A remodel needs a path to the roof for a new vent, and the only clear vertical route is an unused masonry chimney flue that also carries the vent connector for a gas water heater. What does the plumbing code say about running the vent up that flue?
- AIt is permitted where the flue is lined and the vent is carried to the top of the chimney in one continuous piece
A liner does not make a flue an approved plumbing vent path. The prohibition is about mixing the systems, not about the condition of the masonry.
- It is prohibited; a plumbing vent may not terminate in or be run through a chimney or a gas vent
- CIt is permitted where the appliance served by the flue is decommissioned and its connector is capped
Decommissioning the appliance does not convert a chimney into plumbing vent piping. If a plumbing vent is to run in that chase, it runs as its own pipe, terminating on its own.
- DIt is prohibited unless the vent is increased one pipe size for the portion inside the masonry chimney
Pipe size has nothing to do with the prohibition. A larger pipe in a chimney is still a plumbing vent in a chimney.
Why B is correct
The prohibition is absolute because the two systems have incompatible jobs. Keeping plumbing vents out of chimneys and gas vents protects the appliance venting from obstruction and keeps sewer gas out of a flue that discharges near the roof line.
What this question is testing
Whether the candidate knows the prohibition on terminating plumbing vents in chimneys and gas vents and recognizes that conditions attached to the practice do not make it acceptable.
On the job
Old houses tempt everyone with an abandoned flue running straight from the basement to the roof, and remodelers use them as chases constantly. Running a properly sized and supported plumbing vent pipe up through a masonry chase as its own continuous pipe, terminating above the roof, is a different matter from terminating into the flue itself, and it is worth being precise about which one the inspector is looking at. Any flue still serving a fuel-burning appliance is off limits entirely.
Memory technique
Smoke goes up its own pipe; sewer gas goes up its own pipe.
Exam tip
Combustion venting and plumbing venting never share a passage. Treat any option that combines them as wrong.
Where to look it up
IPC Chapter 9, vent terminals, together with the appliance venting requirements in the mechanical and fuel gas codes. The UPC carries the same prohibition in its venting chapter, so the principle survives a code change even though the terminal dimensions do not.