If two or three boilers are connected to a common supply and return header, which type of stop valve would be used?

When two or three boilers are connected to a common supply and return header, a non-return stop valve is the appropriate choice because it ensures that water flows in only one direction and prevents backflow. This is crucial in a system with multiple boilers, as backflow could result in uneven water distribution, inefficiencies, and potential damage to the equipment.

The non-return stop valve allows for the isolation of individual boilers while maintaining the integrity of the entire system. In the event of maintenance or a failure in one boiler, this valve can prevent the flow of water from the other boilers into the isolated unit, ensuring safe and efficient operation throughout the system.

Other types of valves listed do have specific uses but are not suitable in this context. A gate valve is typically used for on/off control and is not designed for preventing backflow. A pressure reducing valve is used to control downstream pressure, which is important in different scenarios but not the main concern when connecting multiple boilers. A butterfly valve is widely utilized for flow regulation and quick shut-off but does not inherently prevent backflow like a non-return stop valve does. Thus, the non-return stop valve is the best option for maintaining system integrity in a multi-boiler setup.