Stephanie Allchurch, Product Development Manager at Altecnic, a member of the Hot Water Association, takes a closer look at G3 compliance.

Many engineers are experienced in installing both boilers and hot water cylinders. However, due to the strict safety requirements for an unvented water heating system, it is a legal requirement for plumbing and heating engineers to have a G3 qualification if they want to install unvented hot water cylinders. 

The guidance to G3, which can be found in the Building Regulations, explains in full what electrical and water safety controls are necessary to demonstrate compliance. 


The installation must be carried out by a competent person. This is defined as a person who has undergone the required training and can has demonstrate competence, through attaining the G3 unvented qualification, supplying evidence of ability, and through prior experience and knowledge.  

Once proven, the person may join an independent competent person scheme which includes insurance liability, code of conduct, and a complaints procedure. Membership of a competent person scheme also removes the need for the installer to notify individual installations directly with Building Control.

Prevention of stored water reaching 100°C

The unvented water heater should have two independent safety controls: 

  • A non-self resetting thermal cut-out to interrupt the supply of heat to the cylinder in the event of it overheating
  • A factory fitted temperature relief valve, or combined temperature and pressure relief valve, which will discharge the water in the event of the cylinder further overheating, preventing the unit from reaching 100°C.

Provision of safe discharge from safety valves

Any safety valve fitted to the unvented water heater must be plumbed to deal with any discharging water from the valves should they:

  • Discharge to drain via a suitable air gap. This is usually a tundish
  • Discharge pipework sized depending on the size of valves and distance of pipe run
  • Discharge where it will not cause danger to persons in or around the building.

They must also:

  • Be visible either at the tundish, final point of discharge, or both
  • Be plumbed using pipe materials that are suitably rated for the temperatures and flow rates that can reasonably be expected from the safety valves.

The guidance to G3 includes information on the correct sizing and installation of discharge pipes.


Following installation, an unvented water heater must: 

  • Be commissioned in accordance with the manufacturer’s instructions and any relevant regulations in force
  • Have the installer’s details recorded on the label affixed to the water heater
  • Have a commissioning certificate completed (for example, Benchmark)
  • Be checked to ensure the correct operation of the system. Explain the system and the functions of any valves and controls to the end-user.

Why is G3 compliance so important?

With no expansion control on an unvented system, if the cylinder is continually heated or over-pressurised and has none of the required safety or control devices as prescribed in G3 or the Water Regulations, the cylinder  could reach a point where it will discharge steam or rupture. Therefore, installation by a competent person who has been trained and understands the requirements, along with the correct controls, is vital for a safe install and continued use.