Expansion Tanks

Expansion tanks (Figures 7-27 and 7-28) are installed in hot-water heating systems to provide for the expansion and contraction of the water as it changes in temperature. Water expands with the rise of temperature, and the excess volume of the water flows into the expansion tank.

Another feature of the expansion tank is that the boiling point of the water can be increased by elevating the tank. In other words, increasing the head (i. e., the difference in elevation between two

Figure 7-27 Open expansion tank. Because the tank is vented to the atmosphere, the pressure at the surface of the water is always that of the atmosphere.



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Points in a body of fluid) increases the pressure. As a result, the water can be heated to a higher temperature without generating steam, which, in turn, causes the radiators or other heat-emitting devices to give off more heat.

Both open and closed expansion tanks are used in hot-water heating systems. The open expansion tank is used on low-pressure systems, and the closed tank is used on high-pressure systems. Air in the tank above the water forms a cushion for increasing the pres­sure. As the temperature of the water rises, the water expands and flows into the tank, thus compressing the air and increasing the pressure.

The relation between pressure and volume changes of the air should be understood. According to Boyle’s law, at constant tem­perature the pressure of a gas varies inversely as its volume. Thus, when the volume is reduced by one-half, the pressure is doubled. This is not gauge pressure, but absolute pressure (the pressure mea­sured from true zero or point of no zero pressure).

In gravity hot-water heating systems, either closed or open pip­ing arrangements can be used. In an open gravity system, the expansion tank is located at the highest point in the system (e. g.,

Expansion Tanks

Figure 7-28 Closed expansion tank. This type of expansion tank is sealed against free venting to the atmosphere.

Roof, attic, or top floor; see Figure 7-14). The expansion tank used in this piping arrangement is an open type with an overflow pipe located at the top. Provisions can be made to return the overflow water to the boiler or to discharge it through outside runoff drains.

In a closed gravity system, a closed, airtight expansion tank is located near the hot-water boiler (Figure 7-28). Higher pressures (and, consequently, higher water temperatures) result as pressure builds up in the system. Pressure-relief valves are installed on the main supply line to prevent the buildup of too much pressure.


Hydronic heating systems are closed ones with expansion tanks located near the boiler.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers