Trade Exam · Module 5
Common Water Supply Problems
Low pressure at one fixture, low or high pressure everywhere, running toilets, dripping faucets, water hammer and hidden leaks. This lesson shows you how to track each one down with a gauge and a few simple tests, and ties the pressure, valve and thermal expansion rules to the 2025 California Plumbing Code sections they come from.
Key Takeaways
- Low flow at one fixture points to that fixture: the aerator, the fixture stop, the supply connector or the valve cartridge.
- Low pressure everywhere points upstream: the main shutoff, the pressure regulator and its strainer, the meter or the service line.
- CPC 608.1 requires at least 15 psi residual at the fixtures; CPC 608.2 requires an approved regulator with a strainer where static pressure exceeds 80 psi.
- Where a regulator, check valve or backflow preventer closes the system, thermal expansion must be controlled with a listed expansion tank or similar device (CPC 608.2, 608.3).
- A T&P relief valve that drips or discharges is a warning sign of excess pressure or temperature, not the fix; its discharge line runs full size with no valves (CPC 608.5).
- A fullway valve is required on the discharge side of the meter and on the cold supply to each water heater (CPC 606.2); control valves go ahead of each appliance and each slip joint or appliance supply (CPC 606.5).
- Dye in the toilet tank separates a leaking flapper or seat from a fill valve that overfills into the overflow tube.
- Check the meter's leak indicator with every fixture off, then close valves one at a time to isolate a hidden leak.
Why This Matters
Supply-side calls are a big share of service work, and the C-36 exam's Service, Repair, and Remodel section lists troubleshooting, isolating leaks and repair. The approach is the same on every call: measure, isolate, then fix. A pressure gauge is the most useful diagnostic tool on the supply side.
Know the Pressure Numbers
The 2025 California Plumbing Code (CPC) gives you the limits to compare a gauge reading against:
- Minimum: CPC 608.1 requires a residual pressure of not less than 15 psi at the fixtures after friction and other losses. If a fixture or fitting needs more, that higher minimum must be provided. If the supply cannot deliver it, a tank and pump or other means is required.
- Maximum: CPC 608.2 requires an approved pressure regulator, preceded by an adequate strainer, where static pressure in the water supply piping exceeds 80 psi. The regulator reduces static pressure to 80 psi or less. Regulators must comply with ASSE 1003 or AWWA C530, be accessible, and have a strainer that can be cleaned without removing the regulator. Regulators 1-1/2 inches and larger do not require a strainer.
- Thermal expansion: CPC 608.2 also requires an approved expansion tank in the cold water piping downstream of each regulator to keep thermal expansion from pushing pressure over 80 psi. CPC 608.3 requires a listed, properly sized expansion tank or similar device on any system with a check valve, backflow preventer or other normally closed device that stops building pressure from dissipating back into the main.
How to read static pressure: put a gauge on a hose bibb, make sure nothing else in the building is running, and open the bibb. That is static pressure. Open a few fixtures and watch how far it drops; that tells you about flow restrictions.
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More lessons in Service, Repair, and Remodel
- 1The Troubleshooting Process
- 2Common Drain Problems
- 3Common Water Supply ProblemsYou are here
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- 5Sewer Line Repair and Replacement
- 6Remodel and Retrofit Work
- 7Emergency Plumbing Response
- 8Fixture and Water Heater Replacement
- 9Retrofit Plumbing Systems
- 10Disposal of Plumbing Materials