Plumbing fixtures are being installed on a level that lies below the elevation of the public sewer serving the building. How must the discharge from those fixtures reach the drainage system?
- ABy gravity, provided a backwater valve is installed on the branch serving them
A backwater valve stops flow coming back toward the building. It provides no lift and does nothing for a fixture whose drain outlet is below the sewer.
- BBy gravity, provided the branch serving them is increased one nominal pipe size
Pipe size affects capacity, not elevation. A larger pipe on a negative grade still will not carry waste upward.
- CBy gravity into the building drain, provided each fixture is separately trapped and vented
Traps and vents protect against sewer gas and pressure changes. They are required in any case and have no bearing on whether gravity flow is possible.
- It must drain into a sump and be lifted into the gravity drainage system by an approved pump or ejector
Why D is correct
Fixtures below the sewer must discharge into a sump and be lifted mechanically. That is the only arrangement that moves the waste from a low elevation into a drainage system running above it.
What this question is testing
Checks that the candidate recognizes an elevation problem as distinct from a capacity or backflow problem, and knows the code answer is mechanical lifting rather than a valve or a larger pipe.
On the job
Basement bathrooms are the classic case, along with floor drains in below-grade mechanical rooms. Keep everything that can drain by gravity off the ejector, because every gallon pumped is a gallon that stops moving when the power fails. Size the pit and pump for the actual fixture load, provide an alarm, and make sure the owner understands that a below-grade bathroom depends on a machine.
Memory technique
Below the sewer means you pump it.
Exam tip
Compare fixture elevation to sewer elevation first. Everything else follows from that comparison.
Where to look it up
IPC Chapter 7, building subdrains along with sumps and ejectors. The UPC requires the same approach for fixtures below the sewer, with its own pump and pit provisions.