Pressure
Pressure is a measurement of force per unit area expressed in pounds per square inch (psi). The minimum required pressure is usually considered to be 15 psi, while the maximum pressure allowed by code is 80 psi. Systems above 80 psi are required to have a pressure reducing valve (PRV) installed to bring the pressure down to below 80 psi to prevent harm to the plumbing system.
A PRV has an adjustable bolt that, when turned, increases or decreases the spring tension. This spring causes the water to use force to push the diaphragm in the regulator up to allow water through, which in turn reduces the outgoing pressure.
To increase the pressure, turn the bolt counter-clockwise (loosen). To decrease the pressure, turn the bolt clockwise (tighten). When setting the pressure, you place a pressure gauge on an upstream hose bibb and turn the dial to increase the pressure. Run some water upstream, such as a bathroom faucet, so that you can read the changes to pressure as you adjust the pressure gauge.
Static pressure is the pressure of the system with no flow. If you place a pressure gauge on a hose bibb and there is no water running in the building upstream this will be the static pressure reading.
Residual pressure is the pressure of the system with downstream flow. If you place the pressure gauge on the hose bibb while water in running in the building, the reading on the upstream pressure gauge is called the residual pressure.
To test a PRV, take a static reading of the system using your hose bibb pressure gauge, then run water in the building to check the residual pressure. You should run multiple fixtures to take multiple residual pressure readings. A drop of more than 5-10 psi indicates the PRV has failed and is restricting the flow of water. A failing PRV is one of the most common causes of pressure problems. If the water pressure cannot be adjusted using the adjustment bolt this also indicates that the PRV has failed, and should be replaced.
Pressure problems such as water hammer can usually be attributed to volume problems. Water hammer most often occurs when there is a combination of high static pressure and a volume restriction in the system. This will cause the high pressure to quickly drop when water is flowed, followed by quickly going back to the static pressure. Water hammer occurs when there is a big, sudden drop between residual and static pressure.
Other than a failed PRV, here are some other common causes of water hammer:
Valve barely open, or dropped gate in a gate valve. A ball valve or a gate valve both can fail in a way that leaves the valve stuck in a shut or partially shut position if the stem disconnects from the ball or gate.
Galvanized piping rusting shut
Failed check valve on a dielectric water heater nipple (heat trap nipples). If only hot water side is affected this is a good one to check for.
Loose washer on a globe valve or faucet stem/cartridge oscillating open/shut. This can happen with multi-turn angle stops that are not fully open.
Malfunctioning quick-acting valve, such as a solenoid valve (irrigation or water leak-sensing valve, for example). This will often appear to be the source of the problem because of water hammer occurring when the irrigation turns on, however more commonly this usually means the volume problem is upstream.
This pressure gauge screws onto a hose bibb. It has a second arm that records the highest pressure sensed. These are used for diagnosing pressure problems that occur intermittently so it may be left on to read how high the pressure goes over a long period.
Restriction in galvanized piping.
