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CYLINDERS: THE BASICS
HOT WATER CYLINDERS: THE BASICS
There are a wide range of cylinders on the market today, but when it comes to selecting the right one, a number of factors need to be considered. Here, the Association of Plumbing and Heating Contractors (APHC), explains some of the basic differences.
WHAT TO CONSIDER
The following requirements need to be understood prior to installation:
■ Quantity of hot water required
■ Temperature required at outlets and the temperature of stored water
■ Cost of installation and maintenance
■ Fuel energy requirements
■ Wastage of water and energy running costs
■ User safety: such as scalding water, over pressurisation and legionella bacteria
TYPES OF HOT WATER STORAGE CYLINDERS
There are essentially two types of cylinder: Direct and Indirect.
DIRECT
This is a hot water storage cylinder which has a large open space inside it. They should only be used in a hot water system where the components are not subject to corrosion by fresh water. They’re therefore not suitable to be installed in combined heating and hot water systems where there is a range of metallic materials such as steel (radiators) which corrode in the presence of fresh water. They tend to be used in modern systems when the hot water is heated by electricity (immersion heaters).
INDIRECT
This type of hot water storage cylinder includes a coil inside it. The coil is used to transfer heat from a boiler or heat pump circuit, keeping the fresh water to be heated in the cylinder and distributed to the taps, completely separate from the heated water supply from the boiler or heat pump circuit.
FURTHER OPTIONS TO CONSIDER
Hot water storage cylinders are either vented or unvented (mains fed). Below are some advantages and disadvantages of each.
OPEN VENTED
Traditionally in the UK, hot water systems were of the open vented type, although unvented hot water storage systems have become more popular.
An open vented hot water storage system is fed from a cold water storage cistern.
ADVANTAGES
■ Simple and cost effective to install
■ Limited mechanical parts so relatively maintenance free
■ Can be useful where there is low incoming cold water mains flow rate and pressure to a property
DISADVANTAGES
■ Can be difficult to site the storage cistern ■ The water flow rate from the taps outlets is determined by the head of water generated by the storage cistern and tends to be low
■ The cistern is often sited in the loft space and can be prone to freezing in winter
DID YOU KNOW?
A hot water cylinder is defined as a 'controlled service or fitting' under Building Regulations.
UNVENTED (MAINS FED)
An unvented hot water storage system is mains fed without the need for a cold water storage cistern. The system is required to include a range of mechanical control devices to limit the water temperature in the cylinder from overheating which could lead to a serious explosion without their correct application and installation. The cylinder requires the installation of a relief pipe which must be carefully sited to warn of a fault occurring within the system.
ADVANTAGES
■ Provides high water flow rate at taps and showers which many customers prefer ■ Can be easier to site than open vented systems (there are limitations due to the requirements for safely siting the relief pipe) ■ Removes the need for a cistern which is often fitted in a loft space
DISADVANTAGES
■ Can be expensive to install ■ Includes mechanical components which are more prone to failure and requires regular servicing ■ Not suited to properties with low flow rate/low pressure supplies without additional and costly components being installed One of the main points that needs to be considered when fitting any type of hot water cylinder is that it’s defined as a ‘controlled service or fitting’ under Building Regulations. This requires the installation to be notified to building control prior to the work taking place, or selfcertified by a company registered with a competent persons scheme.
HANDY HINT
Make sure you fully understand the needs of your customer prior to any installation.
unvented hot water storage system
APHC (Certification) Ltd operates a building regulations competent persons scheme for hot water systems. For further details visit:
