Read the unvented-cylinder diagram: inlet controls, expansion provision, relief valves, tundish and discharge route.
Use the diagram as a reading sequence, not as a drawing to copy from memory. An unvented cylinder is mains-fed and sealed, so the safety idea is defence in depth: control the incoming pressure, allow expansion, stop overheating and make any relief discharge visible and safe.
1. Start at the cold inlet controls
On the cold mains side, read the inlet control group first. The pressure-reducing valve limits the incoming supply pressure to the cylinder design. A check valve prevents heated stored water pushing back into the cold main. An expansion relief valve protects against expansion pressure on the inlet side. A line strainer protects the control group from debris.
2. Find the expansion provision
Stored water expands as it is heated. On a sealed system that expansion needs a normal place to go, usually an expansion vessel or a manufacturer-designed internal air gap. If the expansion provision fails or loses charge, repeated discharge at the relief route is a fault clue rather than normal operation.
3. Separate control from safety
The operating thermostat is the normal temperature-control device. The non-self-resetting energy cut-out is the next layer if normal control fails. The temperature and pressure relief valve is the final protective device on the cylinder itself, opening if temperature or pressure still rise too far.
4. Trace the discharge path
Relief discharge passes through D1 to the tundish. The tundish is there so discharge can be seen and so the pipework has an air break. D2 then carries the discharge to a safe termination. In revision questions, do not treat that route like ordinary overflow or waste pipework: it may carry very hot water and steam from a safety device.
5. Common exam traps
- Calling the tundish an overflow instead of a visible safety-discharge point.
- Using the temperature and pressure relief valve as if it were normal expansion control.
- Ignoring a dripping tundish because the cylinder still produces hot water.
- Confusing a check valve, pressure-reducing valve and expansion relief valve.
- Treating a vented-cylinder mental model as if it applies to a sealed unvented system.
For the full article version, read the unvented cylinder diagram guide. For sizing practice, use the expansion vessel calculator.
*Original revision guidance; confirm device requirements, set points, pipe sizing and discharge detail against current Approved Document G and the manufacturer instructions before real work.*