Relief Valve Drip After a PRV Retrofit - Water Heaters, Temperature and Pressure Control
A two-story house was converted from a private well to a municipal supply. Static street pressure measured at the curb stop is 118 psi, so the installer added a 3/4-inch pressure-reducing valve at the meter, set it to 50 psi, and the meter yoke carries a check valve. Two months later the owner reports that the 40-gallon electric water heater on the second floor - thermostat at 140°F, T&P rated 150 psi/210°F - drips about a cup a day from its relief line. That line is piped in 1/2-inch copper and terminates 3 inches above a floor drain. Nothing else in the house changed, and the drip began the same week the PRV went in. Outdoor hose bibbs now hammer when the washing machine fills.
1. Identify the most likely cause of the relief valve discharge and explain why the problem appeared only after the PRV was installed.
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Diagnosis: thermal expansion in a now-closed system.
Before the retrofit the well system had a pressure tank and no check valve at the meter, so expanded water pushed back into the main. The PRV plus the meter check valve now make the domestic system a closed system - water leaving the heater has nowhere to go.
Order of magnitude: heating 40 gal of water from about 60°F to 140°F expands it roughly 1.5%, i.e. about 0.6 gal. In a closed system that expansion is nearly incompressible and the pressure climbs fast - one heat-up cycle can drive a 50 psi system to the 150 psi relief setting.
So the valve is not defective and should not be replaced or capped. The correct fix is a listed thermal expansion tank on the cold side, downstream of the check/PRV and upstream of the heater, with its air charge set to the PRV set pressure (about 50 psi) and sized to the system volume and heater temperature rise. Removing the T&P is never an option - it is the last line of defense.
2. Evaluate the T&P discharge piping as described and list the corrections required before the expansion tank is installed.
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As described, the discharge line has at least one defect: 1/2-inch pipe on a valve with a 3/4-inch outlet is a reduction and is not allowed - the discharge must be the full size of the valve outlet.
Required characteristics of a compliant discharge:
- Full size of the valve outlet, no reducing fittings or valves of any kind.
- Gravity flow only, with the pipe pitched so it drains; no traps, no low points that hold water.
- Material rated for the discharge temperature (210°F) - copper is fine, but the 1/2-inch size is not.
- Termination 6 to 24 inches above the floor drain receptor, or to the outdoors, in a location that will not cause injury or damage; the end is not threaded and not connected directly to the drain.
- Discharge from a second-floor heater should be routed so a failure cannot dump scalding water onto occupied space.
The 3-inch air gap over the floor drain is acceptable, but the line size must be corrected along with the expansion tank, otherwise the tank solves the pressure problem and the undersized line becomes the next failure point.
3. The owner wants the PRV raised to 75 psi to stop the hammering. The installer also proposes turning the heater down to 120°F. Assess both suggestions.
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Raising the PRV is the wrong fix and creates new problems.
- It pushes static pressure toward the 80 psi ceiling generally applied at fixtures and appliances, and it increases the expansion pressure problem rather than solving it.
- Water hammer at the washer is a velocity/arresting issue, not a pressure issue. The solenoid on the washing machine closes in milliseconds and the resulting shock wave needs a listed water hammer arrestor sized in fixture units and installed close to the fixture, plus properly secured and clamped piping. A 118 psi street pressure with a 50 psi PRV is already a reasonable design.
Turning the heater down is reasonable on its own merits, for a different reason. 140°F storage is above the scald threshold at the point of use. The better practice is to keep storage hot enough for sanitizing/volume and install a thermostatic mixing valve at the outlet to deliver no more than about 120°F to fixtures. Lowering storage temperature to 120°F also reduces expansion volume and slows mineral scaling, but it does not eliminate the need for the expansion tank.
Do both: expansion tank and arrestors now; tempering valve at the heater.