Which valves must be present on a gravity-fed boiler line?

In a gravity-fed boiler line, it is essential to have both a stop valve and a check valve to ensure proper functioning and safety. The stop valve serves to isolate the boiler from the water supply, allowing for maintenance and repairs without draining the entire system. This is critical for managing system integrity and preventing unintentional water flow.

The check valve plays a crucial role in preventing backflow, which could occur if there is a drop in pressure in the supply line or if other events compromise the water flow. By allowing water to flow in only one direction, the check valve protects the system from contamination and potential damage from reversed flow.

Together, these two valves provide necessary controls and safeguards in the gravity-fed system, enabling effective operation and maintenance of the boiler while ensuring safety standards are adhered to.

The other options lack the combination needed for effective operation. Simply having a stop valve does not offer protection against backflow, while two check valves may not facilitate necessary isolation for maintenance. A pressure reducing valve is not fundamental to a gravity-fed system, which relies on gravity rather than pressure for water distribution.