Drainage, Waste, and Vent (DWV) Systems
DWV work is the core of the C-36 trade and the largest share of exam calculation questions. Nearly every item reduces to four decisions: what size, what slope, how far the trap arm may run, and how the fixture is vented.
Fixture units and pipe sizing
Drainage fixture units (DFU) convert unlike fixtures to a common load: water closet 3 (flushometer valve 6), lavatory 1, bathtub or shower 2, kitchen sink 2, floor drain 2, clothes washer 2, dishwasher 2. Table values cap each size: a 1-1/2 inch branch takes 3 DFU, 2 inch takes 6, 3 inch takes 20. A 2 inch branch therefore cannot carry a water closet - a three-fixture bathroom group (3 + 1 + 2 = 6 DFU) needs a 3 inch branch to the WC and a 3 inch or larger building drain.
Slope, traps and trap arms
- 2-1/2 inch and smaller: 1/4 inch per foot. 3 to 6 inch: 1/8 inch per foot. 8 inch and larger: 1/16 inch per foot.
- Trap seal 2 to 4 inches; no S-traps, no double trapping, no bell traps.
- Maximum trap-arm length from trap weir to vent: 1-1/4 inch 5 ft, 1-1/2 inch 6 ft, 2 inch 8 ft.
- Floor drains in garages, mop sinks and similar locations need trap primers.
Venting and cleanouts
Every trap needs a vent. Individual, common, wet, circuit and stack venting are all recognized, and air admittance valves are permitted only where listed, accessible and accepted locally. Cleanouts are required at the base of each stack, at the upstream end of horizontal branches, at aggregate turns of 135 degrees or more, and at intervals along long runs. Fixtures draining below the next upstream manhole need a backwater valve.
Worked example
A 3 inch building drain leaves the building 60 ft from the stack. Invert at the stack is 4.00 ft. Fall = 60 x 1/8 in = 7.5 in = 0.63 ft, so the invert at the property line is 3.37 ft. If the city sewer invert there is 3.60 ft, the drain sits below the sewer and will not flow by gravity - you need more fall, a reroute, or a lift station.
Traps
- Adding fixture units without rechecking branch capacity.
- Trap arms run past the table limit, which siphons the trap.
- Omitting a cleanout at the base of the stack, or a trap primer at a floor drain.
- Venting a fixture into a drain line instead of a vent.