During winterization, what should be closed before proceeding to drain a system?

Closing the main shutoff valve before draining a system during winterization is crucial because it helps to isolate the system from the water supply. This step prevents additional water from entering the system while the draining process is taking place, ensuring that the pipes and components can be fully emptied. By closing this valve, technicians can avoid potential issues related to water freezing in the system, which can lead to pipe bursts or damage over the winter months.

Now, regarding the other options, while it is important to allow air to enter the system by opening the highest level faucets to facilitate proper drainage and to drain from the lowest connection to ensure that all water is removed, these actions should occur after closing the main shutoff valve. Expansion shields also do not directly relate to this initial step in the winterization process, as they are more concerned with accommodating changes in pressure rather than facilitating an effective drain. Thus, closing the main shutoff valve is the essential first step before proceeding with the drain.