Ch.Water FCU-Water connexions

I assume you are talking about chilled water connections (American spelling) to the cooling coil of a fan coil unit.

You can connect the water for either counterflow or concurrent flow to the airflow and it will work both ways, however the correct flow direction should provide a few percent more cooling capacity. There may be local exceptions, but the preferred water flow is typically counterflow.

An air purge vent is normally placed at the high point of the water system, because that is where the air bubbles end up. If the high point of a water system is a horizontal pipe, the air collect at the end of the horizontal flow, where the water begins to flow down and the air gets trapped.