Stopped the Leak, Now a Pressure Spike Surfaces: Two-Fault Decision Tree

Why this matters

You repaired a drip or a running fixture and now the system shows a pressure spike: a relief valve weeps, a fill valve sings, supply lines bang, or a gauge climbs after every water-heater recovery. This is a two-fault scenario. The prior leak was acting as a relief path, bleeding off thermal-expansion pressure before it could build. The moment you sealed the system, expansion had nowhere to go, and a missing or waterlogged expansion tank, or a closed-system condition created by a PRV or check valve, now reveals itself. Closing the call means controlling the expansion, not just chasing the new symptom.

Symptom presentation

After the leak is fixed:

• The water-heater T&P or a system relief valve weeps shortly after a heating cycle, then stops.
• Static pressure reads normal cold but climbs well above the set point after the heater recovers.
• A toilet fill valve hisses or a faucet drips that did not before.
• Banging or a thump on the supply when the heater finishes heating.

The signature is pressure that rises with heating cycles and falls with use. That is thermal expansion in a closed system, previously masked because the leak vented it.

Quick checks

1. Put a gauge on the system (water-heater drain or hose bib) and read cold static. Then run the heater through a recovery cycle and re-read. A rise of 30 to 60 psi or more after heating confirms uncontrolled thermal expansion; on a tight closed system the spike can reach the T&P set point of 150 psi.
2. Determine whether the system is closed: is there a PRV, a check valve, or a backflow preventer on the service? Any of these makes the system closed and requires expansion control. A recent meter swap that added a check is a common trigger for a newly closed system.
3. If an expansion tank is present, tap it. A tank that sounds solid (water all the way up) or feels heavy is waterlogged. A healthy tank sounds hollow in the upper portion and is noticeably cooler on the air side.
4. Check the tank's air-side charge with a tire gauge on the Schrader valve (system depressurized) against the cold static set point. Water weeping from the Schrader confirms a ruptured bladder.
5. Confirm the spike correlates with heating, not with use or time of day. Pressure that climbs after a hot-water draw and recovery, then drops the moment any fixture opens, is thermal expansion; pressure high even cold is a PRV/street issue instead.

Isolation tree

Branch A: no expansion tank present, closed system confirmed.

• The leak was the relief path. Install a properly sized, code-required thermal expansion tank, precharged to the cold static pressure.

Branch B: expansion tank present but waterlogged (solid, no air cushion).

• The bladder failed or the air charge bled off. The tank is full of water and provides no cushion. Replace the tank (or recharge if it is a non-bladder type and the air simply absorbed; bladder failure means replace).

Branch C: expansion tank present, air charge wrong.

• Air precharge must match the cold static set point. If it is far off, expansion control is poor. Depressurize and recharge to the cold static pressure, then re-test.

Branch D: pressure spike is not thermal (rises without heating, or stays high cold).

• This is a PRV/set-point or street-pressure issue, not expansion. Return to PRV diagnosis and verify the regulator holds downstream.

Confirming diagnosis

• The defining test: gauge static cold, force a water-heater recovery (or simply observe over a heating cycle with no draw), and watch pressure climb then hold high. A bleed of one cup of water drops it back. That cycle proves thermal expansion in a closed system.
• Tank diagnosis: with the system depressurized, the air charge read at the Schrader should equal the cold static set point. A waterlogged tank reads near zero air or weeps water from the Schrader.
• Confirm the relief/T&P stops weeping once expansion is controlled. A T&P that still discharges after expansion is fixed is itself failed or the source temperature is too high.

Remediation

• Install or replace the thermal expansion tank sized to the water-heater volume and system pressure. Precharge the air side to the cold static set point before installing, then verify after install. A 40 to 50 gallon heater on typical residential pressure takes a standard 2 gallon tank; larger or higher-pressure systems need more.
• If the system was not previously closed and a recent PRV or backflow device created the closed condition, the expansion tank is now mandatory by code.
• Verify the PRV holds the correct downstream static (50 to 60 psi) so the expansion tank is not fighting an over-pressure baseline. A tank precharged to one pressure on a system regulated to another will not absorb expansion correctly.
• Support the tank properly. A heavy waterlogged tank hanging on a stub of pipe stresses the connection; use a tank tee and bracket on larger units.
• Re-test: cold static correct, post-recovery pressure stays within bounds, no relief weeping, no fixture singing. Run a full heater recovery with no draw and watch the gauge hold.

References

• International Plumbing Code (IPC) Section 607.3, Thermal expansion control on closed water systems.
• Uniform Plumbing Code (UPC) Section 608.3, Thermal expansion tank or device required on closed systems.
• ASME A112.18.1 / CSA B125.1, plumbing supply fittings and pressure ratings.
• IPC Section 504.4, Water-heater relief valve (T&P) required and discharge requirements; ANSI Z21.22 relief-valve standard.
• ASME Boiler and Pressure Vessel Code references for T&P relief device performance on storage water heaters.