An open (open to atmospheric pressure ) central heating system
must have a small water tank called feed and expansion tank at the heights point
in the house. Usually, the F& E tank located in the loft or roof space by the
side of or above large cold water storage tank. It should be about a 1/3 full.
The Expansion vessel may or may not be connected to a water supply. During
heating the water will expand and the F&E tank will accommodate the expanded
water. From the F&E tank some water will evaporate. There is always air
dissolved in the water without it the fish can not survive! When the water is
heated in the boiler the air will separate and try to escape. The air will find
its ways out through the F&E tank as it is open to the atmosphere. As air
escapes water will be filled to that space. The water in F&E tank will be used
to supply (feed) the required amount to the heating system. Then there must be
an arrangement to replenish the lost water. So that there will be always cold
water available in the F&E tank.
If the Expansion vessel is connected to mains cold water
supply, the system will have continues cold water supply. In is known as
Continues Feed System, meaning the cold water is always available to the heating
system.
Normally, the water mains supply is connected to the F&E tank via a ball float
valve arrangement, similar to the toilet cistern. When the water level goes down
the ball float will fall allowing cold water flow in and fill the tank. As water
flow in, float will rise. When the pre-set level is reached the ball valve will
close stopping the water supply.
If there is no water supply connection then the heating system known as Intermittent Fill Open System. As there is no automatic water supply, the heating system user or the maintenance contractor have to check the F & E tank intermittently (once or twice an year before and after winter) and top up the system.
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