4.3 Water Pressure Limits, Booster Pumps, Potable Water Tanks, & Water Hammer (UPC 607–609)
Key Takeaways
Where the supply cannot provide at least 15 psi residual pressure after friction and other losses, a tank and pump or other means must provide it (UPC 608.1).
Static pressure over 80 psi must be reduced to 80 psi or less by a pressure regulator preceded by a strainer; regulators 1-1/2 inches and larger need no strainer (UPC 608.2).
A booster pump on a building supply needs a low-pressure cutoff on the inlet side within 5 feet of the inlet, set for at least 10 psi (UPC 609.8).
Water hammer arresters must be approved mechanical devices meeting ASSE 1010 or PDI-WH 201, installed as close as possible to quick-acting valves (UPC 609.10).
Potable water tank vents and overflows are screened with corrosion-resistant mesh of at least No. 24, and the overflow is at least 16 square inches (UPC 607).
The Pressure Window
| Condition | UPC response |
|---|---|
| Residual pressure below 15 psi after friction and other losses | 608.1: Install a tank and pump or other means to provide at least 15 psi. Where fixtures or fittings need more than 15 psi, provide that higher residual pressure. |
| Static pressure over 80 psi | 608.2: Install an approved pressure regulator preceded by an adequate strainer, and reduce static pressure to 80 psi or less. |
Static pressure is measured with no flow. Residual pressure is what remains at an outlet while water is flowing. The 15 psi floor is a residual value, after the pressure lost to friction, meters, devices, and elevation.
Pressure Regulator Details (UPC 608.2)
- Regulators 1-1/2 inches and larger do not require a strainer.
- The regulator controls pressure to the building's outlets unless the AHJ approves otherwise.
- The regulator and strainer must be accessible aboveground, or in a vault with a sized, sloped drain to daylight. They must be protected from freezing, and the strainer must be cleanable without removing the regulator body or disconnecting the piping.
- When sizing with Table 610.4, use 80 percent of the reduced pressure.
- Install an expansion tank in the cold water distribution piping downstream of each regulator, sized and installed per its listing (Section 3.4).
Elevation: Physics Versus the Sizing Table
A column of water exerts 0.433 psi per foot of height, and 1 psi lifts water 2.31 feet. Use this physics value when calculating actual pressure at a fixture. Table 610.4 sizing is different: 610.8 adds or subtracts 1/2 psi per foot, a deliberately conservative allowance. On the exam, read what the question asks: "pressure loss due to elevation" is 0.433 psi per foot, while "Table 610.4 pressure range" uses 1/2 psi per foot.
Example: A fixture is 70 feet above a 45 psi main in a six-story building. Elevation alone consumes 70 × 0.433 ≈ 30.3 psi, leaving about 14.7 psi before any friction losses. That is below the 608.1 minimum, so a booster system is required.
Booster Pumps (UPC 609.8)
Where a booster pump (other than a fire pump) is connected to a building supply or underground water pipe:
- Install a low-pressure cutoff switch on the inlet side, not more than 5 feet from the pump inlet.
- Set the cutoff for not less than 10 psi.
- Install a pressure gauge between the shutoff valve and the pump.
The cutoff prevents the pump from pulling the public main down toward a vacuum. That condition could collapse piping or cause backsiphonage at other connections. Modern variable-frequency-drive booster packages still need this cutoff. Low-pressure zones in tall buildings are often fed through pressure-reducing stations so lower floors stay at or below 80 psi.
Potable Water Supply Tanks (UPC 607)
| Section | Requirement |
|---|---|
| 607.1 | Install per the manufacturer's instructions; support per the building code |
| 607.2 | Tanks, coatings, and liners that supply drinking water meet NSF 61 |
| 607.3 | Tightly covered and vented; the vent is screened with corrosion-resistant mesh of at least No. 24 |
| 607.4 | Overflow of at least 16 square inches, screened with at least No. 24 mesh |
| 607.5 | Pressurized tanks need a listed pressure relief valve discharging per 608.5. A tank located above the fixtures it serves needs a CSA Z21.22 vacuum relief valve |
Water Hammer (UPC 609.10)
When a valve closes quickly, the moving water column stops almost instantly. Its momentum becomes a pressure spike that travels back and forth through the piping. As a rough physics estimate (the Joukowsky relation), stopping water instantly produces about 60 psi of surge for every 1 foot per second of velocity. That is why an 8 ft/s line can see a surge of several hundred psi, producing banging pipes, broken hangers, and failed connectors.
Section 609.10 requires:
- Water hammer arresters wherever quick-acting valves are installed. Examples include clothes washer and dishwasher solenoids, ice maker valves, electronic faucets, and flushometers.
- Arresters must be approved mechanical devices meeting ASSE 1010 or PDI-WH 201.
- Install them as close as possible to the quick-acting valve.
- 609.10.1: Listed mechanical devices are located and installed per the manufacturer's specifications.
A field-built air chamber (a capped vertical stub) is not an approved mechanical device. Its air dissolves into the water over time and the chamber becomes waterlogged.
PDI-WH 201 sizing
PDI-WH 201 sizes arresters by the fixture units they serve:
| PDI size | Fixture units |
|---|---|
| A | 1–11 |
| B | 12–32 |
| C | 33–60 |
| D | 61–113 |
| E | 114–154 |
| F | 155–330 |
Related Valve Rules
- 606.7: A single control valve is installed ahead of an automatic metering valve that supplies a battery of fixtures.
- 609.5: Unions are installed within 12 inches of regulating equipment, water heaters, conditioning tanks, and similar equipment that must be removed for service.
The static pressure at a new building's supply is 92 psi. What does UPC 608.2 require?
A relief valve set at 80 psi on the building supply
Nothing, because the limit is 100 psi
A pressure regulator preceded by a strainer, reducing static pressure to 80 psi or less, with an expansion tank downstream
A booster pump with a low-pressure cutoff
A booster pump is connected to a building's water supply. Where must its low-pressure cutoff be installed, and at what setting, under UPC 609.8?
On the discharge side, set at 5 psi
On the inlet side, not more than 5 feet from the pump inlet, set for not less than 10 psi
Anywhere in the building, set at 15 psi
At the water meter, set at 20 psi
A new commercial laundry has several solenoid-operated washers. What does UPC 609.10 require for water hammer control?
A pressure regulator set to 40 psi
Larger pipe sizes only
Capped 18-inch air chambers at each washer
Approved mechanical water hammer arresters meeting ASSE 1010 or PDI-WH 201, installed as close as possible to the quick-acting valves
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