Servicing an unvented cylinder: annual safety checks

An unvented cylinder stores hot water under pressure, and its safety depends on devices that must keep working, so it needs a periodic service. Use this as a revision checklist; the work requires the relevant G3 competence and the manufacturer schedule.

1. Inspect the installation: confirm it still matches the design, look for leaks, corrosion or signs of past discharge, and check labelling is in place.
2. Check the expansion provision using the manufacturer’s method. An external vessel’s pre-charge is checked with its water side depressurised; an internal air gap has its own recharge procedure. Reduced charge can leave insufficient usable expansion capacity, but the symptom alone does not establish that the vessel needs replacing.
3. Check the operating controls: confirm the control thermostat holds the water at its set temperature in normal use.
4. Check the safety controls: confirm the independent over-temperature protection and the relief valves are present and function as intended.
5. Operate the relief valves: exercise the temperature and pressure relief valves to confirm they open and reseat, taking care with the hot discharge.
6. Check the discharge route: confirm the tundish, the D1 and D2 pipework and the termination are intact, continuous, correctly falling and terminate safely.
7. Record and hand back: complete the service record and tell the user about anything they should watch for, such as a discharge from the outlet.

The point of the service is that an unvented cylinder is only as safe as its working devices, so each layer is checked. Service to the manufacturer instructions and current guidance, and only within your qualifications.