Open vented systems comprise a Feed and Expansion (F&E) tank located at the highest point in the system. They work exclusively with heat only (also known as ‘regular’) boilers. (NB regular boilers can also work with sealed systems.)
When water is heated it expands. An F&E tank will take up the increased volume of water in the system from expansion. As the system cools, the F&E tank ‘feeds’ water back into the system.
The ‘open vent’ is a pipe which rises from the heating system to above the tank and hooks over into it. The open vent pipe is a safety mechanism which will release excess pressure in the system in the event of an over-heat fault situation.
When water is lost from the system, which occurs naturally via evaporation, a supply pipe and float valve will regulate an amount of fresh water back into the system.
This is the traditional set up for a central heating system and was found in the vast majority of homes until the introduction of the modern combi in the 1980s.
Open vented systems are still operating in many homes across the UK today and in some cases remain a more suitable set up.