6.2 Water Supply Fixture Units and Estimating Demand

Why fixture units exist

No building ever runs every fixture at once. If you sized a 30-unit apartment building's water service for the simultaneous full flow of every fixture, you would install a main three times larger than anything the building will ever use.

The water supply fixture unit (wsfu) is a weighted probability index. Each fixture gets a load value reflecting how much water it uses, how fast, and how often. Add the load values, then convert the total to gallons per minute through a demand curve that flattens sharply as the count rises — the Hunter curve, tabulated in Table E103.3(3).

Do not confuse wsfu with drainage fixture units (dfu) from IPC Table 709.1. They are different systems with different numbers for the same fixture. A private lavatory is 0.7 wsfu on the supply side and 1 dfu on the drainage side.

Table E103.3(2) — load values assigned to fixtures

Footnote a carries two rules worth memorizing:

1. For fixtures not listed, assume loads by comparing to a listed fixture using water in similar quantities at similar rates.
2. The separate hot and cold water loads are each three-fourths of the total load for the fixture. Check it: a private lavatory is 0.7 total and 0.5 each side; 0.75 x 0.7 = 0.525, rounded to 0.5. A private bathtub is 1.4 total and 1.0 each side; 0.75 x 1.4 = 1.05.

The hot-plus-cold sum therefore exceeds the total (0.5 + 0.5 = 1.0 versus a total of 0.7), because hot and cold are rarely drawn at full rate simultaneously.

The bathroom group shortcut

A "bathroom group" bundles a water closet, a lavatory and a bathtub or shower into a single value. A private bathroom group with a flush tank water closet is 3.6 wsfu. Adding the same three fixtures individually would give 2.2 + 0.7 + 1.4 = 4.3 wsfu. The grouped value is lower because the code recognizes that one person uses one bathroom at a time.

Note the enormous jump for flushometer valves: the same private bathroom group goes from 3.6 to 8.0 wsfu. A public water closet goes from 5.0 to 10.0. That reflects the instantaneous 25 gpm draw of a flushometer valve.

Table E103.3(3) — converting load to demand

The table has two separate sets of columns: systems predominantly for flush tanks and systems predominantly for flushometer valves.

Three lessons from these numbers:

1. Demand is strongly nonlinear. Going from 10 to 100 wsfu — a tenfold increase in fixtures — raises flush-tank demand only from 14.6 to 43.5 gpm, a threefold increase.
2. Flushometer systems demand much more at low counts. At 10 wsfu, flushometer demand is nearly double flush-tank demand.
3. The two curves converge. At 1,000 wsfu both read 208 gpm, because with enough fixtures the statistics dominate the device type. The flushometer column does not even begin until 5 wsfu.

Worked example: a 12-unit apartment building

Each unit has one private bathroom group (flush tank), one kitchen sink and one clothes washer. The building also has a shared laundry with two 15-pound public washing machines and one service sink.

Total (combined) load:

Demand: this is a flush-tank building, so read the flush tank column. 80 wsfu gives 38.0 gpm and 90 wsfu gives 41.0 gpm. Interpolating at 87.8 wsfu gives roughly 40.3 gpm; sizing conservatively at the next tabulated value, use 41 gpm.

Hot water side: use the hot column of Table E103.3(2) for the hot branch. Bathroom group hot is 1.5 each, kitchen sink hot 1.0, private washer hot 1.0, public washer hot 3.0, service sink hot 2.25:

(12 x 1.5) + (12 x 1.0) + (12 x 1.0) + (2 x 3.0) + 2.25 = 18 + 12 + 12 + 6 + 2.25 = 50.25 wsfu hot, which reads about 29 gpm from the flush tank column.

The five-step E104.1 procedure

1. Supply load in wsfu from Table E103.3(2), using the combined column for the service and main distribution pipe.
2. Minimum daily static service pressure from the local water authority, adjusted for:
   • 0.5 psi deducted per foot the highest outlet is above the source (added per foot below);
   • where a PRV is installed, the available pressure is 80 percent of the minimum daily static pressure at the source, or the PRV set pressure, whichever is smaller;
   • deduct all pressure losses from special equipment (backflow preventer, filter, softener) per the manufacturer;
   • deduct pressure in excess of 8 psi required by special fixtures such as a temperature-controlled shower or a flushometer tank water closet.
3. Maximum developed length = actual pipe length from the source to the most remote fixture, including hot (through the water heater) or cold branches, multiplied by 1.2 to account for fittings.
4. Read the service, meter and main distribution size from Table E104.1 at the intersection of the pressure range, developed length column and total demand.
5. Size each distribution pipe by working back from the most remote outlet on each branch, accumulating the related hot or cold column values. No branch or main needs to be larger than the main distribution pipe established in step 4.