Air Management and Pressurization Part 2 – Air Control versus Air Elimination

By Chris Edmondson

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 Air must be managed in every type of closed hydronic system, either by (1) routing it to a specific place in the system where it can’t create problems like pipe corrosion or air blocking of components, or (2) by continuously eliminating it.

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 These are the two basic approaches to air management known as “air control” and “air elimination.”  Both rely on an air separator to separate air bubbles from the system water, and both require an expansion tank with an air cushion.  This air cushion is necessary to accommodate the expansion of water as it is heated; it also helps keep the system operating within the proper pressure range. 

Air Control

Air control systems account for approximately 5 - 10% of air management systems.   This is an older approach to air management, but still common enough for contractors and engineers to be familiar with its unique operating principles. 

 Air control systems rely on what is known as a standard type tank.  In a standard tank there is no separation or barrier between the water and the air in the tank. The air that fills the empty space above the water serves as a cushion that allows the water to expand as the system temperature increases (remember – water grows when it is heated!) while still keeping the system pressurized as the system cools and the water volume decreases.   Standard tanks are designed to receive the air that is separated out of the water by an air separator, and therefore should always be installed above the separator.  This can be structurally challenging given the size and weight of an expansion tank, which is why this approach to air management continues to be phased out as systems are upgraded.

Air Elimination

Air elimination is a more modern approach to air management.  It is also typically less costly to install because it eliminates the problem of elevating and supporting a heavy tank. 

 Air elimination systems use expansion tanks that have an internal bladder or diaphragm that keeps the water separate from the air cushion. With a bladder or diaphragm tank there is no reason to maintain air within the system beyond that which is already within the expansion tank. The air cushion inside the expansion tank is pre-charged with air to flex back and forth with water expansion.  Thus any entrained air is separate from the system through the air separator and completely eliminated via a high capacity air vent installed on the air separator. 

 Bladder/diaphragm tanks needn’t be located above the air separator since they are not there to collect air.  Rather, they can be installed right on the floor right beside the air separator.  They can also afford to be quite a bit smaller, taking up less room in an equipment room. Given these two advantages, it’s not surprising that 90 to 95% of new systems are designed to be air elimination systems. 

 There are, however, a couple of piping measures to keep in mind.  It is best to a avoid connecting a bladder tank directly above or below the pipe.  This could lead to trash plugging the drain or the formation of an air pocket if an air trap is made. It is also recommended to provide a 12” minimum anti-thermo-siphon loop to prevent gravity heating of the tank. 

 Click here to watch a webinar on Air Control Versus Air Elimination.  And stay tuned for more in our series on Air Management and Pressurization!