Perhaps one of the least understood functional areas of a closed-loop hydronic heating system is the expansion tank. This one item receives the least amount of attention of all the components in the heating system.

Much of this inattention is due to a general lack of knowledge. Here’s some information to help you to better understand the operation of expansion tanks.

Water, as we all know, expands when it is heated. Conversely, it contracts when cooled. If we don’t have some means to compensate for all this expansion and contraction, the fluid pressure in our systems will fluctuate wildly.

The first expansion tanks on the old gravity systems were nothing more than copper-lined, open-top tanks located above the highest part of the heating system. As the water expanded, it filled the tank in the attic with the expanding fluid. If the tank overflowed because of excessive fill, the excess water spilled into the gutter of the attic. No big deal.

When the water cooled down, the tank volume decreased but still maintained a full heating system. Filling of the system was done via manual fill, and a gauge in the mechanical room indicated how many “feet” of water were above the boiler. A mark on the gauge usually indicated cold fill level.

As time went on, and systems evolved, a need arose for higher operating temperatures. In order to achieve this, a “closed” pressurized system had to be created. Hence, the open expansion tank went the way of the dinosaurs.

People knew early on that they must have an air charge inside the tank because water is incompressible. The early tanks were basically a cylindrical steel tank, just like the vessel in a gas-fired water heater. They were connected to the boiler because this is where it was thought that the air that was in suspension in the water had the greatest potential of coming out of solution.

The air was recovered from the water and sent to the expansion tank to maintain the “cushion” that could be compressed when the fluid volume of the system expanded.

This type of system is called an “air in” system. Other than initial purging, all the air that was in the system will remain in the system and is supposed to be recovered at the boiler and sent back into the tank. Many fittings were developed to ensure that this was what happened, and that the air didn’t escape from the tank when the system cooled down.

This type of system should never have an auto vent installed anywhere on the system. If it does have one, remove it. It’s letting the air that’s supposed to be recovered and sent back to the cushion escape. Consequently, the expansion tank becomes water logged, and then the relief valve starts popping off.

Conversely, a system with a captive air bladder type of expansion tank is considered an “air out” system and must have an air elimination device installed. This device should be installed at a place where the velocity of the water is the lowest and the potential for air coming out of suspension is the greatest.

This “air out” method is the “new” type of system. Its operation is much quieter than the old “air in” systems and the potential for ongoing internal corrosion of ferrous components is held to a minimum. Remember, rust never sleeps.

See you next month for the second installment of Expansion Tanks 101. Until then, keep your caps dry and Happy Hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at guruofbtus@ mindspring.com or by phone at 303/778-7772.