Unvented Cylinders
What is an Unvented Cylinder?
An Unvented Cylinder is a hot water storage system that has a hot water storage cylinder that does not incorporate a vent pipe to atmosphere.
In simple terms, a system of pipework and components operating above atmospheric pressure, normally fed from the mains and sealed to the atmosphere. (no vent pipe).
As no vent pipe is provided, the system differs from a conventional system in that the operation of the vent pipe needs to be performed in other ways.
Unvented Cylinders are covered by section G3 of the Building Regulations. G3 of the Building Regulations came into force on November 11th 1985.
Advantages and Disadvantages of unvented systems
Advantages:
- Balanced pressures at hot and cold taps for showers.
- Higher water pressures available at the hot taps.
- Eliminates risk of contamination as there is no storage cistern. (tank in the loft).
- the hot water storage cylinder can be sited anywhere in the house, are very suitable for one storey dwellings, e.g. flats and bungalows.
- Quicker to install, no cold storage tanks in the loft, less pipework.
- Small diameter pipework may possibly be used.
- Gives architects and service designers greater flexibility of design.
Disadvantages:
- Normally there would be no back-up of stored water should the main water be shut down for any reason.
- If the mains water pressure is low then all water outlets are affected.
- The possible need for long runs of safe discharge pipe with high temperature capacity.
- The need for higher level of maintenance compared with conventional systems.
- Possible noise problems due to velocity in small pipes.
It is important that you employ a fully qualified installer
for Unvented Cylinders
Do not let the following statements put you off considering Unvented Cylinders. As long as they are installed correctly and with the proper safety precautions in place Unvented Cylinders are as safe as any other pressurized system. It is important therefore that your installer is properly qualified and that you check his or her credentials before they start work.
What is the danger with Unvented Cylinders?
The danger is temperature related rather than pressure. An Unvented hot water system normally operates above atmospheric pressure and unless adequate protection is provided to prevent an overheat situation, the system has the potential for an explosion of quite catastrophic proportions.
See an Unvented Cylinder explode!!
At normal atmospheric pressure water boils at 100°C. At this temperature water changes from a liquid to a gas (steam). As the system pressure increases the boiling point of water increases. For example, a system running at atmospheric plus 1 bar (14 lb/sq") the boiling point of water is 121°C. Unvented hot water systems may be pressurized to 2.5 or 3 bar above atmospheric pressure. At 3 bar above atmospheric pressure the boiling point of water will be around 145°C. If you open a tap at this temperature the pressure drops the water turns to Steam instantly and the cylinder will explode. I repeat, WILL EXPLODE!!!