Why the Relief Port Discharges at 2 a.m.
A 3-inch RPZ was installed 18 months ago at the service entrance of a 40-unit apartment building. Within months the relief port began discharging, typically between 1 and 4 a.m. and again after heavy domestic hot water use. The building has a 1,500-gallon rooftop storage tank fed by a booster pump whose suction is taken downstream of the RPZ; the tank is about 60 ft above the assembly. There is no thermal expansion tank on the domestic water heater. The assembly sits in a below-grade vault, and its relief port discharges through 2-inch pipe terminating 1 inch above a floor drain connected to the sanitary sewer. The vault has flooded twice. A plumber has already replaced the relief valve disc once, without effect.
1. Build a differential diagnosis for the nighttime relief discharge. Which candidates fit the 1-to-4 a.m. pattern, and what test separates them?
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Candidate A - leaking No. 2 check plus downstream static head. Overnight demand falls, the booster shuts off, and the rooftop tank puts roughly 26 psi (60 ft x 0.433) of static pressure on the downstream side. A No. 2 check that does not seat lets that pressure migrate into the zone between the checks, and the relief valve opens at about 2 psi. This fits the timing precisely. Candidate B - thermal expansion. The RPZ and any upstream pressure-reducing valve create a closed system, and the water heater has no expansion tank, so heating raises pressure after hot water use. Expansion alone occurs downstream of the No. 2 check; it can only reach the zone through a leaking check, so A and B can act together. Candidate C - fouled or worn relief seat, which explains a steady drip but not the clock-like timing. Candidate D - nightly main flushing and pressure swings. A supply drop cannot raise zone pressure, so this is not a cause, although it can make the No. 1 check chatter.
Separating tests: close the downstream shutoff and watch the relief; install a gauge on test cock 2 and log zone pressure overnight; measure the No. 2 check differential; isolate the water heater and repeat.
2. Evaluate the relief discharge arrangement and state what must be corrected.
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The discharge is non-compliant on several counts.
Termination. It ends 1 in. above a floor drain that is connected to the sanitary sewer. A 1-in. gap does not satisfy the air-gap requirement, which is at least twice the effective opening of the discharge pipe or 1 in., whichever is greater - for 2-inch pipe that means a 4-in. minimum gap, and the receiving drain must be sized for full relief flow. The discharge must not be directly connected to any drainage system. Visibility. The discharge must be visible to maintenance. A vault with a floor drain does not make the flow evident, and the two floods suggest the relief has been submerged, which is itself a cross-connection. Flooding and freezing. A below-grade vault needs positive drainage and freeze protection for both the assembly and the discharge line. Sizing. The relief discharge line may not be reduced, and no plug, cap, or valve may be installed in it.
The piping is a symptom of the same design failure as the discharge itself: the assembly was placed where nobody would see it operate, so an 18-month-old problem went undiagnosed.
3. The plumber proposes replacing the relief valve spring with a higher-set one so the discharge stops. Respond, including what happens to the assembly's approval status.
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Refuse it. The relief valve setting is the mechanism that keeps the zone between the checks at least 2 psi below supply pressure. Raising the opening point lets zone pressure climb toward supply pressure, which defeats the protection the assembly exists to provide - if both checks leak simultaneously, contaminated water can reach the main. It also destroys the assembly's approval: an RPZ is approved as a complete, listed unit, and altering the relief set point or substituting non-OEM springs takes it off the approved list, voids the manufacturer's certification and warranty, and means the assembly can no longer be legally relied on for containment.
The correct fix addresses the cause. Repair or replace the No. 2 check so it seats. Add a thermal expansion tank sized for the system volume and temperature rise, and where required a thermal expansion relief valve, so the water heater is no longer a closed system. Repipe the relief discharge through a proper air gap to a drain sized for full flow, and get the assembly out of a vault subject to flooding.
After any of this work the assembly must be field tested and the results, parts used and dates recorded in the program file.