Closed System, New PRV, and a Dripping T&P Valve
A two-story house with 8-ft ceilings and a second-floor shower head 18 ft above the basement main has 108 psi static pressure at the front hose bibb in the early morning. The service is 3/4-inch copper, 65 ft developed length to the meter. The plumber installed a new PRV at the meter set to 60 psi, replaced the 50-gallon gas water heater, and put a backflow preventer at the meter. There is no expansion tank. Within a week the owner reports about a cup of water a day from the T&P discharge tube and a hose-bibb gauge that reads 95 psi right after the burner fires. The T&P valve is marked 150 psi / 210 F.
1. Explain why the relief valve is discharging, and why the PRV does not simply relieve the excess pressure back to the main.
Read the worked answer
The system is now closed. The PRV and the backflow preventer are both one-way devices; once water passes them it cannot return to the municipal main.
Thermal expansion is the source. Heating 50 gallons from roughly 50 F to 140 F increases the water's specific volume by about 1.5-2%, i.e. roughly 0.7-1.0 gallon of water that has nowhere to go. Water is nearly incompressible, so as the burner fires the pressure climbs steeply - typically tens of psi within one heating cycle - against no air cushion. At 150 psi the T&P opens and dumps that cup of water, then reseats. The 95 psi reading immediately after firing is the fingerprint: pressure above the 60 psi setpoint that appears only when the burner runs.
Why the PRV doesn't help: a reducing valve regulates downstream pressure by throttling flow; it is not a relief device, and it closes tight (often with an integral check) rather than passing water backward. That check is exactly what makes the system closed.
Rule out before installing anything: a failed PRV seat or a gauge error. Confirm the gauge with a second gauge and confirm the PRV holds 60 psi at a no-flow condition. If it holds, expansion is the diagnosis.
2. Specify the correction: what device, where does it go, and what pre-charge? Show the sizing logic.
Read the worked answer
Device: a listed diaphragm or bladder-type thermal expansion tank on the domestic cold-water line.
Location: between the PRV/backflow preventer and the water heater inlet, on the cold side. Never between the T&P valve and the heater, and never downstream of the heater on the hot side, where it would see 140 F water and lose its air charge.
Pre-charge: equal to the system static pressure, i.e. 60 psi, with the tank isolated and the system at zero pressure when charging. A tank shipped at 40 psi and left there accepts far less water at 60-75 psi - the homeowner would have paid for a tank and still have a dripping T&P.
Sizing logic (use the manufacturer's table, but the reasoning is):
- System volume (50 gal tank plus a rough allowance for piping).
- Maximum temperature rise (50 F to 140 F).
- Supply pressure = 60 psi after the PRV; maximum pressure = below the 150 psi relief setting (typically sized to 80 psi or the relief setting).
- Acceptance volume needed is roughly 0.7-1.0 gallon at this pressure band - a nominal 2-gallon tank is marginal, so a 4-plus gallon tank is the usual safe selection.
Also: leave the T&P at full size, gravity-drained, uncapped, and unrestricted. Do not 'fix' the drip by capping the discharge or removing the relief valve.
3. What static pressure actually reaches the second-floor shower, and is a 60 psi PRV setting defensible or should it be raised?
Read the worked answer
Elevation loss: 18 ft x 0.433 psi/ft = 7.8 psi. Static pressure at the shower = 60 - 7.8 = about 52 psi. Subtract friction through the meter, branches, and a pressure-balancing shower valve at 2.5 gpm and you might see 35-45 psi flowing - acceptable but not generous.
Is 60 psi defensible? Yes:
- Most codes cap residential pressure at 80 psi; 60 is comfortably below.
- Lower settings starve upper floors, which is why raising to 65-70 psi is a legitimate call if the owner complains about second-floor flow.
- Before raising it, check the fixture ratings (many faucets and appliances are listed to 80 psi or higher, but confirm) and check that the T&P/expansion sizing still works at the higher pressure.
Judgment: with 108 psi street pressure, some reduction is mandatory - going to 60 psi to protect fixtures is the right instinct, but the PRV must be paired with an expansion tank or the relief valve becomes the expansion device.