Plumbing Mathematics, Sizing, and Fixture Units
Plumbing Mathematics, Sizing, and Fixture Units
This domain tests the calculations behind sizing and layout. Expect questions on fixture units, slope, fall, pressure, volume, and unit conversion.
Core rules
- Drainage fixture units, DFU, convert fixtures to a common load: a water closet is commonly 3 DFU, a lavatory 1, a bathtub 2, and a shower 2.
- Water supply fixture units, WSFU, are used with a demand curve to estimate flow; do not add WSFU directly as gallons per minute.
- Slope is fall divided by run; express it in in/ft or percent. A 1/4 in/ft slope is about 2 percent.
- Pressure and head: 1 psi equals about 2.31 ft of water column.
- Volume: 1 cubic foot equals 7.48 gallons; convert dimensions to feet before multiplying.
Worked example
A 4 in building drain runs 60 ft at 1/8 in/ft. Fall equals 60 times 0.125, or 7.5 in. If the same run were required at 1/4 in/ft, fall would be 15 in. The reasoning: multiply the length in feet by the slope in inches per foot, and keep the units consistent.
Traps that cost marks
Using 1/4 in/ft for 3 in and larger pipe when 1/8 is permitted, treating DFU and WSFU as interchangeable, forgetting to convert inches to feet, and using developed length as the horizontal run.