Unvented cylinder commissioning and safety checks

This checklist concerns a common domestic mains-fed unvented cylinder, with no open vent or loft feed cistern. Unvented describes the absence of the open vent; a dedicated cistern supply is possible in another designed arrangement. Water stored under pressure can be heated above 100 °C without boiling and flashes to steam with explosive force if the pressure is lost, which is why installing one is controlled under the Building Regulations. Commissioning checks each specified control and safety function under the manufacturer’s required conditions before handover.

Before you start

1. Confirm the route: in England, installing an unvented storage vessel of more than 15 litres is notifiable. An installer registered with a competent person scheme covering the work self-certifies it; otherwise notice goes to building control before work starts.
2. Confirm competence: the installer needs the skills and knowledge for unvented systems. A training certificate is evidence of competence, not authority to self-certify.
3. Check the mains: the cylinder has no pump and is pressurised by the mains, so low or variable pressure and flow may make it unsuitable.
4. Plan the discharge: if there is no safe termination with discharge visible at the tundish, do not proceed until one is resolved.

Commissioning

1. Fill, vent trapped air and check for leaks before any heat is applied.
2. Check the cold inlet controls and the expansion provision, including the vessel charge or built-in air gap.
3. Check every heat source has its own control thermostat and its own non-self-resetting cut-out, working independently of the thermostat so one fault cannot disable both. On a boiler coil the cut-out closes the motorised valve; an immersion heater has its own thermostat plus a separate thermal cut-out. A timer is not temperature protection.
4. Energise and confirm the control thermostat holds the set temperature.
5. Confirm the order of protection: control thermostat, then cut-out, then the temperature and pressure relief valve as the final safeguard.
6. Check the discharge: tundish visible; D2 vertical for at least 300 mm, then falling continuously at not less than 1 in 200 (5 mm per metre) to a safe termination. A high-level discharge onto a roof needs a surface that withstands hot water, at least 3 m from any plastic guttering that would collect it.
7. Leave the completed commissioning record, the manufacturer's instructions and the compliance paperwork, and explain what a discharge means.