Which statement correctly differentiates a service shut-off valve from an isolation valve, and where are they typically located?

• A. Service shut-off controls the main incoming supply; isolation valves control individual fixtures or branches; service shut-off near the street or meter, isolation valves at fixtures or branch lines.
• B. Service shut-off controls the meter reading; isolation valves control the main incoming supply; located at the street.
• C. Service shut-off controls individual fixtures; isolation valves control the entire building; located in the basement.
• D. Service shut-off has no function; isolation valves are only for hot water lines; located at the roof.

Answer: (A)

The main idea is that there are two levels of control in a plumbing system. A service shut-off valve is the main valve that stops water entering the building from the supply, so it shuts off water to the entire system. An isolation valve is a valve placed at individual fixtures or branch lines, allowing you to stop water to just that fixture or branch without affecting the rest of the building. The service shut-off is typically located where the supply enters the property—often near the street or at the water meter. Isolation valves are located at or near each fixture or appliance—for example under sinks, behind toilets, or on the supply line to a washer—so you can service that item without shutting off the whole building. Other options mix up what each valve controls or where they’re found, which isn’t correct.