IOP Plumbing: Unvented Hot Water Storage 
Richard Hanson-Graville MA FIOP 
Technical Director, Dedicated Pressure Systems (DPS) Ltd. 
Tel 01372 803643, Fax 01372 803678, Email
10th October 2001

What is an unvented cylinder?
Unvented hot water cylinders store water supplied directly from the mains water supply, and heat it using either electrical heating elements or with heat from a boiler.  This provides high pressure hot water supplies capable of feeding outlets with high flow rates of hot water. The cylinders store water under relatively high pressures, typically in the region of 2 to 3 bar (20 to 30 metres head), and as such can supply water to outlets at 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, keeping loft spaces clear, and pipework to a minimum.

Mains Water Supply: 
Unvented cylinders are limited by the 'power' of the mains water supply that feeds them, and in order to function properly require both adequate mains pressure and flow rate. Supply pressure should typically be in the region of 2 bar, preferable more, and capable of supplying the required flow rates.  A Pressure Reducing Valve is required on all unvented cylinders to limit the pressure of the incoming mains water to a safe level at which the cylinder is approved to operate.   The difference between pressure and flow is important to understand - a mains water pressure of 5 bar is no good if it is supplied through 100 metres of 15mm supply pipework.  Mains supply pipe sizes and flow rates, as well as pressures, must be checked.  

Unvented cylinders are typically made from copper, stainless steel, or glass lined steel. Each has its benefits, and the type of cylinder to be used will often depend upon the application. 

1. Copper is used extensively to manufacture domestic water cylinders, mainly because it is easy to cut, bend, fold and braise, and it is very easy to add additional bosses or work specials.  Copper unvented cylinders will usually have a operating pressure in the region of 2 bar. 

2. Stainless Steel is a much stronger material than copper, and is capable of withstanding higher pressures. Such cylinders typically have an operating pressure around 3 bar.  Stainless steel is also highly resistant to corrosion giving it a greater life expectancy, however welds may be susceptible to corrosion from alkaline water.

3. Glass Lined Steel cylinders are made up of a mild steel cylinder that is coated on the inside with a layer of glass (similar to the enamel coating on a cast iron bath) to provide resistance to corrosion.   Cylinders typically have operating pressures in the region of 3.5 bar, but can often take far higher pressures.  These type of cylinders are cost effective, typically being mass produced on the continent and in the USA. Care must be taken not to crack the glass lining around bosses, and it is wise to check that primary coils are supported to avoid movement during transit that may stress and possible crack the glass lining. Cylinders should also be fitted with powered anodes to further protection from corrosion. 

In summary, all three types of cylinder are sold in the UK to work with operating pressures between 1.8 and 3 bar to fall in line with typical domestic mains supply pressures.  In more commercial applications, where higher pressures are required, steel cylinders may be required.

Unvented cylinders must provide a means to accommodate the expansion of the water in the cylinder as it heats up, and manufacturers will typically use one of  two methods:

1. External Expansion Vessel:
  An expansion vessel is basically a trapped volume of pressurised air, held by a rubber diaphragm within a steel vessel.  The vessel is connected by pipework to the unvented cylinder, typically onto the mains cold inlet pipework, and as water in the cylinder expands it is pushed into the expansion vessel, compressing the air within the diaphragm.  As the air is compressed, its pressure, as well as that of the water in the cylinder, will increase - the larger the expansion vessel, the smaller the increase in pressure.  Minimum recommended sizes must be adhered to, as must the appropriate charge pressures.  As a general rule, the vessel charge pressure should be 0.2 bar below the setting of the pressure reducing valve.

2. Internal Air Bubble:  Another way of accommodating expansion is by trapping a bubble of air in the top of the cylinder.  This bubble compresses as the water expands, much in the same way as air in an expansion vessel.  This method makes for a simpler design, without the need for having to fit or ever replace an expansion vessel, however annual servicing of the cylinder is usually required to maintain the bubble and prevent safety discharges from occurring.

Safety Requirements: 
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 100C, 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 'flash' rapidly to steam. The higher the pressure at failure the more steam, and cylinders 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 to allow water to be discharged during heat up if the means of accommodating 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 to allow 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 and Tundish to allow water to be released from the store under fault conditions.  Discharge pipes are typically 15mm up to the Tundish, and then on in 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 store is fitted with a Motorised Isolating Valve which will close when an Overheat Thermostat (manually reset) fitted to the store detects 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, and 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.  The 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. 

Solar and Solid Fuel Systems:
It is possible to use solar panels or a solid fuel boiler to heat an unvented cylinder.  The problem with doing this is the need to protect from overheating.  System should only be designed by an approved unvented system designer, and may include such features as a heat dump to radiators, as well as additional motorised isolation valves.

Tank Fed Unvented Systems: (this is the first paragraph to go if short on space)
It is not always the case that an unvented cylinder need be fed from the mains.  They may equally well be supplied with pressurised water from high level water tanks, or from a pumped water supply.  Indeed, where the mains water supply is not adequate to match the instantaneous demands of the user, cold storage tanks will be a necessity.  The use of a booster pump will provide the required water pressure to feed the unvented cylinder.  This has the advantage over a pumped vented system of not requiring a vent pipe to be run from the cylinder back up to the cold tank. 

Pre-Fabricated Systems:
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. Figure 2 shows one of the new generation of unvented cylinders that is supplied with the safety controls factory fitted within the unit casing.  Such an approach can cut down installer error, reduces the risk of transit damage to safety controls, and makes for a tidier installation. 

Further Information:
WRAS (Water Regulations Advisory Scheme) Water Regulations Guide - available from the WRC (Water Research Council) 01495 248454
G3 Building Regulations
WMA (Water-Heater Manufacturers' Association)
Kingsmark Design:  01633 244165
DPS (Dedicated Pressure Systems)  01372 803 675