Unvented Hot Water Systems

Unvented hot water systems are now being chosen over more traditional water heating solutions for new build and refurbishment projects.

The advantages of this system are:

  • The system is fed directly from the mains cold water supply; therefore no additional tanks are required. This reduces pipework and allows the roof space to be used for additional storage or living accommodation.
  • The unit can be installed in any suitable location as the system does not rely on gravity for the adequate flow of water.
  • A high pressure flow of water is delivered even if more than one tap or shower is being used.
  • The system can be connected to solar water heating systems and/or solar panels.
  • Noise is reduced as there is no refilling of a storage cistern.
  • The risk of freezing and burst pipes in roof spaces is eliminated.
  • An immersion heater can be used to supplement the heating of the water reducing the dependence on the boiler.


Because the unvented cylinder operates under high water pressures, built in safety measures are required these include:

  • A combination valve – This is positioned on the incoming cold water pipe and incorporates 3 features; a pressure reducing valve, which keeps the pressure at a constant level; a line strainer, which filters incoming cold water ensuring it is clean and free from any grit that may damage the system and a single check valve, which prevents contamination of the water supply from backflow.
  • An expansion vessel – This stores the water produced when the water in the cylinder is heated and expands. The expansion space can also be provided within the cylinder, this is known as an ‘air gap’ or ‘bubble top’ system. The internal air bubble is created when the system is commissioned.
  • Thermostats – The thermostats are in place to prevent the temperature of the water in the system exceeding 99°C. These include a control thermostat which is set to maintain the temperature of the water between 60-65°C; this gives the first level of protection against the overheating of the water. A second thermostat incorporates a thermal cut-out which switches off the immersion heater and shuts off water from the boiler if the control thermostat fails and temperatures reach between 85-89°C. This thermostat has manual reset feature in place which cannot be self-resetting.
  • A temperature and pressure relief valve is fitted near the top of the cylinder. This is set to 90°C + and is designed to remove pressure from the system preventing the water temperature exceeding 99°C. It provides protection against the failure of the pressure reducing valve, failure of the expansion vessel or the loss of the internal air bubble.

The temperature and pressure relief valve releases water under fault conditions via a discharge pipe to a tundish where the water released becomes visible. The tundish must be positioned within 500mm of the valve in a safe and visible position so that the householder will be alerted to a fault condition.

Discharge pipes are typically 15mm up to the tundish, and then 22mm from the tundish to a safe discharge point outside the building. The pipe from the tundish should never be less than the size of the tundish outlet. It should be vertical, at least 300mm long before any elbows or bends and be installed with a continuous fall of at least 1 in 200 thereafter.

The discharge pipe should be made of metal or other suitable material which can be shown to withstand the high temperatures.

The Local Authority Building Control must be notified of the installation of an unvented hot water storage system as they are subject to the legal requirements of Building Regulation G3. The system must be installed by a ‘competent person’ who holds the relevant qualification for the installation of unvented hot water cylinders. The installer must be able to provide a card issued by a body such as the Institute of Plumbing (IoP) or the Construction Industry Training Board (CITB).

If the installer is registered with a competent person scheme for this work, building control do not need to be notified in advance. Instead installers will self-certify their work and give the owner a certificate issued by the competent persons scheme operator.