12.2 Part 3 Worked Example: Accumulating Water Supply Fixture Units, Cold and Hot
Key Takeaways
Table 610.3 values represent the total load on the cold water building supply; hot and cold branch loads may each be taken as three-quarters of the listed value (note 3).
Flushometer fixture units depend on how many valves a section serves: 40 for one water closet, 70 for two, and 20 for one urinal, with water closets and urinals added together (Table 610.10).
A water heater's cold inlet carries the hot-side fixture units of the fixtures it serves.
The first hose bibb counts 2.5 fixture units and each additional hose bibb 1.0 on shared piping, but each hose bibb branch is sized for 2.5 (Table 610.3 note 8).
In the example office, the building supply carries 99.5 fixture units: 90 for the flushometers plus 9.5 for the other fixtures.
The Rules for Counting
- Use the right column of Table 610.3 (private, public, or assembly). An office restroom is public.
- Flushometer valves use Table 610.10, counted by the number of valves on each pipe section:
| Valves on the section | Water closets (FU) | Urinals (FU) |
|---|---|---|
| 1 | 40 | 20 |
| 2 | 70 | 35 |
| 3 | 90 | 45 |
| 4 | 105 | 53 |
| 5 or more | 115, plus 10 per valve over 5 | 58, plus 5 per valve over 5 |
Water closet and urinal values are added when both are on a section (Example 610.10: 2 WC plus 2 UR = 70 + 35 = 105). 3. Hot and cold separation (note 3): For fixtures with both hot and cold, the listed value is the building supply load. Each separate hot or cold branch may be counted at 3/4 of the listed value. Cold-only fixtures (drinking fountain, hose bibb, flushometers) count their full value on the cold side. 4. Hose bibbs (note 8): the first counts 2.5 and each additional counts 1.0 on shared piping. Each hose bibb's own branch is sized for 2.5. 5. Continuous demands (note 5) are added in gpm, separately from fixture units.
Example Values (Table 610.3, Public)
| Fixture | Listed FU (public) | Cold side (3/4) | Hot side (3/4) |
|---|---|---|---|
| Lavatory | 1.0 | 0.75 | 0.75 |
| Service or mop basin | 3.0 | 2.25 | 2.25 |
| Kitchen sink, domestic type | 1.5 | 1.125 | 1.125 |
| Drinking fountain | 0.5 | 0.5 (cold only) | — |
| Hose bibb | 2.5 | 2.5 (cold only) | — |
| 2 flushometer water closets + 1 flushometer urinal | 70 + 20 = 90 | 90 (cold only) | — |
Segment Ledger
Cold side
| Segment | Serves | Fixture units |
|---|---|---|
| S1 building supply (meter to hose bibb tee) | Everything | 90 + 2 × 1.0 + 3.0 + 1.5 + 0.5 + 2.5 = 99.5 |
| HB hose bibb branch | Hose bibb | 2.5 |
| S2 (hose bibb tee to water heater tee) | All but the hose bibb | 90 + 2.0 + 3.0 + 1.5 + 0.5 = 97.0 |
| S3 cold main to restroom | Flushometers plus the cold side of the other fixtures | 90 + (0.75 × 2 + 2.25 + 1.125 + 0.5) = 95.375 |
| S4 flushometer branch | 2 WC + 1 UR | 90 |
| S4a single water closet branch | 1 WC | 40 |
| S4b urinal branch | 1 UR | 20 |
| S5 cold branch to lavatories, sinks, fountain | Cold side only | 1.5 + 2.25 + 1.125 + 0.5 = 5.375 |
| S6 cold feed to water heater | Hot side of the lavatories and sinks | 1.5 + 2.25 + 1.125 = 4.875 |
Hot side
| Segment | Serves | Fixture units |
|---|---|---|
| H1 hot main | Lavatories, mop sink, kitchen sink | 4.875 |
| H2 hot to lavatories | 2 lavatories | 0.75 × 2 = 1.5 |
| H3 hot to sinks | Mop sink and kitchen sink | 2.25 + 1.125 = 3.375 |
Why S2 is 97 and not 100.25
Upstream of the water heater tee, the pipe carries each fixture's combined hot and cold demand, and Table 610.3's listed value already represents that. Downstream of the split, each side is counted at 3/4. Adding the two sides back together (95.375 + 4.875 = 100.25) double-counts. Always use listed values for piping that carries both the hot and cold demand, such as the building supply and the main ahead of the heater.
Private Occupancy Example (A Quick Contrast)
A house has 2 tank water closets (2.5 each), 2 lavatories (1.0 each), 1 tub/shower (4.0), 1 shower (2.0), a kitchen sink (1.5), a dishwasher (1.5), a clothes washer (4.0), and 2 hose bibbs (2.5 + 1.0).
- Building supply: 5.0 + 2.0 + 4.0 + 2.0 + 1.5 + 1.5 + 4.0 + 3.5 = 23.5 FU.
- Hot main: 0.75 × (2 × 1.0 + 4.0 + 2.0 + 1.5 + 4.0) + dishwasher. A dishwasher is hot-only, so count its full 1.5. That gives 0.75 × 13.5 + 1.5 = 10.125 + 1.5 = 11.625 FU.
A Quick Self-Check
Before moving on to sizing, confirm three things on your ledger:
- Flushometer sections: every section serving flushometers uses Table 610.10's cumulative value for the valves downstream of that point. The branch past the first water closet serves one valve (40), and the branch serving both serves two (70).
- Hot plus cold does not equal total. Downstream of the heater tee, the hot and cold sides are each 3/4 of listed values. Upstream, the listed total applies.
- No fixture is missing. Count the fixtures on the drawing and compare with your ledger. A missed hose bibb or drinking fountain changes the building supply total.
Common Counting Errors
- Treating each flushometer water closet as 40 on a section serving two. Two valves on one section = 70, not 80.
- Using public values in a house, or private values in an office.
- Forgetting the cold feed to the water heater, which carries the hot-side load.
- Counting the second hose bibb at 2.5 on the shared main. It is 1.0, but its own branch is still sized for 2.5.
- Applying the 3/4 rule to the building supply. The building supply uses full listed values.
A pipe section serves two flushometer water closets and one flushometer urinal. How many fixture units does Table 610.10 assign to that section?
80
90
100
60
A private lavatory is rated 1.0 fixture unit in Table 610.3. How many fixture units does it add to a separate hot water branch under note 3?
0.75
1.0
1.5
0.5
The example building's main ahead of the water heater tee serves every fixture except the hose bibb. Which total should be used for it?
The listed Table 610.3 values, 97.0
Only the flushometer value, 90
The hot value only, 4.875
The sum of the 3/4 hot and 3/4 cold values, 100.25
Sections you finish are checked off in the contents.