Sizing a sealed heating expansion vessel (worked example)

Why vessel volume exceeds water expansion

In a sealed system, an expansion vessel contains a compressible gas cushion separated from the heating water by a flexible diaphragm. As the water expands, more enters the vessel and compresses the gas. Only part of the shell volume is available to accept that expansion between the chosen cold and hot pressure limits.

A simplified worked example

Use these chosen figures to practise the arithmetic:

1. System water content: 100 litres.
2. Expansion factor for this example: 0.04, or 4%, for the assumed temperature change. Expansion volume is 100 × 0.04 = 4 litres.
3. Acceptance fraction, the share of the vessel that can take water at the chosen pressures: 0.5.
4. Expansion-only vessel volume: 4 / 0.5 = 8 litres.

Eight litres comes from these simplified figures; it is not a selection for a real installation. The manufacturer’s method may also need a water reserve and other allowances. Use the real fluid, temperature range, water content, pressure limits and vessel sizes before choosing one.

Pre-charge and cold fill are different settings

The gas pre-charge is measured at the vessel’s Schrader valve with the water side depressurised as instructed. Set it for the static head and the vessel and appliance requirements. Some vessels, such as Altecnic's Reflex range, are factory pre-charged with nitrogen, which diffuses through the diaphragm much more slowly than compressed air, so the vessel holds its charge for longer and less oxygen is present to corrode it. Check the pre-charge at least at every annual service. The cold fill pressure is a separate setting, and may need to be above the pre-charge to give the specified reserve. Neither is the relief valve setting. Vaillant’s procedure for the internal vessel of an ecoTEC plus VUI, for example, drains the boiler first, then reads the charge at the vessel’s valve with a gauge. If water comes out of that valve, the diaphragm has let water through to the gas side and the vessel is replaced; topping up the charge would not fix it.

The acceptance fraction depends on the pressures; it is not a fixed figure for every vessel. Keep the normal hot pressure below the relief setting by the margin the sizing method requires.

Reading the system gauge

Many domestic boiler circuits use around 1–1.5 bar cold and a 3 bar relief valve, but use the installed appliance’s figures. Check an odd reading again under the same conditions. A new system often needs topping up once or twice in its first weeks, because air coming out of the fresh water escapes at the automatic air vent and the radiator vents and the pressure falls a little; after that it should hold. If the cold pressure keeps dropping, investigate. If it rises too far when hot, check the vessel’s usable capacity, the filling and the protection. The gauge cannot tell you the diaphragm has failed or that a new vessel is needed.