How the gas cushion takes up expansion, what the pre-charge should match, how to check it, and what weeping on every heat-up means.
On a sealed unvented system there is no open vent to take the extra volume when water heats, and the check valve stops it going back to the main. Without somewhere to go, the pressure would rise sharply on every heat-up until the relief valves discharged. The expansion vessel, or an internal air gap built into some cylinders, is the normal provision; the relief valves are only back-ups.
How the vessel works
A flexible diaphragm separates the water side, connected to the system, from a gas side pre-charged with air or an inert gas. As the water heats, it pushes on the diaphragm and compresses the cushion, taking up the extra volume and limiting the pressure rise. As it cools, the cushion pushes the water back. The charge does not maintain tap pressure and is not a water-hammer arrestor.
Why the pre-charge matters
Set the pre-charge to the manufacturer's specified value. Where that value corresponds to the regulated cold inlet pressure, the vessel starts accepting water as soon as the pressure rises above that setting on heat-up.
- Too high: water is kept out until the pressure climbs further, so it enters late.
- Too low or flat: less usable cushion is left, so the pressure spikes on heat-up and can lift the relief valve.
Check the pre-charge with the water system isolated and fully depressurised. Water pressure on the diaphragm can affect the gas-side result, so the unloaded check is needed before comparison with the specified pre-charge. A serviceable vessel with a reduced charge can be recharged; a failed diaphragm or damaged vessel needs repair or replacement.
Reading the symptom
- Weeps on every heat-up: lost charge or a failed vessel is the classic cause. A failed pressure-reducing valve letting the inlet pressure climb can do the same, so check both.
- Weeps even when cold: check whether the reducer is allowing excessive downstream pressure, whether the relief valve is correctly rated, and whether its seat is damaged or dirty. Timing guides the checks but does not prove the cause.