AHG Solutions Guide_Edition 2.7

Expansion Tanks & Pressurisation Systems
Contents
Introduction to Expansion Tanks
& Pressurisation Systems
EXPANSION &
DEAERATION
Why do we need
Expansion tanks?
Expansion Tanks & Pressurisation systems are critical to
the successful operation of heating and cooling circuits and
must essentially fulfil three fundamental tasks:
• Maintaining the pressure within permissible limits at
every point of the system, that is, the permissible working
pressure must not be exceeded as well as maintaining
a minimum pressure to prevent vacuum, cavitation and
evaporation of the system liquid, e.g. in circuits with
superheated water, solar systems.
• To prevent a negative pressure at the highest points of the
installation in order to avoid the intrusion of air into the
piping network.
• Compensation of volume fluctuations of the heating or
cooling water due to temperature fluctuations.
• Providing a water seal to prevent system-related water
losses.
With any heating or chilled water system there is expansion
and contraction as the system water is heated or cooled.
Because water is incompressible, the pressure in the
system will increase or decrease accordingly. To prevent
damage to the system components, expansion tanks are
installed which contains the excess volume as the water
heats and expands and allow that water to fill the vacuum
created when the system water cools and contracts.
Careful calculation, commissioning and maintenance are
the prerequisite for the correct functioning of the overall
system.
Expansion tanks in open vs.
closed systems
Open header or feed tanks were an early form of
expansion tank which were usually copper lined open
tanks installed above the highest point of the system as
the tanks relied on gravity to maintain system pressure.
Apart from the connection to the system, the tanks
were fitted an overflow outlet and a mains water inlet
controlled by a floating ball valve which allowed the
system water to be replenished automatically if needed.
While functional, open header tanks fail to provide
an optimal solution to the expansion problem due to
several key issues.
• Because the tanks are unsealed, the system water is
constantly exposed to the air which allows the system
water to absorb oxygen. Oxygen in the system leads to
corrosion and decreased pump efficiency.
• Depending on the design of the tank, there may be the
potential for backflow if the water level rises above
the mains inlet, leading to contamination of the water
supply.
• With any open tank, there is the potential for overflow
which is normally piped to the nearest drain. Given that
the tank needs to be elevated, if the drain becomes
blocked or the tank leaks, the resulting spillage may
cause damage to surrounding building surfaces.
• Because the system is open, system pressure is limited
to the head pressure achieved by the height of the
tank. This may negatively impact pump performance
due to insufficient NPSH (Net Positive Suction Head),
potentially resulting in cavitation and pump failure.
• Many modern boilers require controlled system
pressures and water quality and are therefore not
suited to open vented systems.
To overcome these issues, most heating and cooling
systems are now designed as closed systems which
allows system pressure to be controlled and gases
to be eliminated, resulting in greater efficiencies and
improved performance across all components of the
system.
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