Turning a system water content into an expansion volume and then a vessel size, with the steps shown.
A sealed central heating system has no open vent, so an expansion vessel takes up the extra volume as the water heats. This worked example shows how the vessel size is estimated. The numbers are chosen to show the method.
Step 1 - system water content. Estimate the total water in the boiler, pipework and radiators. For this example take it as 100 litres.
Step 2 - expansion volume. Multiply by an expansion factor for the temperature the water is heated through. Using a factor of 0.04 (4 per cent) for this example: 100 x 0.04 = 4 litres of expansion.
Step 3 - allow for the pre-charge. A vessel cannot accept its whole shell volume of water, because part of it is the pre-charged air cushion behind the diaphragm. Manufacturers express this with an acceptance factor; using 0.5 for this example means the vessel only usefully accepts half its size.
Step 4 - vessel size. Divide the expansion volume by the acceptance factor: 4 / 0.5 = 8 litres. So you would select a vessel of at least 8 litres, then round up to the next available size.
Step 5 - set it up. The vessel pre-charge has to suit the system fill pressure for the calculation to hold, so the charge is checked and set when the vessel is fitted. Use the correct expansion factor for the design temperature, and the manufacturer acceptance and charge data, for a real installation.
Inside the vessel, and typical pressures
A flexible diaphragm divides the vessel into two sides: one holds a pre-charge of compressed air (or nitrogen), the other connects to the system water. As the heated water expands it pushes against the diaphragm, squeezing the air side and taking up the extra volume, which is why the pressure only climbs a little rather than shooting up. The air side has a Schrader valve — the same fitting used on a tyre — so an engineer can check or top up that pre-charge, done with the system drained down on the water side.
A typical sealed system is filled to about 1 to 1.5 bar when cold, and its pressure relief valve commonly opens at around 3 bar. A cold reading near zero usually means the system has lost water somewhere; a hot reading close to or above the relief valve's setting points to too little expansion capacity, often because the vessel has lost its pre-charge or failed.