The hot water then travels through a copper coil in the hot water cylinder, heating the surrounding water without actually mixing with it. A direct cylinder system uses an internal element, often in the form of an immersion heater. This is required for properties which have no access to gas, such as off-grid homes. There are two main ways of doing this, the first of which is the bubble top unit. This type of cylinder uses an internal air bubble that is produced and trapped at the top of the cylinder when it is installed.
The other type is an external expansion unit, which utilises an expansion vessel to contain the expanded hot water. Since unvented hot water tanks operate at a higher pressure than vented systems, and have many additional safety features installed, they need to be fitted and maintained by specially qualified technicians.
This means that in order to benefit from the superior performance offered by unvented water cylinders, homeowners must ensure they are installed by G3 certified heating engineers. This provides high pressure hot water supplies to feeding outlets. The increased pressure helps deliver better quality showers without the need for pumps.
With unvented systems there is no need for a cold storage tank which helps free up space and requires much less pipework.
As unvented cylinders work from the power of the mains water supply, the mains pressure and flow rate must be adequate. Supply pressure should be around 2 bars and able to supply the required flow rates. A pressure-reducing valve is needed on all unvented systems to control the pressure of the incoming mains water supply to a safe level.
Good plumbers will check the pipe sizes and flow rate of the mains supply in addition to the pressure. Unvented systems have to provide a means to accommodate the expansion of the water in the cylinder as it heats up.
Typically, two such methods are used — an external expansion vessel and internal air bubble. The former is basically a trapped volume of pressurised air and is held by a rubber diaphragm within a steel vessel. This is linked by pipework to the unvented cylinder and as water expands, it's pushed into the expansion vessel, compressing the air within the diaphragm. Because the air is compressed, its pressure, together with that of the water in the cylinder, will increase.
With the internal air bubble method, air is trapped in the top of the cylinder. An unvented cylinder is connected directly to the mains water supply.
It uses electricity or gas to heat water to a pre-set temperature. The act of heating water causes it to expand, which creates pressure within the cylinder.
However, because water expands when it is hot, there is a need to account for this additional space needed. An expansion vessel is attached somewhere to the system to handle this need. Skip to main content Need help? Call us on Sign in or register. VAT Ex. How do they work? What are the benefits? Do I need a new cylinder? Types available Hot water cylinder capacity varies between 90 and litres for domestic supply.
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