Like a conventional hot water cylinder fed from a tank in the roof, an unvented cylinder contains hot water (heated by a boiler or immersion heater) which directly supplies the hot taps. Unlike a conventional cylinder however, the water in the unvented cylinder comes directly from the cold water main, and is at (or nearly at) mains pressure.
To contain this pressure the cylinder has to be physically much stronger than those used in gravity-fed systems. Unvented cylinders are made of thicker copper or stainless steel, and are usually more expensive than cylinders designed for vented operation.
This article describes a typical unvented cylinder installation of a size appropriate for meeting the hot water needs of a domestic property. There are some small unvented cylinders on the market (with a water capacity of 15 litres or less) designed for just providing hand wash facilities. These are in some circumstances subjected to fewer regulations and less complex system installation requirements. However they are not the primary focus of this article.
Considerations for installation
When installed in appropriate situations, unvented cylinders are good at providing high flow rates of hot water, and can have the capacity for larger properties with multiple baths and showers. Hot water is supplied at (or near) mains pressure, and will drive showers without the need of additional pumps.
When considering an unvented cylinder, there are several factors to check first. Failure to do this could result in unsatisfactory performance, or a system not well suited to the actual requirements.
To function well, you need adequate mains pressure and an adequate flow rate (these are not the same thing). Ideally your mains pressure should be in the range of 2 - 10 bar (systems with less pressure may still work, but you will lose performance, and may find you get better results with a conventional header tank).
The mains flow rate should be a minimum of 20 Litres Per Minute (lpm) - (although more is better). The mains supply will need to meet all the household demands concurrently - hot and cold.
Advantages of Unvented cylinders
- Mains pressure hot water to all outlets
- High flow rates of hot water at "mix temperature" , ideal for properties with multiple showers / baths
- Fast reheat 
- No requirement for a header tank in the loft
- Work well in properties without adequate height available for gravity fed systems (flats, chalet style houses etc)
- Can be heated by multiple sources, e.g. gas boiler, immersion heater, solar collector.
-  i.e. Water that is supplied to the outlet hotter than is required so that it can be mixed with cold before use. This means the hot supply does not need to supply water at the full rate of use and hence there will have more spare capacity.
-  Most cylinders are equipped with large high efficiency indirect coil heat exchangers that can transfer heat at over 20kW (if the boiler is capable of supplying it). The reheat time of electric only cylinders will be dependant on the number and power of immersion heaters fitted.
- More complex controls required than with a conventional vented system or combination boiler
- More expensive component costs (although total system price may be similar depending on the labour required)
- Need an adequate and reliable mains water supply (they can't supply water without a mains feed)
- Need annual servicing, possibly incurring extra cost
- Can't be heated by solid fuel stoves etc.
Selecting cylinder size
Typical unvented domestic cylinders start at around 125L and many makers do a range of sizes up to 300L or more. The smaller sizes being adequate for properties that have a single shower/bathroom. Larger sizes become more appropriate for larger properties with more baths and showers to cater for. Specifying the "right" size is not an exact science. See the Sizing a hot water cylinder article for more details.
The components of an unvented cylinder
In addition to the cylinder itself, there are several other components needed to form a functional system.
- Temperature and pressure relief valve
- Expansion relief valve
- Pressure limiting valve (usually combined with a filter / strainer)
- Connection for a secondary return (often provided by an extra tapping on the cylinder)
- Balanced pressure cold water outlet
- Main cold supply inlet
- Outlet to DHW
- Tundish 
- Drain Point
- Expansion Vessel
Connections to the primary heating flow and return are shown to the left (with two port zone valve).
- A check valve (which is usually implemented as part of the pressure reduction valve).
- A pressure reduction valve typically set to around 3 bar (although other pressures are sometimes used).
There may also be a cold water outlet point after the pressure reducing valve to supply e.g. shower mixing valves which work better when their hot and cold supplies are at the same pressure.
- A pressure relief valve (typically set to 6 bar), this is mounted on the inlet to the cylinder.
The above items are often found combined into a single multifunction unit.
- A combined over pressure and temperature relief valve. This is always on the side of the cylinder. It will typically be set to 7 bar and 90C. The thread that is used for this fitting is unique to this type of device to avoid the hole being used for any other purpose and so that the only way to block the hole is to install the valve.
(The reason for the temperature limiting valve in addition to a pressure limiting one is so that if a heating source fails to cut out, it is possible for the pressure rise will remain quite modest until the water reaches well over 100C (even 1 bar will raise the boiling point to around 120C). So neither pressure relief valve would open and yet the contents of the cylinder would be lethal and will 'flash' boil to steam when pressure in the system is reduced by opening a tap. Should this valve operate, it will dump some of the contents of the cylinder (containing water at 90C!) into the relief discharge pipework. The valve will only close when cooler water from the mains supply has reached it. This is also why there are stringent requirements for the size, length and routing of the discharge pipe.
The thermostats for the immersion heater(s) (if fitted) need to be of a type that includes a second higher temperature cut-out requiring manual resetting if it operates.
The connection to a boiler (if provided) must be via a zone valve connected controlled by both a normal cylinder thermostat (or equivalent electronic sensor). It should also be connected via also a second high temperature cutout.
The pressure relief and over temperature valves discharge into an emergency relief discharge pipe. They are usually combined and connected to a tundish, which in turn feeds the discharge pipe. The tundish provides an air gap required between anything connected directly to a mains water supply, and a drain so as to prevent any possibility of back contamination of the mains supply. It also provides a visible indication of any loss of water via the safety valves. The discharge pipe must be metallic (or certified as capable of handling a significant flow of near boiling water) and of at least 22mm diameter.
The maximum effective length of pipe from the tundish is also specified:
|Minimum size of discharge
pipe pipework from tundish
|Effective length of
each elbow or bend
There must be a vertical drop of pipe after the tundish (minimum length of 300mm) before any bends. The discharge pipe must terminate outside of the building in a safe place such as a gully where it can't cause injury. (other discharge positions may be acceptable so long as measures are taken to minimise risk of injury to bystanders)
An expansion vessel usually required to absorb the expansion of the water as it is heated. The volume of this will vary with the size of the cylinder. This is also normally connected to the multipurpose valve described above (although other fitting locations are also allowable, see the manufacturers instructions for your particular cylinder for recommendations).
The expansion vessel is pre pressurised with air (or nitrogen) contained in a bladder. There is typically a shrader valve (like that on a car tyre) fitted to the vessel to allow its pressure to be checked, and if necessary replenished).
Note some unvented cylinders may have an internal pressure relief bladder or "bubble" - although these are less common now.
(Note, that some small unvented storage cylinders of 15 litres or less, may not be required to have these, since the small amount of expansion can be accommodated by backing the water back into the mains. The only requirement is to check that all stop valves upstream are not of the 'loose jumper' type which could act as a non-return valve and block the expansion. However consult manufacturers documentation for specific details).
- The indirect heating coil connection must contain at least one 2-port zone valve.
- The use of solid fuel heating is forbidden.
- Most manufacturers will require the fitting of an anti scaling device of some description when used in hard water areas.
Note that qualification for working on these units is known informally as a "G3" card since Part 3 of the building regs approved document G used to specifically cover just the installation of unvented hot water vessels. Later versions of approved document G however now cover all hot water storage systems, not just unvented systems.
Safety of unvented systems
A properly installed and maintained system will be as safe as (or in many cases safer than) any other hot water system.
Water expands as it is heated. Since the outlet of the cylinder is to DHW taps which are normally closed, and the inlet is from the cold water main which may incorporate non-return (check) valves or other devices preventing expansion back into the supply pipe, measures have to be provided to accommodate the expansion of the hot water which could otherwise give rise to enormous pressure in the cylinder. These take the form of some type of container of gas which can be compressed as the water expands. This may be arranged as a bubble of air in the cylinder or a separate expansion vessel.
At normal atmospheric pressure water boils at 100C. At higher pressures the boiling point increases so that pressurised water can be heated to over 100C and remain liquid. However if the pressure is released it will rapidly turn to steam, expanding greatly and causing a steam explosion. In the days of steam the explosion of boilers or pipework in locomotives, shipping and industry was rightly feared for the destruction and loss of life it caused. If the water in an unvented hot water cylinder is allowed to rise above 100C then the reduction of pressure when a tap is opened could cause a steam explosion. For this reason unvented systems must have safety systems to control the temperature and pressure of the water, these systems must be checked annually, and their installation and servicing must be carried out by technicians with adequate knowledge and training for these tasks.
Regular service and maintenance of unvented systems is vital and should typically be carried out annually. Maintenance tasks should include:
- Cleaning and inspection of the strainer in the cold water supply to the Pressure Reduction Valve
- Inspection of pressure / temperature relief valve and expansion relief valve
- Manual operation of each relief valve to ensure that water flows freely to the tundish, and that they reseat correctly.
- Checking of the pressure in the expansion vessel, and topping up if required
There are many manufacturers of unvented cylinders including: Telford, Albion, Heatrae Sadia (the market leader whose model Megaflow is used as a synonym for all other makes and models), Santon, Gledhill, Elston, IMI, & Vaillant.