What should be done to the stop valve in the return line of a boiler when the outlet stop valve is closed?

When the outlet stop valve of a boiler is closed, it is necessary to close the stop valve in the return line as well. The closure of the outlet stop valve prevents the flow of water from the boiler, and if the return valve remains open, it could create a situation where the boiler is unable to manage the pressure and temperature effectively.

Closing the return line stop valve helps maintain proper water circulation, ensuring the boiler remains in a safe operational state. It prevents the backflow of water, which could lead to pressure drops or overheating in the boiler. By managing the flow of water through closing the stop valve in the return line, operators can avoid potential hazards and ensure proper functioning of the system.

Therefore, the correct answer emphasizes the importance of coordination between the outlet and return valves to maintain safe boiler operations.