An air separator is a device used to trap, separate, and eliminate air from the water in a hot-water space heating boiler. Air separators are not installed on boilers used only for heating the domestic water supply.

The air separator is screwed into the boiler supply nozzle. The separator traps the air in the top of the boiler section where the water is hottest and where it travels a long horizontal path at low velocity, permitting the air bubbles to separate. The trapped air escapes through a 3/4 inch tapping into an air cushion expansion tank or through an automatic float vent on systems using diaphragm expansion tanks.

An air separator can only expel air which reaches the boiler. However, sometimes air pockets remain trapped in the piping or radiation, thereby impending the water flow and reducing the heating performance of the boiler. Such air pockets can be eliminated by installing several different purge fittings in the heating system. A manual air vent is installed on the highest point of the system (automatic air vents are not generally required).

Some boilers are constructed with built-in air separators, which make the addition of an external unit unnecessary. In this design, the air is diverted to an automatic air vent. A variation of this design provides for the diversion or air to an expansion tank. The manufacturer refers to this device as an air eliminator. Its function is similar to the air eliminator used on pipe lines, but it differs in design and construction.

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