What action signifies the official commencement of the 3-hour period for startup in a boiler?

The action that signifies the official commencement of the 3-hour period for startup in a boiler is placing refuse on the grates. This step is crucial as it indicates that the boiler is being prepared for operation and that the combustion process is about to begin. The placement of refuse, or fuel, signifies that the combustion will soon start, which is critical for measuring the startup duration accurately.

The other options do not mark the official start of this critical timeline. Lighting the burner is an action that occurs after the refuse has been placed, and though it leads to combustion, it does not initiate the startup period itself. Monitoring the temperature increase is a process that takes place during the operation of the boiler, rather than marking its start. Opening the main steam valve typically occurs once the boiler is operational and is not a step in the initial firing process.

Thus, placing refuse on the grates is the definitive action that defines the beginning of the startup phase, making it essential for tracking the operation timeline in a boiler system.