What the feed and expansion cistern, open vent and filling loop do, and how a sealed system replaces them.
Open-vented systems: the feed and expansion cistern
An open-vented system fills and tops up from a feed and expansion (F&E) cistern, usually in the loft. It does two jobs: it keeps the circuit topped up with water, and it gives the water somewhere to go as it expands when heated. That is why the cistern is deliberately left only part-full when the system is cold — the space above the water is there for the water to rise into as it warms, then drop back as it cools. A float-operated valve holds the level, so a part-full cold cistern is normal, not a fault.
The open vent pipe
The open vent (expansion) pipe rises from the system and discharges harmlessly over the F&E cistern. It keeps the system open to the atmosphere and gives expanding water, or steam from an overheating boiler, a safe route out instead of letting pressure build up in the pipework. An open vent must rise continuously with no valve fitted anywhere in it, because a valve could be shut by mistake and remove the system's only pressure-relief path. The cold feed pipe that supplies the system from the cistern is normally narrower than the vent (commonly 15 mm feed to at least 22 mm vent) and also rises continuously; always check the appliance's own installation manual for its exact minimum sizes.
Sealed systems: no cistern, no open vent
A sealed system replaces the F&E cistern and open vent with an expansion vessel and a pressure relief valve. The vessel's gas cushion takes up the water's expansion instead of a cistern, and the relief valve is the safety device that discharges if the pressure still goes too high. A sealed system has no route to atmosphere in normal use, so it depends on the vessel and relief valve working correctly, and it is filled and topped up differently, through a filling loop rather than a float valve.
The filling loop
A sealed system is filled, or topped back up, through a temporary filling loop: a flexible connection with backflow protection between the mains and the heating circuit. Opening it lets mains water in and the pressure gauge rises; once the system reaches its normal cold-fill pressure (commonly around 1 to 1.5 bar), the loop is closed and then disconnected. It is only ever a temporary connection — leaving it connected, or open, keeps cross-connecting wholesome mains water to the heating circuit and lets the pressure keep climbing.