Where should a leak detector probe be placed to check for leaks into the water box with water removed?

To check for leaks into the water box when water has been removed, placing the leak detector probe through a drain valve is the most effective method. Drain valves are typically situated at the lowest points of a water system and are designed to allow for drainage of water, which means they provide direct access to check for leaks within the water box.

When using a leak detector, it is essential to access areas where air might infiltrate or where leaks could potentially occur. The drain valve allows for probing into the water box without interfering with the water flow, providing a clear pathway for detecting any leaks that might be present.

Other options, such as the rupture disc or the vent valve, do not offer direct access to the water box itself and may not be suitable for leak detection in this context. A test plug could be used, but it is also likely to be less accessible than a drain valve, and may not provide as effective a seal as needed for accurate leak detection. Thus, using the drain valve ensures the best chance of identifying leaks effectively.