Vented hot water cylinder system
This is the traditional hot water system found in most UK households and is a low pressure system.
There is a cold water storage tank (header tank) in the loft and a copper hot water cylinder in the airing cupboard. Water in the cylinder is topped up from the header tank in the loft which in turn is kept topped from the mains. Water in the cylinder is heated either by an electric immersion heater (direct) and/or by the central heating boiler (in-direct). Hot water in the cylinder is used to supply the hot taps.
The header tank also acts as an expansion tank. As the water in the hot water cylinder heats up it expands and excess goes back to the header tank and a vent allows trapped air to escape to prevent build-up of pressure.
UNVENTED HOT WATER CYLINDER SYSTEM
An unvented system consists of a pressurised hot water cylinder fed directly from the mains water supply. It supplies hot water at cold water mains pressure to taps and showers.
Cold water from the mains is delivered to the base of the cylinder, the water is heated. Then when a tap or shower is turned on the mains water pressure pushes the hot water out. To allow the water that is heated in the cylinder to expand as it is heated. An expansion vessel is used and the system includes a pressure relief valve.
Benefits of Unvented Hot Water Cylinders:
- – Hot water flows at cold water mains rates . A substantial improvement over vented systems and compared to a condensing combi boiler.
- – More flexibility over where the cylinder is installed space . Unvented cylinder can be installed anywhere (for example in a garage). Freeing up space in the airing cupboard and there is no need for header tank in loft.
- – No risk of frozen pipes in the cold loft in winter.
Disadvantages of Unvented Hot Water Cylinders :
- – The hot water flow depends on the cold water pressure . So if the cold water goes out, you also lose hot water. With a ventilated system hot water from the hot water cylinder would still be available until it came out.