Plumber Red Seal · Question
A plumber is sizing a common vent for two water closets installed back-to-back in a commercial washroom. Each water closet has a drainage fixture unit (DFU) value of 6. If the common vent also serves a lavatory (2 DFU) located nearby on the same floor, what should be the minimum nominal pipe size for the common vent according to the NPC?
Each water closet has 6 DFUs, so two would be 12 DFUs. Adding the lavatory's 2 DFUs brings the total to 14 DFUs. According to NPC Table 2.5.7.1.(2) for a common
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Question: A plumber is sizing a common vent for two water closets installed back-to-back in a commercial washroom. Each water closet has a drainage fixture unit (DFU) value of 6. If the common vent also serves a lavatory (2 DFU) located nearby on the same floor, what should be the minimum nominal pipe size for the common vent according to the NPC?
Answer options:
- 50 mm
- 65 mm ✅ 75 mm
- 100 mm
Correct answer: 75 mm
Explanation: Each water closet has 6 DFUs, so two would be 12 DFUs. Adding the lavatory's 2 DFUs brings the total to 14 DFUs. According to NPC Table 2.5.7.1.(2) for a common vent, a vent pipe serving 14 DFUs requires a minimum nominal pipe size of 75 mm. Options 50 mm and 65 mm are too small for this DFU load.
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