The boiler vent valve is used for the purpose of allowing:

The function of the boiler vent valve primarily pertains to managing air within the boiler system. When filling a boiler with water, it’s essential to allow the air trapped inside to escape. This is because air in the boiler can lead to inefficiencies in heating and result in water hammer, which can damage the system. The vent valve facilitates the expulsion of air, ensuring that the boiler can properly fill with water for efficient operation.

While the other choices mention aspects related to boiler operation, they do not accurately capture the specific purpose of the vent valve. For instance, smoke escape pertains to a chimney or stack management system, while draining water is typically managed through different valves designed specifically for that purpose. Excess pressure release is generally handled by pressure relief valves specifically designed for that function to maintain safety within the boiler system. Thus, understanding the purpose of the vent valve in allowing air to escape while filling is crucial for effective boiler operation.