Unvented Hot Water System

There are no cold water storage tanks, vent pipes or expansion tanks with this cylinder, instead cold water is supplied at mains pressure into the base of the cylinder, allowing the supply of hot water and cold water to the entire property at mains pressure. A pressure vessel allows for expansion as the water is heated, via a flexible diaphragm.


Working with high pressure water

Special measures are necessary to accommodate expansion as the high pressure water heats up, to  ensure there is no dangerous build-up of pressure within the unvented storage vessel, and to discharge hot water safely away from the unit if necessary. If the vessel fails, an expansion release valve protects the system by releasing water via the discharge pipe.

  • A normal thermostat maintains the temperature of the water below 65°C.
  • A second thermostat switches off the immersion heater and shuts off water from the boiler should the main thermostat fail and the temperature reaches 90°C.
  • If both thermostats fail, and the water reaches 95°C, a temperature release valve discharges all water to the outside.


Water bylaws and building regulations

The installation of an unvented hot water system must comply with both the Water Regulations and the Building Regulations. You must also notify your local Building Control Office. If you are planning on replacing your existing hot water cylinder with an unvented vessel, ensure that the new cylinder has all the necessary safety devices, and it is installed by a competent heating engineer. A “competent” person is described as one holding a current Registered Operative identity card for the installation of unvented domestic hot water storage systems. In the UK, cards are issued by such organisations as the Institute of Plumbing (IoP) or the Construction Industry Training Board (CITB) after an operative has been assessed by a recognised training body. Requirements for the unvented storage water heating system itself and for its installation are laid down in the UK Building Regulations, Schedule 1 Part G3 which states that: A hot water storage system that has a hot water storage vessel which does not incorporate a vent pipe to the atmosphere shall be installed by a person competent to do so, and there shall be precautions:

  • to prevent the temperature of stored water at any time exceeding 100°C


  • to ensure that the hot water discharged from safety devices is safely conveyed to where it is visible but will not cause danger to persons in or about the building.

Pros Provides hot water at mains water pressure throughout the property Does not require cold water tanks and ventillation pipes Works with existing pipework Does not rely on gravity at all so can be located anywhere within a property Cons Requires specialist installation Often more expensive than a vented system