Proper Air Management in a Hydronic Systems

Whether you are troubleshooting an old forced hot water system or installing a new one, you must consider air that will be present in the system. The two basic types of air management in hydronic systems are air control and air elimination. When installed properly, both are effective at preventing air problems.

Air Control

The air control method has been around for more than 40 years and has proven quite successful. It uses a standard steel tank in which the air and water actually touch. The air inside the tank acts like a spring pushing down on the water to keep the system pressurized.


The air cushion in the tank is compressed by the heated water that expands into the steel tank. The compressing of the air in the tank causes the system pressure to increase, but through proper sizing methods, the pressure increase won’t reach the relief valve’s setting. Without this cushion of air, the pressure in the system will rise rapidly when the water is heated, causing the relief valve to dump water onto the floor. Therefore, for this system to work properly, it is important that a cushion of air be maintained in the compression tank.

Unfortunately, one of the characteristics of this system is that the water and air “see”each other inside the compression tank. We know that water can absorb air into solution: the hotter the water and the lower the pressure, the more air will come out of solution in the form of bubbles; as the water temperature cools down or the pressure increases, the water will be able to absorb more air. On the next firing cycle, if the air that comes out of solution isn’t directed back into the compression tank — and is instead vented out of the system — the tank will start to lose its cushion. Then it is only a matter of time before the tank becomes waterlogged.

For an air control system to work properly, it is important to use some type of air separating device that “catches” the air when it comes out of solution and then immediately directs it back up into the steel compression tank. Do not use automatic vents in an air control system. They will do a good job of venting air out of the system that really belongs back in the expansion tank. Also, don’t forget to install a device known as an Airtrol Tank Fitting (ATF).

It does a great job of preventing the cooler, “air-filled” water from sliding out of the tank in gravity flow. Finally, make sure the line connecting the tank to the air separator is pitched up towards the tank without any pockets or places where the air can get trapped.

Air Elimination

The air elimination method has also proved to be quite effective. This method uses a diaphragm or bladder-style expansion tank instead of a standard steel tank. The tank is pre-charged with air on one side of the membrane that separates the system water from the air.


Any air that is released from the water needs to be vented out of the system through automatic air vents. This tank style also gives you a lot of flexibility regarding installations. When a standard steel tank is used, it is necessary to locate the tank somewhere above the air separator so that the separated air can flow by bouyancy back into the tank; however the diaphragm tank can be located anywhere because it already has a charge of air in the tank. The air pressure in the tank must be pre-charged to the same pressure as the system’s fill pressure. When the tank is under-charged, cold system water will enter the tank even before the boiler has heated up the water. The result is an under-sized tank causing the relief valve to discharge water onto the floor. When checking an existing tank’s air charge, make sure you isolate the tank from the system. If you don’t, you will just read the pressure of the system at the point where the tank is connected. An effective air separator, such as the B&G EAS (see illustrations above), is also very important for successful air elimination; the difference is that now all of the separated air must be vented out through a high capacity automatic vent.

Where to find out more…


Both methods of air management have proven to be quite successful, but they must be installed properly to work. If you have any questions regarding air management in hydronic systems, contact your local Bell & Gossett representative. They are well trained in all aspects of hydronics. Ask for a copy of B&G’s training manual TEH-1196, Air Management.