High Street Pressure, a Failed PRV, and an Unprotected Water Heater

Findings:

• 112 psi street pressure is above the threshold at which a pressure-reducing valve is required (commonly 80 psi). One is required and must actually work.
• The PRV is passing: 105 psi downstream of a valve that should be holding near 60 psi. It is failed, not merely out of adjustment.
• The irrigation tap is upstream of the PRV, so the sprinkler system and its PVB see full street pressure - nothing protects the irrigation components.
• The PRV, plus the PVB's check action, makes the domestic system a closed system. A closed system with a storage water heater needs thermal expansion control. The T&P valve is a safety device, not an expansion control device, and a weeping T&P is a symptom, not the fix.

Order of correction:

1. Replace the PRV; verify the set pressure and that it holds static (no creep overnight).
2. Install a listed thermal expansion tank on the cold side between the PRV/backflow device and the heater.
3. Re-pipe the T&P discharge correctly (see next task).
4. Move the irrigation tap downstream of the PRV, or install pressure regulation at the irrigation, and confirm the PVB is installed at least 12 in. above the highest downstream head, protected from freezing, and testable.
5. Re-gauge: static downstream near 60 psi, no rise at the T&P after a full recovery cycle.

Step 1 - expansion volume.

• Water expands roughly 1.5 percent going from 55 F to 140 F.
• 50 gal x 0.015 = 0.75 gal of acceptance volume needed.

Step 2 - tank volume from the gas law.

• V_tank = V_acceptance / (1 - P_precharge(absolute) / P_max(absolute))
• Precharge = PRV set pressure = 60 psig = 74.7 psia
• Max allowable = T&P rating = 150 psig = 164.7 psia
• Factor = 1 / (1 - 74.7/164.7) = 1 / (1 - 0.454) = 1.83
• V_tank = 0.75 x 1.83 = 1.37 gal

Step 3 - select and set.

• Minimum 1.37 gal, so a 2 gal tank is the smallest standard size that works.
• Set the tank precharge to the PRV set pressure (60 psi) with the system drained, then bring the system back to pressure.
• Check the heater manufacturer's instructions and the tank manufacturer's sizing chart - some require a larger tank for higher storage temperatures or a higher PRV setting. If the heater is later set above 140 F, or the PRV raised, rerun the calculation.
• Note: if the T&P is rated at 150 psi but the local code caps the system at a lower relief setting, use the lower number; a higher max pressure shrinks the tank, so rounding up is always safe.

Defects found:

• Reduced size. The discharge is 1/2 in. copper on a valve with a larger outlet; the relief discharge must be the full size of the valve outlet, never reduced.
• Terminated in a crawlspace, where it discharges where nobody sees it, it can scald anyone servicing the space, and the moisture damage goes unnoticed.

Requirements for the replacement:

• Same nominal size as the valve outlet, no reducers, no thread at the discharge end, no cap or plug.
• Gravity flow only: no traps, no low pockets, no valves, no tees to other piping.
• Material rated for the temperature of the discharge (copper, CPVC, or listed relief piping) - not PEX, not PVC.
• Terminate within 6 in. of the floor or outside the building, in a location that will not scald anyone and is visible to the occupant, and where the discharge cannot damage structure.
• Route it so a full-flow discharge will not splash onto electrical equipment, and so the outlet does not discharge into a crawlspace or an occupied area.
• Confirm the T&P is the correct rating for the heater's input and tank, and that the temperature probe is long enough to sense the top of the tank.
• Test the valve after installation and tag the date.