"Where vent pipes from either a primary or secondary system terminate over their respective cold water feed cisterns they should rise to a height above the top water level in the cistern to prevent discharge occurring under normal operating conditions."
The calculation given is V = 0.04H + 0.15 where V is the height of vent pipe above the top water level and H is the height from the top water level to the cold water inlet to the hot water cylinder. V & H are in metres. This calc only relates to gravity circulation systems. If a circulation pump is installed due allowance should be made for induced head.
The end of the vent pipe must not be below the top water level in the cistern.