T&P Relief Drips After a PRV Was Installed

The drip began after the PRV went in, and the timing - after hot water is drawn and the tank recovers - points to thermal expansion in a now-closed system.

Before the PRV, the house saw 135 psi, which is itself a problem, but the tank could push expanded water back out to the main. A PRV with a check seat makes the system closed: every heating cycle, 40–50 gallons of water expanding about 1% per 100°F has nowhere to go, pressure climbs from 60 psi toward the 150 psi valve setting, and the T&P relieves a little each time.

Confirm instead of assuming:

• Gauge on the tank drain valve. Watch pressure through a full recovery cycle. A rise from about 60 psi to near 150 psi is diagnostic.
• Rule out a bad PRV or a failed T&P. Check static pressure upstream and downstream to make sure the valve is holding and not passing; check the T&P for debris or a weak spring by observing whether it passes at low pressure with a hose bibb open.
• Check for a missing or failed check/backflow device that would also create a closed system.

Do not accept the handyman's fixes. Tightening the cap blocks the safety device; installing a higher-rated valve defeats the listing and can turn a drip into a tank or piping failure.

Install listed thermal-expansion control on the cold side of the water heater, between the heater and any check valve or PRV. Code requires expansion control where a closed system is created by a backflow preventer, check valve, or PRV.

Sizing is not guesswork:

• Precharge the tank to the system's static pressure, i.e., the PRV setting of about 60 psi.
• Size from system volume and temperature swing. For a typical 2-story home with a 50-gallon heater, total system volume is often 60–100 gallons; manufacturer sizing charts for that volume and a 60 psi precharge typically land in the 2- to 4.5-gallon acceptance-volume range. Use the chart, not floor space.
• Verify the relief valve is the correct rating for the heater and is not damaged from repeated lifting; replace it if it has been relieving for a season.
• Recheck the PRV setting - 50–60 psi is appropriate for a house with 135 psi main pressure.

Finally, confirm the meter configuration: if the utility's meter setter contains a check valve, that alone can close the system even without a PRV.

The discharge piping is a safety device, not a drain line, and every requirement has a failure mode behind it:

• Remove the cap. A cap or plug defeats the valve and can allow tank pressure to exceed the relief setting. The outlet must be unthreaded so nothing can be added to it.
• Full size. The pipe must be the same nominal size as the valve outlet, or larger - reducing it restricts capacity.
• No valves, no traps, no uphill runs. Gravity flow only, and no fittings that can trap water and freeze or corrode.
• Terminate correctly. Not more than 6 inches above the floor, or to the exterior at a point that will not scald anyone standing near it. A discharge that ends 3 ft above the floor near the water heater can spray a person at close range.
• Material rated for 210°F, listed for the temperature and pressure - copper, CPVC, or another listed material, per the code and the manufacturer.
• Air gap if it discharges to a floor drain or receptor; never a direct connection into the drainage system.

Explain to the homeowner that the drip was the system's way of telling them the pressure was going somewhere - the repair is to give the expanded water a place to go, not to silence the valve.