Set the expansion vessel pre-charge equal to the cold-water pressure feeding the cylinder — the pressure-reducing-valve setting, typically 3.0–3.5 bar on an unvented system. Check the charge once a year with the water side isolated and depressurised, and top up through the Schrader valve with an ordinary tyre pump. A flat vessel is the single most common cause of a dripping tundish.
What pressure should the expansion vessel be charged to?
The rule is simple: pre-charge equals the cold fill pressure at the vessel. On an unvented cylinder that means the setting of the inlet pressure-reducing valve, most commonly 3.0 or 3.5 bar; the correct value is printed in the cylinder's installation manual and on the inlet control group. The pre-charge is the air (or nitrogen) pressure behind the diaphragm measured with no water pressure on the wet side — measuring it against a pressurised system reads falsely high and is the most common servicing error.
| System | Where the vessel sits | Typical pre-charge |
|---|---|---|
| Unvented DHW cylinder | Potable side, after the pressure-reducing valve | Equal to PRV setting, usually 3.0–3.5 bar |
| Sealed heating circuit / buffer tank | Heating water side | Static height ÷ 10 + 0.3 bar, often 0.75–1.5 bar |
| Solar thermal loop | Collector circuit | Sized and set separately — see sizing a solar expansion vessel |
Why must the pre-charge match the incoming water pressure?
Because the expansion vessel can only accept water once system pressure exceeds the gas charge. Water expands roughly 2% when heated from 10 °C to 65 °C, so a 200 L cylinder generates about 4 litres of expansion water with nowhere to go except the vessel. If the pre-charge is far below mains pressure, incoming water half-fills the vessel while cold, using up acceptance volume before heating even starts. If it is above mains pressure, the diaphragm stays pinned and the vessel accepts nothing until the system is already over-pressurised. Matching the pre-charge to the cold fill pressure means the vessel is empty of water when cold and absorbs the full expansion volume with the smallest possible pressure rise.
How do I check and recharge an expansion vessel?
An annual check takes ten minutes and needs a tyre-pressure gauge and a pump:
- Isolate the cold supply to the cylinder and open a hot tap until flow stops, releasing system pressure.
- Unscrew the cap on the vessel's Schrader valve (identical to a car tyre valve) and measure the charge.
- If low, pump the vessel back to the manual's figure — a foot pump or 12 V tyre inflator is fine.
- If water spurts from the Schrader valve, the diaphragm has ruptured and the vessel must be replaced.
- Close the tap, reopen the supply, and watch the pressure gauge through one full reheat cycle.
Diaphragms lose a little air permanently through diffusion — a small annual top-up is normal, a vessel that is flat again within weeks is failing.
Why does a flat expansion vessel make the tundish drip?
With no gas cushion, the roughly 2% expansion volume has no room, so pressure spikes with every reheat until the expansion relief valve lifts and dumps water through the tundish — classically a drip that appears only when the cylinder heats. That symptom, its diagnosis and the discharge rules are covered in why an unvented cylinder discharges through the tundish. Some unvented cylinders use an internal air gap ("bubble top") instead of an external vessel; those are restored by draining the cylinder briefly to re-establish the air pocket rather than by pumping. Persistent discharge after a correct recharge points to the other usual suspects listed under common unvented cylinder problems.
How often should the charge be checked?
Once a year, alongside the other unvented safety checks — UK Building Regulations Approved Document G (G3) requires unvented storage systems to be installed by a competent person, cylinder manufacturers specify an annual service, and EN 12897 is the product standard those cylinders are built to. Storage tank manufacturers typically tie warranty conditions to that annual service because a flat vessel quietly fatigues the relief valve and stresses every joint in the system with each heating cycle.
Frequently asked questions
Can I pump up an expansion vessel with a car tyre pump?
Yes. The Schrader valve is the same fitting as a car tyre, and a foot pump or small electric inflator reaches 3–3.5 bar easily. Always depressurise the water side first, otherwise the gauge reading is meaningless.
What happens if the pre-charge is too high?
The diaphragm stays pressed against the water connection and the vessel accepts no expansion water until system pressure exceeds the charge. Pressure then spikes during reheat and the relief valve can discharge, exactly as with a flat vessel.
How do I know the diaphragm has failed?
Press the pin in the Schrader valve: air should hiss out. If water comes out instead, the diaphragm is ruptured and recharging is pointless — the vessel must be replaced.
Do I check the vessel with the water pressure on?
No. The pre-charge must be measured with the cold supply isolated and a hot tap opened to drop the water-side pressure to zero. Measured against a live system, the gauge simply reads system pressure, not the gas charge.