Central Heating Open Vented System Basics
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| Open Vented (used on older systems and cast iron
boilers) means that there is an expansion tank fitted Usually fitted in the loft next to the header tank which feeds the
water down the “Cold Feed” into the central heating system. The
water level in the tank should be very low when the system is
cold (the water level is maintained by a ballcock), but as the
system warms up the water expands and its level in the tank rises.
There is also usually a safety pipe, fitted just above the tank and
shaped like a hook pointing into the tank.
This is called an “Expansion Pipe” or “Vent Pipe”.
1 Header tank for the
hot water
2 Expansion tank for
the Central heating
3 Air separator (if
fitted)
4 Pump
5 Motorised valve
6 Indirect Cylinder
7 Boiler
8 & 9 Heating Flow &
Return
Diagram is of a fully
pumped open vented "Y" Plan system
Sometimes the ballcock sticks so the
expansion
tank is empty, this will cause the highest
radiators to go cold or more commonly the cylinder to go cold,
especially on gravity hot water systems
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