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.
feature of the expansion
tank is that the boiling point of the water can be increased
by elevating the tank. In other words, increasing
(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.
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.
(hydronic) hot-water heating systems
are closed systems with expansion tanks located near the boiler.