A worked first calculation for unvented expansion volume, with sizing left to the manufacturer data.
On an unvented hot water system there is no open vent to take up expansion, so the design has to provide for it deliberately. This worked example shows the first calculation.
The principle. When water is heated it expands by a small percentage of its volume. That extra volume has to be accommodated, usually by an expansion vessel or, on some units, a built-in air gap, otherwise the pressure would rise until a safety device operated.
Step 1 - water content. Take the total water content that gets heated as 200 litres for this example.
Step 2 - expansion factor. Use an expansion factor of 0.04, that is 4 per cent, for this worked example. The real factor depends on the temperature the water is heated through, so confirm the correct value for the design.
Step 3 - expansion volume. Expansion volume = water content x factor = 200 x 0.04 = 8 litres. That is the extra volume the system must accommodate.
Step 4 - size the device. The expansion vessel, or built-in provision, must be able to accept at least that 8 litres at the system working pressure. A vessel only uses part of its shell volume for water, because the rest is the pre-charged air cushion, so the vessel chosen is larger than the expansion volume and is selected from the manufacturer sizing data using the charge and maximum pressures. Use that data, and the correct expansion factor, for real designs.
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