6.1 Pressure Limits, Pressure Reducing Valves, and Water Hammer

Two different pressures

Water Supply items regularly hinge on telling these apart:

• Static pressure — pressure with no flow. Governed by IPC 604.8, capped at 80 psi.
• Flow pressure — the residual pressure at the fixture supply outlet while the fixture is flowing at its design rate. Governed by IPC Table 604.3.

A house can have 78 psi static and still fail Table 604.3 at the top-floor shower, because undersized piping and long developed length eat the pressure once water moves.

Table 604.3 — required capacity at fixture supply outlets

IPC 604.3: the distribution system must be designed and sized so that under peak demand the capacities at fixture supply outlets are not less than Table 604.3. For fixtures not listed, use the manufacturer's installation instructions.

Read the pattern: 8 psi is the ordinary case, 20 psi applies where a mixing or pressure-balancing valve introduces a pressure drop, and 25, 35 and 45 psi belong to flushometer valve devices that need pressure to drive the flush. The blowout water closet at 45 psi is the highest requirement in the table, and the 25 gpm flow rate for flushometer water closets is the highest flow rate.

Footnote b: where the shower mixing valve manufacturer indicates a lower flow rating for the valve, the lower value applies.

Static pressure and the 80 psi ceiling

IPC 604.8:

Where water pressure within a building exceeds 80 psi static, an approved water pressure-reducing valve conforming to ASSE 1003 or CSA B356 with strainer shall be installed to reduce the pressure in the building water distribution piping to not greater than 80 psi static.

Exception: service lines to sill cocks and outside hydrants, and main supply risers where pressure from the mains is reduced to 80 psi or less at individual fixtures.

Two design requirements follow:

• 604.8.1 Valve design: the PRV must be designed to remain open to permit uninterrupted water flow in case of valve failure. Fail-open, not fail-closed — a fail-closed regulator would strand a building without water and, worse, without fire-fighting supply.
• 604.8.2 Repair and removal: PRVs, regulators and strainers must be constructed and installed to permit repair or removal of parts without breaking a pipeline or removing the valve and strainer from the line.

And remember the knock-on effect covered in Chapter 4 of this guide: installing a PRV closes the system, so IPC 607.3 then requires a thermal expansion control device downstream of it.

Inadequate and variable pressure

• IPC 604.6 Variable street pressures: where street main pressures fluctuate, the building distribution system shall be designed for the minimum pressure available. You size for the worst case, not the average.
• IPC 604.7 Inadequate water pressure: wherever pressure from the street main or other source is insufficient to provide the Table 604.3 flow pressures, a water pressure booster system conforming to Section 606.5 shall be installed.

Water hammer — IPC 604.9

The flow velocity of the water distribution system shall be controlled to reduce the possibility of water hammer. A water-hammer arrestor shall be installed where quick-closing valves are utilized. Water-hammer arrestors shall be installed in accordance with the manufacturer's instructions and shall conform to ASSE 1010.

Three separate obligations in one paragraph: control velocity, install arrestors at quick-closing valves, and follow the manufacturer for placement.

The physics: water hammer pressure rises with velocity, not with static pressure. A column of water moving at 8 feet per second that is stopped instantly can spike the line by several hundred psi. That is why designers hold distribution velocity near 8 feet per second or less on cold water and lower on hot — and why Table 604.10.1 gives manifold capacities at 4 and 8 feet per second. Quick-closing valves include solenoid valves on clothes washers, dishwashers and ice makers, and flushometer valves.

Arrestors are sealed, engineered devices under ASSE 1010. A capped vertical stub of pipe is not an arrestor; the air charge in an open standpipe dissolves into the water within weeks and the device stops working.

Worked pressure budget

A three-story house sits on a street main whose minimum daily static pressure is 55 psi at the meter. The highest fixture, a balanced-pressure shower valve, is 28 feet above the meter. A water softener in the line costs 8 psi.

Following the Appendix E104.1 method:

1. Start: 55 psi minimum daily static pressure.
2. Elevation: deduct 0.5 psi per foot of rise. 28 ft x 0.5 = 14 psi. Remaining: 41 psi.
3. Special equipment: deduct the softener's manufacturer-published loss of 8 psi. Remaining: 33 psi.
4. Special fixture allowance: deduct pressure in excess of 8 psi required by the special fixture. The balanced-pressure shower valve needs 20 psi flow pressure, so deduct 20 - 8 = 12 psi. Remaining: 21 psi.
5. Use 21 psi to select the pressure range column in Table E104.1 — the 30 to 39 psi band does not apply; you are below it, which is a signal the design will need larger pipe, a shorter developed length, or a booster.

That is the honest arithmetic behind IPC 604.7: when the remaining pressure cannot support the Table 604.3 requirement through the piping you can install, a booster system becomes mandatory rather than optional.

Checks an inspector will make