However, because air can be absorbed in water, some means of restricting the flow of cooler water (gravity circulation) from the tank into the system is needed without restricting passage of free air into the tank. Compression tank size can be reduced considerably by reducing vapor pressure if this is achieved.

B&G Airtrol Tank Fittings fill this need. The cross section of the ATF fitting (right) illustrates how air bubbles can rise directly into the tank, but water flow is restricted by a baffle or trap. ATF fittings are available for tanks up to and including 24" diameter.

Both the ATF and ATFL are equipped to help establish the correct initial air level in the compression tank when filling a system. After a system has been thoroughly cleaned and flushed, it is then filled with water.

In the process of filling, air will be trapped in the compression tank. Usually too much air is trapped within the tank, since additional air will be collected in the process of heating and deaerating the system water. A manual air vent is furnished on the ATF, separately with the ATFIL, which will permit excess air to be released from the tank prior to heat-up of the system.