Which of the following actions should NOT be taken when firing a cold boiler?

When firing a cold boiler, the correct procedure is crucial for ensuring safety and efficiency. Among the actions listed, the opening of blowdown valves should not be taken at this stage.

Blowdown valves are used to remove sediment and impurities from the boiler water. However, when the boiler is cold, opening these valves can lead to a significant drop in water level and may cause damage to the boiler itself. Additionally, it could introduce air into the system, which is not desired when starting up a cold boiler.

On the other hand, setting the water level to about 1/3 of the gauge glass is an important step, as it helps to ensure there is sufficient water in the boiler to prevent overheating during the initial stages of firing. Venting is also critical in this context as it allows any trapped air to escape and helps to establish proper circulation. Prepurge is necessary to clear out any combustible gases that may have accumulated before ignition, ensuring a safe startup process.

In summary, while the other options involve necessary steps for the safe operation of a boiler when it is cold, opening blowdown valves is counterproductive and poses risks that should be avoided.