What is the difference between a stop valve and a service valve in domestic pipework?

• A. They are identical in function.
• B. A stop valve shuts off water to a supply line; a service valve isolates a portion of pipe or a fixture for maintenance.
• C. A stop valve is used to filter water; a service valve heats water.
• D. A stop valve is only used for gas lines.

Answer: (B)

The key idea is how valves control water for maintenance versus overall shut-off. A stop valve is placed on a supply line to shut off water to that line or section, typically used to stop flow to a part of the system in an emergency or when replacing that section. A service valve, on the other hand, is meant to isolate a specific portion of pipe or a fixture so you can perform maintenance or repair without shutting down water to the rest of the system. This targeted isolation is very handy for fixing a faucet, toilet, or a short length of pipe while other areas stay pressurized.

So the statement that describes one valve stopping flow to a line and the other isolating a portion of pipe or a fixture for maintenance is the best fit. The idea that they are identical, or that one filters water or heats it, or that they’re only for gas lines, doesn’t align with how these valves are actually used in domestic plumbing.