When should the air vent or cock be closed during operation?

The air vent or cock should be closed during operation at 25 psi. Closing the air vent at this pressure is essential for maintaining the system's efficiency and safety. When steam or hot water systems operate, they generate pressures that can lead to steam formation, which can push air out of the system. Proper venting allows trapped air to escape initially, ensuring that the system is correctly filled and that steam can circulate effectively.

Once the pressure reaches 25 psi, the introduction of steam and the subsequent rise in pressure means that the system transitions to a fully operational mode. At this point, leaving the vent open can cause steam loss, reduce operational efficiency, and might even hinder the heating process. Additionally, an open vent at higher pressures can risk possible steam escape or safety hazards. Hence, closing the air vent at 25 psi is a standard practice to ensure the system operates safely and efficiently under normal operating conditions.