Open vented stored hot water systems




Open vented stored hot water systems

		With open vented systems a large volume of cold water is stored in a header tank located at a higher level than the cylinder usually in the attic. The water storage cylinder usually located in the airing cupboard is fed at the base of the cylinder by the header tank. Water pressure and flow within the system is driven by gravity, i.e. the weight of stored water in the header tank is usually sufficient to push water down the pipe that feeds the water storage cylinder and back up to any tap or shower outlet, providing it is lower than the stored water level.


		When the water is heated it rises to the top of the cylinder where it can be drawn off through the hot water supply pipes and tap or shower outlets, the water is automatically replaced in the cylinder via the header tank. Heating causes the water within the cylinder to expand, a vent pipe allows a safe route for excess pressure, air bubbles and steam should the system overheat, it runs from the top of the cylinder back up to the cold water storage header tank where its open vent is located just above the water level.

Advantages

Disadvantages

Easy to maintain

Low maintenance costs especially with electric heating

Relatively easy to install

Less to go wrong when compared with a Combination boiler or pressurized cylinder

Less risk of being without a hot water supply due to breakdown

Power shower capability depending on choice of cylinder

Gain the traditional airing cupboard

Tanks in the attic risk of freezing

Water tanks, cylinders and pipework take up space and look unsightly

Need to pre heat hot water to match demand

Hot water availability is restricted by the heat recovery time period and size of cylinder.






There are a variety of differing designs of water storage cylinders though the two most important distinctions are between direct and indirect heated water. Follow the links below to find out more:


		Direct water heating and cylinder This is the simplest form of water heating, it gets its name from the fact that the water drawn from the hot taps has been directly heated via the boiler.


		Indirect water heating and cylinder With an indirect water heating system and cylinder, the water that circulates around the boiler, pipes and radiators is kept totally separate from the stored water in the cylinder that is used for bathing.


		Single feed indirect cylinders Single feed indirect cylinders commonly known as primatic cylinders were introduced and used widely until the mid 1970's. One method of identifying this type of cylinder is by the absence of a smaller header tank that feeds the central heating system.


		Combination direct / indirect cylinders A combination water storage cylinder has its own cold-water header tank fitted on top of the hot water cylinder. They are available as direct and indirect water storage cylinders.