The traditional British domestic hot water supply system, in which water is heated and stored for use, is based on cold water feed from a storage cistern to a heated cylinder that is vented to the atmosphere. The flow rate and pressure of the hot water discharged from the hot water outlets is then dependent on the height of the storage cistern above the outlets.
However increasing demand to remove cisterns from the roof space means greater difficulties in meeting consumer demands for high performance hot water systems, from traditional open vented systems. This is particularly the case with the growing market for high performance showers.
Unvented domestic hot water systems can, however, meet these expectations by utilising cold water supplied directly from the mains to the heated storage cylinder. Included with the cylinder is a package of safety devices and controls to regulate temperature and pressures within the system. This type of system can provide flow rates in excess of 25 litres/min thus meeting BS6700 specification for design flow rates of 18 litres/min whilst retaining the facility for simultaneous draw-off from other hot water outlets.
Means of Water Heating:
The means of heating the water in the storage cylinder may be either a) direct, which entails one or more immersion heaters in the cylinder, or b) indirect, in which case the contents of the storage cylinder are heated by a primary heater of the coil heat exchanger type.
In both cases the heat source is subject to thermostatic control. Should the thermostat fail the energy input to the storage cylinder, either from an immersion heater or a coil heat exchanger, will be interrupted by an energy cut-out
Advantages of the Unvented System:
The advantages of the unvented system are:
- Balanced hot and cold water pressures.
- Improved shower performance.
- Less pipework and no storage cistern.
- Reduced risk of frost damage to system.
- Saving on installation time and costs.
- Excellent heat transfer capability.
- Allows the use of a wider range of modern tap fittings and showers.
- Flexibility of vessel siting and hence system design