Maximum Drainage Fixture Unit - DFU - loads in sanitary piping systems.
The relationship between volume flow and fixture units is not constant, but varies with the number of fixture units. A Fixture Unit is not a flow rate unit but a design factor based on the rate of discharge, time of operation and frequency of use of a fixture. A fixture unit is equal to one cubic foot of water drained in an 1 1/4 pipe over one minute (≈ 7.48 US gpm, ≈ 0.47 l/s).
Maximum Drainage Fixture Units (DFU) in Stacks and Horizontal Fixture Branches
|Pipe Size||Maximum Drainage Fixture Units (DFU)|
|Horizontal fixture branch||Stacks3) less than three stories in height||Stacks more than three stories high|
|Total for stack||Total for one story|
0) No water closet permitted
1) Maximum two water closets
2) Maximum six water closets
3) Stacks - vertical runs of plumbing waste or drainage lines
The values are based on the major US plumbing codes.