5.1 Water Supply Fixture Units (WSFU) & Demand

The load factor for water piping

SPS 382.40 (6) (a) says the load factor for intermittent fixtures "shall be computed in terms of water supply fixture units as specified in Tables 382.40-1b and 382.40-2." Fixture units are then converted to gallons per minute with Table 382.40-3 (or Table 382.40-3e for treatment devices). The water side is completely separate from the drainage dfu values in Table 382.30-1. A lavatory is 1 dfu and also 1.0 wsfu, but a nonpublic water closet is 4 dfu and only 2.0 wsfu (flush tank).

Each table gives hot, cold and total columns. Use the hot or cold column when sizing piping that carries only that water, and the total column for the building supply and water service.

Nonpublic fixtures (Table 382.40-1b)

Fixtures not listed may be assigned a value by comparison with a listed fixture that uses water at similar quantities and rates.

Public fixtures (Table 382.40-2), selected values

Commercial dishwashers and large-capacity commercial washers use the manufacturer's gpm (footnote b).

Converting wsfu to gpm (Table 382.40-3)

The conversion has two curves. Use the flushometer column when the system is predominantly flushometer water closets or syphon-jet urinals, and the flush tank column when it is predominantly flush-tank closets or washdown urinals.

The curves meet at 1,000 wsfu. Values between rows may be interpolated. Example: 15 wsfu on the flush-tank curve lies halfway between 8 and 14 gpm, or 11 gpm.

Table 382.40-3e is a separate conversion for water treatment devices serving an individual dwelling (for example, 8 wsfu = 6.5 gpm, 40 wsfu = 9 gpm). It may not be used for hose bibbs, hydrants or high-flow fixtures. Devices that treat water to meet Table 382.70-1 use Table 382.40-3 instead.

Continuous-flow devices (SPS 382.40 (6) (b))

Equipment demanding a continuous flow, such as a cooling tower makeup, a process line or a lawn irrigation zone, is computed at its anticipated flow rate in gpm. Add that gpm to the gpm you get from converting the intermittent fixtures. Don't convert it to fixture units.

Worked example: a single-family home

The house is predominantly flush tank, so 19.5 wsfu converts to about 13.7 gpm (interpolating between 10 wsfu = 8 gpm and 20 wsfu = 14 gpm). The hot-water piping carries only the hot column: 2 x 2.0 + 1.0 + 1.0 + 1.0 + 1.0 = 8.0 wsfu.

Water heater sizing alternate (SPS 382.40 (6) (c))

For a water heater serving a single residence or living unit used by one household, an alternate method is allowed:

1. Minimum heater flow rate = 0.65 x the calculated hot-water gpm from Tables 382.40-1b and 382.40-3.
2. A storage heater's flow rate may be based on 70 percent usable storage plus recovery rate, with a 10-minute minimum draw.
3. An instantaneous heater must deliver a rise that provides 110 degrees F at the terminal fixture.
4. The method may not be used for heaters serving high-flow fixtures (more than 4 gpm at 80 psig), hose bibbs, hydrants or fixtures needing 1/2-inch or larger supplies, or for sizing hot-water distribution piping.

Exam traps

• Mixing up dfu (drainage) and wsfu (supply) values.
• Using the flushometer curve for a house; a house is predominantly flush tank.
• Using the nonpublic table for a restaurant or office, where public values (water closet 3.0 tank or 6.5 flushometer) apply.