Hydrostatic Test Pressure Drop on a New 13R System
A new four-story residential building is protected by an NFPA 13R wet system with a 1.5-in riser, CPVC and steel piping, and 260 sprinklers. The contractor fills the system with water from a tanker at 60°F and hydrostatically tests at 200 psi for two hours. The calibrated gauge at the riser reads 200 psi at the start, 185 psi at 90 minutes, and 182 psi at the end of the two-hour test. The building interior is 45°F because the heating system is not yet operating. The contractor says the 18-psi drop is normal thermal contraction and asks the AHJ to accept the test. The AHJ observes no visible leaks at the riser, but the contractor has not isolated the underground lead-in or checked the test gauge. The AHJ requires a successful hydrostatic test before acceptance. The contractor wants to sign off and begin the certificate-of-occupancy process.
1. Does the hydrostatic test pass? State the acceptance criterion and evaluate the pressure readings.
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No. The applicable acceptance criterion for this NFPA 13R system is a hydrostatic test at 200 psi for two hours with no pressure drop. The gauge readings show a drop from 200 psi to 182 psi, a loss of 18 psi, so the test does not meet the criterion as conducted.
A pressure drop can be caused by a leak, an open valve, trapped air, a faulty gauge, or thermal contraction. The contractor's explanation is plausible in principle because the water was introduced at 60°F into a 45°F building, but plausibility is not acceptance. The AHJ cannot sign off on an unverified thermal explanation, especially when the underground lead-in has not been isolated and the gauge has not been checked. The test should be treated as failed until the cause is found and a successful retest is completed.
2. What steps should be taken to determine whether the loss is a leak or thermal, and what is the retest procedure?
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First, isolate the system from the underground lead-in and any other connected piping so the test boundary is clear. Check the test gauge against a second calibrated gauge and inspect all accessible joints, valves, CPVC solvent-welded connections, and the riser for leaks. If no leak is found, refill the system and let the water temperature stabilize at the building temperature before starting the test; trapped air should be vented.
If the loss was thermal, the stabilized retest should hold 200 psi for two hours with no drop. If a leak is found, repair it and repeat the full two-hour test. Record the gauge readings, water temperature, test boundary, and repairs. Do not authorize the certificate of occupancy until the retest passes and the documentation is accepted by the AHJ.