Vented and pressurised heating systems
In a vented heating system at the highest point in the system there is at least one header tank. A header tank is a small tank, usually black, that is usually installed in the loft. It has a loose fitting lid and is partially filled with water. At one side a water supply with a ball valve (like the one in some toilet cisterns) makes sure that the tank stays full to the right level, and an overflow pipe makes sure that the tank does not overfill. A pipe runs out of the bottom of the tank and connects to the heating system.
The header tank allows for expansion and contraction in the heating system, for example, say you light your stove and have it roaring at full blast - the water in the system starts to heat up and expands. This makes the water level in the header tank rise (because the tank is connected to the heating system through the pipe at the bottom), and, should it rise up far enough, the excess would then pour out the overflow pipe. When the water contracts again the ball valve in the header tank falls and fresh water flows into the tank to replenish the system.
An open vent pipe is needed in a system with a wood boiler stove. This is a pipe running off from the hot water pipe of the stove boiler (the flow) which runs up to, and bends over the top of the header tank. If the water in the stove boiler boils the steam can then easily escape into the header tank through this pipe. The water lost due to the steam coming out would be replaced by fresh cold water entering the system when the ball valve drops. This header tank and ball valve should be made of metal in a solid fuel system.
In an unvented system there is no header tank and the water in the pipes is under pressure. Modern heating systems run by gas boilers tend to be unvented ( or pressurised). One givaway is that there is often a pressure gauge on the boiler measured in 'Bar'. When the water in a pressurised system expands the pressure in the system increases. Most stoves are not suitable to be directly linked up to a pressurised system because with a stove it is possible to boil the water in the boiler. Some stoves can be linked directly to pressurised systems with the incorporation of an expansion vessel - which expands to cope with pressure increases, a pressure release valve, and a heat dump system but certainly at the moment these stoves are the exception to the rule.