Most engineers know that when you change the temperature of a fluid, its volume changes. This fact is most important concerning hydronic systems, where temperature can change and systems are closed to the atmosphere.

The typical example is a closed-loop hot water heating system where temperature can vary over 100°F over the season. The typical solution for this fluid expansion is to select a tank with the admittance volume to permit the hydronic medium to expand while the system maintains a similar pressure. This component is called an expansion tank, and several types and piping locations are recommended for its position in the hydronic loop.

An Overview

Hydronic systems are pressurized so that at their highest point, they maintain a positive pressure relative to their surroundings. This permits fluid to be pushed to the top of the system, allowing for the removal of unwanted air. In some cases where the pumps are located at the top of the system, the positive pressure is needed to meet the net positive suction requirements of the pumps.

There are many rules of thumb about where to install the expansion tank in the system. Expansion tanks are not always at the point of common pressure, not always at the pump suction, are not always used for air elimination, or at the point of lowest pressure, or installed where the pressurized makeup water is connected. Consequently, the expansion tank can be installed at any point in the system, and it will meet the needs of expanding volume.

Design Considerations

Engineers need to assess many factors determining style and tank location. Over the years, systems requiring expansion tanks have evolved; many are now variable flow, have antifreeze in addition to water, and new tank technologies are available. Proper location must also consider structural and annual maintenance requirements.

Originally in hydronic systems, vented tank vessels were set above the highest piping point in the system. At this location, they aided in air removal since air is less dense and collects naturally at this point. Air vent piping from several high points can be routed to the tank for air removal. The pressure of the system is set based upon the tank’s elevation above the system, since it is vented to atmosphere. Installation conditions could affect providing overflow piping on a vented expansion tank. The design challenge with this style tank is to find a location inside the building that will properly pressurize the system specifically with 10 to 12 ft of elevation above the highest piping loop. In many systems, where the proper space and elevation are available, a vented expansion tank may be considered.

Expansion Tanks And You

Expansion tanks are available as closed vessels, where pressurized air over fluid provides the volume for fluid expansion. The air pressure can be adjusted as required by the system pressure at the point of connection to the system.

Tanks are available in many sizes and pressure ratings and have options such as sight glasses, drain fittings, and air admittance valves. Multiple tanks can be configured for large systems requiring greater expansion volume.

A popular design for small systems is for the tank to be located between the boiler and the pump suction, and to install an air removal fitting at the piping connection to assist air that has escaped the cushion, to be recaptured and returned to the expansion tank. When used with an air removal fitting, closed tanks should be mounted above the piping system to permit air to rise into the tank. This application is widely used in residential systems. This style tank should be installed near a drain, as they commonly lose the air cushion and must be emptied of fluid to restore operation.

Expansion tanks are available with internal bladders that separate the air and fluid with a membrane. This prevents the air cushion from escaping and permits many options for locating the tank. The tank can be set at any elevation relative to the piping system.

Multiple tanks can be configured where desired. The air pressure must be charged while the tank is empty; therefore, it is still important to consider drain and air admittance needs during design. A pressurized bladder tank can be located on the floor, to provide for easier installation and structural savings. The membrane separates the air cushion from the fluid, which is desirable with many antifreezes (such as glycol). Air removal is provided independent of the installation of the expansion tank when bladder-style tanks are utilized.

Selecting the style of expansion tanks is important if they are to be combined with an air removal design. Bladder-style expansion tanks should not be used with air removal fittings; they can be installed anywhere within the system where space allows, if they meet the pressure requirements for fluid temperature and volume admittance.ES