Airtrol Tank Fittings help establish the proper initial liquid level in the compression tank when filling the hydronic system. They help maintain proper system pressurization by directing released air to the compression tank during system operation and permit cooler liquid flow to the boiler during the off cycle as system liquid cools. Airtrol Tank Fittings must be used with the appropriately sized In-Line Airtrol or Rolairtrol air separator.