Unvented cylinder commissioning and safety checks

This checklist is for a common domestic mains-fed unvented cylinder, with no open vent or loft feed cistern. "Unvented" means it has no open vent; some designs are fed from a dedicated cistern instead. 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 every specified control and safety function, under the conditions the manufacturer sets, 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 assess pressure and flow under the expected demand. Also check water quality, outlet compatibility and the proposed heat source against the cylinder instructions.
4. Plan the discharge: if there is no safe termination with discharge visible at the tundish, do not go ahead until there is.

Two examples of installation checks

Megaflo Eco: the inlet controls accept up to 16 bar mains pressure and reduce it to 3 bar for operation. The suggested minimum supply is 1.5 bar and 20 litres/minute; simultaneous outlets may still give poor flow. The instructions exclude water with chloride above 250 mg/l and ascending-spray bidets needing a type A air gap. A heat source without effective thermostatic control needs additional suitable safety measures. Hot-water fittings and pipework must withstand at least 8 bar at 80 °C.

UK Cylinders direct with an external vessel: provide at least 22 mm mains pipework through to the cylinder and water treatment where hardness exceeds 200 ppm as calcium carbonate. Feed mixers and showers from the inlet set's balanced cold connection. Where that cannot be done, the instructions allow a separate 3.0 bar reducer on the cold supply to each affected mixing fitting, without restricting the cylinder's flow. For electric heating, the maker recommends trace heating instead of secondary circulation; an unavoidable return needs the specified potable-water circulator, non-return valve and cold-feed connection.

These are the named manufacturers' arrangements. Check the actual model before applying a figure or connection detail.

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. On the UK Cylinders direct cylinder, for example, the thermostat is supplied at 60 °C and the cut-out operates at 80 °C, isolating both the live and the neutral.
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.