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. The float valve keeps the water at the set cold level, leaving room for expansion. A part-full cistern can be normal, but the level does not tell you the valve and feed are working. The guidance to the Water Fittings Regulations asks for the cistern to hold all of the circuit's expansion water, and for its water level, with the circuit in use and the water expanded, to stay at least 25 mm below the level at which it would overflow into the warning pipe.
The open vent pipe
The open vent rises from the system and ends over the F&E cistern, giving water or steam a way out to atmosphere if the system overheats. That discharge can be dangerously hot, and pumping over in normal use needs investigating. The open vent does not take away the static head from the cistern or the pressure differences the pump creates.
Follow the appliance’s open-vented layout. A common regular-boiler arrangement specifies at least 22 mm for the vent and 15 mm for the primary cold feed, both rising continuously from the appliance with no valve in either pipe. Check those sizes and the cistern height for the installed model, and never let an isolation valve block the feed or the vent.
One regular boiler's instructions show the figures to look for. For an open-vented primary on a Worcester Greenstar Ri, the feed and expansion pipe and the open vent are close-coupled, no more than 150 mm apart. The cistern's water level must be at least 250 mm above the highest point of the heating system, measured from the top of the appliance or the highest pipework, whichever is higher, with room left in the cistern for expansion. Where the cylinder's primary return and the heating return share a common return pipe, the cylinder return joins it last, which prevents reverse circulation through the cylinder circuit.
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. It is also filled and topped up differently: through its filling arrangement, not an F&E cistern’s float valve.
The Greenstar Ri instructions also show a typical sealed layout: an expansion vessel sized for the system and fitted close to the boiler on the heating return, a pressure gauge, a 3 bar pressure relief valve and an automatic air vent. No valve may be fitted that could isolate the boiler from its expansion vessel or relief valve. Closed, such a valve would leave the boiler heating water with nowhere for the expansion to go and no relief protection.
The filling loop
A common filling arrangement uses a removable flexible hose between the mains and the heating circuit, with suitable backflow protection. Open its manual valves only while filling, watch the gauge and stop at the appliance’s specified cold pressure, often around 1 to 1.5 bar. Close the valves and disconnect the hose as instructed. A hose left connected does not raise the pressure by itself, but valves left open, or passing, can let water in.
Some boilers instead have an integral keyed, keyless or automatic filling device. Follow that device’s instructions; there may be no hose to remove. Backflow protection must suit the assessed risk of the heating fluid. A double-check valve may be acceptable for a system classified as fluid category 3, but not for every system.