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Pressurised Hot Water

In traditional open vented systems, as the water is heated, it simply expands into the water tank in the loft. If there's a fault with the thermostat, the water will continue heating and may eventually reach 100°C, and boil. The boiling water will turn to steam and evaporate into the atmosphere.With an unvented system, the water is typically held under pressure at 3 bar. If the thermostat fails, and the water continues heating, it could reach 100°C. In a vented system the water would boil and evaporate, but remain at 100°C.
With an unvented system, at say 3 bar pressure, the boiling point of water would be 134°C, so the pressure and temperature will continue to increase. At above 100°C, the water would instantly flash into steam when released into normal atmospheric pressure, for example if opening a tap!. This can be extremely dangerous to both people and anything in the surrounding area. It is critical that unvented systems are installed with the correct safety controls to ensure the water never reaches boiling point, and if the temperature or pressure exceeds safe predetermined limits, the safety valves will release to rapidly reduce the temperature and/or pressure.


In order to ensure safety and health for occupants, the government of the UK enacts strict regulations regarding building design, construction and other areas. This includes hot water, hot water heaters, and other areas related to the supply and flow of water through buildings. It is crucial that these regulations are followed at all times, as they ensure a supply of fresh, wholesome water uncontaminated with bacteria, waste and other materials

The specific regulations relating to the supply of water are known as Part G (Sanitation, Hot Water Safety, and Water Efficiency), and an approved document was published by the government in April 2010. Within the document, the regulations are broken down into six separate sections, which are as follows:

G1: Cold Water Supply - This section is constrained to the supply of cold water to and throughout a building.
G2: Water Efficiency - This section is limited to the efficiency of water flow to and throughout a building.
G3: Hot Water Supply and Systems - This is the most important section for EasyFlow and for consumers investing in hot water heaters, and details the rules that must be followed by hot water heater manufacturers, installers and professional service companies.

G4: Sanitary Conveniences and Washing Facilities - This section is concerned with the flow of and distribution of water through washing and sanitary systems.
G5: Bathrooms - This section details the rules laid out for water flow and distribution throughout a bathroom and its associated fixtures and appliances.
G6: Food Preparation Areas - This section details the rules that apply to water flow and distribution through areas in which food will be prepared.

Obviously, we’re most concerned with section G3, as it deals specifically with hot water, hot water storage and other related issues. Once, G3 related strictly to unvented water storage, but the new requirements relate to all hot water systems, and there are new provisions included to prevent scalding injuries. Sections two and three of G3 are the most important.

Within section two, you’ll find enhancements to the requirement that hot water systems, expansion vessels and storage tanks are “fit for purpose “. They must be capable of withstanding the stress of temperature and pressure increases through normal use, or that might occur during a failure that can be “reasonably anticipated “.

What does reasonably anticipated mean? Really, it simply means any situation that the designer or installer of the hot water system could foresee occurring through normal usage of the system. For instance, lime scale building up within pipework would be a foreseeable potential issue.

However, it is one where the only real option to remedy the system is annual draining and flushing of the system to remove the lime scale build up.

Another example could be the ongoing heating of a thermostat. Thermostats must be made from material capable of holding up in situations in which they remain at high temperatures for long durations, without degrading, breaking down or being damaged. It includes, but is not limited to concepts like normal wear and tear, temperature resistance, pressure resistance, and the like.

The third section requires that any part of a hot water system incorporating a hot water storage vessel be fitted with safety devices that prevent the water from exceeding 100 degrees centigrade. In addition, any water discharged from a heated vessel must be discharged safely and visibly.

Discharge pipework can be located through an exterior wall.

Note that in all instances, the discharged water must be contained and protected. This is generally accomplished by the use of metal grids, specially designed outflow shields, and other methods.

Most vented hot water heaters contain at least three safety systems, and the tundish provides visibility for discharged water. This section also quires that all hot water systems are installed and maintained in accordance with manufacturer instructions. This goes hand in hand with the recommendation from manufacturers that hot water heaters be professionally serviced at least once per year.

The actual wording from the document regarding sections two and three can be found below:

(2) A hot water system, including any cistern or other vessel that supplies water to or receives expansion water from a hot water system, shall be designed, constructed and installed so as to resist the effects of temperature and pressure that may occur either in normal use or in the event of such malfunctions as may reasonably be anticipated, and must be adequately supported.

(3) A hot water system that has a hot water storage vessel shall incorporate precautions to:(a) prevent the temperature of the water stored in the vessel at any time exceeding 100c; and (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.

If you need assistance with any hot water heater repairs, or to schedule hot maintenance, or to install a new hot unvented cylinder,

Please call DJG Plumbing and Building on 07830382814

G3 Building Regulations

Unvented cylinder heating systems must be installed by a competent and qualified G3 qualified engineer; this is a legal requirement in the UK and all installations must be certified and registered with the local council.


As the name suggests, unvented cylinders are sealed to the atmosphere, whereas traditional vented cylinders are open to the atmosphere; this is significant and is further explained below (Pressurised hot water). 
The water contained within the cylinder is stored under relatively high pressure (typically 3 bar which is equivalent to 30 metres head), and this pressure is created by the inlet water supply (usually the incoming water main). Unvented cylinders do not generate or increase water pressure, they simply use the inlet pressure to supply the outlets. Like traditional vented systems, the heat source can be electric immersion heaters, a boiler, some other heat source or any combination of the aforementioned.

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DJG Plumbing and Building
Jacklin Green
Woodford Green
EssexIG8 0TY
Phone: 07830382814