Expansion tanks 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 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.
There are both open and
closed types of expansion
tanks used in hot-water
heating systems. The
open expansion tank is used on low-pressure systems and the
closed tank 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, it
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 to half, the pressure is doubled. This is
not gauge pressure, but absolute
pressure (the pressure measured from true zero or
point of no 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. rood, attic, top floor, etc.). 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
boiler or to discharge it through outside run-off drains.
In a
closed gravity system, a closed, airtight expansion tank is located near the hot-water boiler. 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.
Forced (hydronic) hot-water heating
systems are closed systems with expansion tanks located near the boiler.