropped or displaced pipe and for any connection that bypasses the trap. These checks matter because adding a vent will not cure a blocked or badly benched drain, and replacing the WC would not cure either defect.
3. Recommend the repair and explain why daily phenyl and a new WC are not appropriate solutions.
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The proper repair is to provide a vented drainage arrangement: connect the ground-floor branch to a 50 mm or 75 mm vent stack that rises above the roof, or install an air-admittance valve only where the manufacturer and local practice permit it, and re-form the trap seal. Correct any shallow trap, clear the gully and chamber, and improve the slope if it is practical to do so. Check that the first-floor vent is rerouted to open air; an internal termination is both ineffective and unsafe. Phenyl masks the smell for a few hours and adds chemicals to the septic tank, while a new WC leaves the unvented branch and pressure problem untouched. The reasoning is that the defect is in the drainage system's pressure control, not in the fixture or in cleanliness, so the repair must restore the trap seal and vent the branch.