• Yes, you should have your unvented hot water serviced at least once a year.

    Just like you have your gas boiler serviced.

    Watch the video below to see what could happen should something go wrong.

    Wow, Did you see that?

    That just shows you what could happen if some form of failure occurred.

    Let’s look at the unvented hot water cylinder and explain how it works.

    Unvented hot water cylinders store water supplied directly from the incoming mains water supply.

    They are heated using either electrical heating elements (immersion heaters) or from a boiler. 

    This provides high-pressure hot water supplies capable of feeding all the hot outlets with a high flow rates of hot water.

    These unvented hot water cylinders store water under relatively high pressures.

    Typically the pressure is in the region of 2 to 3 bar in the United Kingdom.

    As such, they supply water to outlets at a high level, such as in a loft space, and at similar pressures to the mains cold water supply. 

    The increased pressures also make for better quality showers, without the need for pumps. 

    With both hot and cold water services fed from the mains.

    There is no more need for a cold water storage tank.

    This keeps loft spaces clear, and pipework to a minimum.

    Unvented hot water cylinders are limited by the ‘power’ of the incoming mains water supply that feeds them.

    In order for them to work properly, they require both adequate mains pressure and flow rate.

    The incoming supply pressure should typically be in the region of 2 bar.

    Preferably more, and capable of supplying the required flow rates. 

    A Pressure Reducing Valve is required on all unvented cylinders.

    This wil limit the pressure of the incoming mains water to a safe level at which the cylinder is approved to operate.  

    Which is normally 3 bar.

    The difference between pressure and flow is important to understand.

    Incoming mains water pressure of 5 bar is no good if it is supplied through 100 metres of 15mm supply pipework. 

    The incoming mains supply pipe sizes and flow rates, as well as pressures, should always be checked.  

    This ensures an unvented hot water cylinder is viable.

    There are two basic dangers that unvented systems must address. 

    First is over-pressurisation, caused by a failed pressure reducing valve, or by backpressure, from a faulty mixer valve for example.

    The other is overheating.

    If an unvented cylinder should ever overheat and reach 100°C.

    Then instead of boiling away as it would with a vented system.

    The water will continue to rise in temperature and pressure until the cylinder can no longer hold the pressure and splits. 

    At this time, the sudden reduction in pressure resulting from the split may cause water to turn rapidly to steam.

    The higher the pressure at failure the more steam.

    ylinders capable of taking higher pressures will fail more dramatically or even explode.

    To prevent this from ever happening, there are safety requirements that have to be fulfilled. 

    As well as the mains Pressure Reducing Valve to limit the incoming water pressure, additional protection must be taken. 

    An Expansion Relief Valve is required.

    This allows water to be discharged during heat up.

    This is if the expansion has failed to operate correctly.

    Both of these valves are often combined into a single inlet control device.  

    A Temperature and Pressure Relief Valve, fitted near the top of the cylinder, is required.

    This allows water to be discharged when store temperatures or pressures start approaching unsafe levels. 

    Any water discharged in this way will typically be replaced by incoming cold mains water that will prevent store temperatures from rising further.  

    Both relief valves make use of a Discharge Pipe.

    The  Tundish allow water to be released from the store under fault conditions. 

    Discharge pipes are typically 15mm up to the Tundish, and then increase to 22mm to a safe discharge point outside.

    Certain systems layouts and longer pipe runs will require larger pipe sizes as laid down in the G3 Building Regulations . 

    To complete the array of safety controls.

    All indirect unvented cylinders require that the flow from the boiler to the cylinder is fitted with a Motorised Isolating Valve.

    The motorised valve is normally supplied with the cylinder from the makers.

    This will close when an Overheat Thermostat thats fitted to the cylinder detects an overheat. 

    Likewise, immersion heaters must have a built-in overheat thermostat.  

    Only those safety controls supplied by the manufacturer of a cylinder should ever be used.

    Only those specific to the model and size of unit.

    In addition to the use of approved safety controls.

    Installers must also be competent and qualified to install unvented cylinders and check the operation of these controls. 

    G3 Building Regulations provide a clear indication that installers must carry certification from either the CITB, IOP, or other approved body.

    In general, cylinders must be approved and supplied with certified safety controls. 

    A recent trend in the market for hot water systems is the growing demand for pre-fabricated systems that are factory fitted with the controls and wiring. 

    Such systems can make installation much simpler for the installer and can cut down installation time considerable. 

    Pre-fabricated systems also ensure that controls have been fitted as per requirements and to a ‘standard’ pipework layout.  

    Such an approach can cut down installer error, reduces the risk of transit damage to safety controls, and makes for a tidier installation. 

    If you watched the video above you will see how dangerous they can be.

    So do you need to have your unvented hot water serviced?

    A big big yes.