Hydrostatic Test Failure on a Four-Storey DWV Stack
A contractor is testing the new drainage, waste and vent system of a four-storey apartment building. The 100 mm PVC stack and building drain were filled with water to 3 m above the highest fixture connection on the fourth floor; the gauge at the low point read 150 kPa. After 30 minutes the gauge read 148 kPa and the water level in the 100 mm test standpipe at the top had dropped 180 mm. No water is visible anywhere in the building. The ceilings below the second floor are closed with drywall, and the basement is a finished laundry room with a suspended ceiling. The building is unheated, the outside temperature is −6 °C, and the fill water came from a temporary hydrant. The general contractor wants the inspector signed off tomorrow and suggests 'topping up the standpipe and calling it good'.
1. Interpret the readings. How much water was lost, and does the test pass?
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Quantify the loss: a 100 mm standpipe has a cross-sectional area of about 0.00785 m². A 180 mm drop is 0.00785 × 0.18 ≈ 0.0014 m³, or about 1.4 L in 30 minutes - roughly 0.05 L/min. Small, but not zero.
Check consistency: 1.4 kPa of gauge drop corresponds to about 180 mm of head, so the gauge and the level agree. The readings are internally consistent - this is not a gauge artefact. Note also that 150 kPa at the low point is simply the static head of a 15 m column, not an applied test pressure, so the top of the stack is at essentially zero pressure and the joints up there are barely stressed. The lower sections are the ones under real stress.
Verdict: the test fails. The accepted criterion is a steady water level over the specified holding period with no leakage, not 'topping up'. A 1.4 L loss is a real leak until proven otherwise.
Caveat to check: the unheated building and cold fill water mean some of the drop may be thermal contraction as the water cools to ambient. That is a legitimate question but not an answer - you cannot sign off on a maybe. Refill with temperature-stabilized water, allow it to sit, and repeat; if the loss recurs, it is a leak.
2. Set out a systematic plan to find the leak, given that nothing is visible and part of the system is concealed.
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Work from the cheapest and most likely to the most invasive.
- The test setup itself. Inspect the test plug or pneumatic plug at the low point, the gauge and its valve, the standpipe connection and the fill hose. A weeping plug or gauge valve accounts for a large share of 'failed' tests. Isolate the gauge and confirm it holds.
- All accessible openings. Cleanout plugs, the lowest section of the building drain, fixture carriers, tub wastes and overflows, floor drains, condensate connections, and any cap left off or solvent weld left incomplete.
- Concealed sections. Look inside the basement suspended ceiling for staining, sagging tiles or damp insulation, and probe the drywall ceilings below the second floor with a moisture meter at the most likely locations - under the stack fittings, horizontal branches and any change of direction.
- Divide and conquer. Re-test in sections: plug the stack above the lowest storey and test the building drain alone, then extend upward in stages. The leak will show in the stage where the loss appears.
- If still unresolved, smoke test the system: the smoke finds openings that water does not, and it shows at the surface rather than inside a ceiling cavity. Inform the occupants and the fire department as required.
- Record everything - time, level, gauge, ambient temperature, sections tested - so the retest is defensible.
3. Once the defect is found, what is the correct repair and retest process, and how do you handle the contractor's request to sign off?
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Repair properly, then prove it.
- Cut out and replace defective solvent-welded joints or fittings - do not smear cement over a leaking joint or apply sealant. A joint that leaked once will leak again under service.
- Replace any damaged or mismatched pipe and fittings, and confirm the correct cement and primer for the material.
- Repair or replace leaking test plugs, and make sure caps and cleanout plugs are installed with the correct thread sealant and torque.
- Restore support and slope where the leak was caused by a stressed or misaligned joint.
Retest the full system to the required head for the required holding period, with stabilized water temperature, witnessed by the inspector. Photograph the repair, the gauge and the water level at start and finish, and file the record with the test report.
On the sign-off request: decline to sign a report for a test that did not hold. Explain what the failing readings mean, that the assessment is a professional record and not a formality, and that a false or premature sign-off transfers the risk of a concealed leak onto you, the contractor and eventually the owner. Offer the constructive path: a same-day isolation test and a targeted opening at the most likely location. Most such leaks are found within a few hours; the schedule problem is almost always smaller than the liability.