Sizing the expansion for an unvented cylinder

On an unvented system there is no open vent, and the check valve on the inlet stops expansion going back to the main. The design has to take up the extra volume deliberately, or the pressure rises on every heat-up until a relief valve opens.

The calculation

1. Water content. Take the total heated water content as 200 litres for this example.
2. Expansion factor. Use 0.04, that is 4 per cent, for this example. The real factor depends on the temperature the water is heated through, so confirm the correct value for the design.
3. Expansion volume. 200 litres × 0.04 = 8 litres. That is the extra volume the system must accommodate.
4. Device capacity. The expansion provision must accept at least 8 litres as pressure rises from the cold condition to the permitted hot condition. Only part of a vessel's shell takes water, because the rest is the gas cushion, so the vessel chosen is larger than 8 litres. Select it from the manufacturer's sizing data using the charge and maximum pressures.
5. Pre-charge. Set the gas charge to the manufacturer's specified value. Where that corresponds to the regulated cold inlet pressure, the vessel starts taking water as soon as the pressure rises on heating. Check it with the water side depressurised.

What goes wrong

• Charge too high: water is kept out of the vessel until the pressure climbs further, so less of its capacity is usable.
• Charge too low or lost: the vessel can take less of the expansion. If what is left cannot take the heat-up, the pressure can reach the relief setting. How much charge was lost is only part of the diagnosis.
• No provision at all: the pressure rises sharply every time the water heats, the safety valves relieve repeatedly and the system is stressed.

If a valve weeps every time the water heats up, investigate. Check the expansion provision, the regulated pressure and the valve, then recharge, repair or replace the faulty part as the findings and the manufacturer’s instructions require.