For proper system operation, the air cushion (Compression Tank) should be the only air space within a closed hydronic system. This air cushion can then provide adequate pressurization for all fluctuations of the system. Because air can be absorbed in water, some means of restricting the flow of cooler water from the tank into the system is needed without restricting the passage of air into the tank. Compression tank size can be reduced by reducing vapor pressure if this is achieved. The Airtrol can do this by allowing the air to rise in into the top of the tank, but the water flow is restricted by a ball check or trap.