Tel. 01206 432 999
Various Thermal Stores and Cylinders

Unvented Cylinders

Telford Unvented Cylinder in white airing cupboard

This section is titled "Unvented Cylinders" but what exactly is a Unvented Cylinder?

The unvented cylinder is not particularly a new technology and can provide much better Domestic Hot Water pressure to the conventional "Open Vented" Cylinder.

With an open vented cylinder, the water is forced out of the hot taps by the static head pressure and that is influenced by the height distance between the cold water storage to the hot water outlet.

Conventionally, with the cold water storage in the loft, greater pressure and flow are delivered to the ground floor, than the top floor.

If our Bathroom is on the top floor, it could mean that we have only 2 metres height difference on an open vented cylinder and this equates to around 0.2 bar of pressure.

The low pressure could result in baths taking an excessive time to fill and cause showers to perform below expectations.

The cold main into your property is likely to have over 1.0 bar of standing pressure and is often between 3.0 to 4.0 bar, increasing the suitability of an unvented cylinder.

If the pressure is above 4.0 bar, it is worth considering a pressure reducing valve for reliability and consistance of performance.

Good standing pressure alone is not a complete indication that the cold main is suitable for an unvented cylinder, as the operating pressure and flow must also be verified.

The requirements for the installation dictate how much pressure and flow from the cold main is required, a one bathroom flat will have a different requirement to a six bathroom hotel.

Once the cold main has been confirmed as suitable, we have the potential to deliver Domestic Hot Water at the pressure and flow, nearly equal to the pressure of the cold main. If an unvented cylinder can now be considered, the advantage of high pressure showers and fast filling baths at your finger tips.

If you wish for your Unvented Cylinder to be repaired, replaced or serviced, simply call us on 01206 432 999.

Do you own an existing Unvented Cylinder and are interested in Heat Lab Ltd improving the delivery of your Domestic Hot Water?

The performance of the Unvented Cylinder can be improved primarily by increasing the water pressure available to the cylinder inlet. This can be done by increasing the diameter of the cold water main (not always possible/effective) or install some form of boosting equipment, as detailed below;

All of these products are available from ourselves and have a thorough description in their relevant section of this website or just click on the description above.

To appreciate the how the unvented cylinders operate, we have information below outlining the operation of the equipment and the components involved.

The Technical Side of Unvented Cylinders

This section will explain the key differences between each type of Cylinder and will also show schematics for their conventional installations.

Open Vented Cylinder

Open Vented Cylinder Schematic

The Open Vented Cylinder has been used since the inception on Hot Water Systems and generally gives years of service without problems.

The main problem with the Open Vented Cylinder is a potential lack of water pressure and/or flow.

The cylinder can be heated directly by an immersion heater or indirectly from a heating appliance on a "S", "Y" or "W" plan.

The installation is flexible in design, allowing boosting pumps to be installed on Showers or any other outlet requiring superior performance.

It is referred to as an "Open Vented Cylinder" as the cylinder is open to the atmosphere and is vented to the highest point in the system, the Cold Water Storage Tank.

The vent pipe is shown in red on the schematic on the left and runs from the top of the cylinder, to over the top of the cold water storage. This pipework must raise vertically and also, be over a minimum diameter.

When the cylinder is heated, by whatever means, the water inside the cylinder expands and if not allowed to escape, will cause the cylinder to fail. The expansion is intended to travel up the vent pipework and will also cause the water level within the cold water storage to rise.

A recent addition has been the addition of a temperature and pressure relief valve on the cylinder itself. This is a safe guard to prevent the cylinder failing in the event the vent and feed pipework to the cylinder becoming blocked.

With open vented type hot water cylinders, the system relys on the height difference between the cold water storage tank and the hot water outlet, every one metre of height difference equates to approximately 0.1 bar of pressure.

Unvented Cylinder

As you can see from the schematic below for the Unvented Cylinder, there is no Cold Water Storage Tank at the highest point in the system and as a result, less space is generally required for installation.

Unvented Cylinder Schematic

The main reason for specifying an Unvented Cylinder is increased Domestic Hot Water Performance and this is acheived by the cold water main pushing domestic hot water out of the outlets.

The Cold Water Main must be capable of providing sufficient pressure and flow for the requirements of the property.

If the water pressure and/or flow is below the level required, please refer to our products and solutions for Cold Water Boosting in the links above.

When the unvented cylinder is heated, by whatever means, the water inside the cylinder expands but as the name suggestes, there is no vent for the water to expand.

The expansion is therefore dealt with in another way, by the expansion vessel and the movement of an air bladder within. Hover over the image above and it will show the water expanding into the potable expansion vessel.

The sizing is identical for both types of cylinder, the capacity is calculated upon the requirement of the property and not the user. Therefore the number of baths, showers, hot water outlets and expected use are all required to finalise the require capacity.

Safety and Controls

The commissioning of an Unvented Cylinder MUST be conducted to ensure the safety of the equipment. Failure to correctly commission the unvented cylinder will potentially cause a substantial failure.

Unvented Cylinder Combination Valve

The valve pictured to the right is inlet control valve or combination valve and has five functions;

The next two components from left to right are seperate but work together for functionality and safety;

Unvented Cylinder Thermostat and 2 Port Valve

Collectively, these components form the "Energy Cut Out".

The operation 2 port valve is controlled by the Thermostat "calling for heat" and in the event of the thermostat failing to on, the high limit thermostat will activate causing the 2 port valve to close stopping the heating of the cylinder.

This high limit thermostat must not reset automatically and will therefore require resetting as needed manually, after the cause for the overheating has been rectified.

Unvented Cylinder Temperature and Pressure Relief Valve

The component to the right is a Temperature and Pressure Relief Valve, it is the last component to prevent a failure of the unvented cylinder due to overheating.

This valve has two crutical safety factors, to stop over pressurisation and heating. The pressurisation element is by conventional means of the water pressure acting on a sealing rubber washer which is spring loaded and the second lifts the seat when the water temperature exceeds 96 Deg C by allowing the cold water to enter the cylinder and the hot water to be discharged safely to the drain.

The last factor is why a Temperature and Pressure Valve is not required on a Thermal Store, the body of water is not repressurised by the cold main.

Immersion High Limit Cut Out Unvented Cylinder

To the left is the Immersion Heater Thermostat and the connecting mains electricity cable, of which must be rated for high temperatures encountered.

The Immersion Thermostat must feature a high limit cut out (circled) and it must not automatically reset.

If the high limit thermostat activates, the reason for the overheating must be investigated and rectified prior to resetting.

Immersion heaters are now commonly connected to Solar PV Installations and controlled by a Solar Controller, similar to an "Immersun" diverter. This enables the Immersion heater to consume any energy that would be otherwise be exported to the mains national grid, effectively providing free Domestic Hot Water and also, a very low carbon footprint.

If an Immersun controller is used, it is often worthwhile specifying two 3kw Immersions, one in the bottom of the cylinder and the second in the mid section.

The upper immersion is the primary heating element and is utilised initially, especially useful in periods of low solar gain. The secondary Immersion is located in the bottom section of the cylinder and is used in periods of high solar gain, where it is possible to heat the complete cylinder.

The installation of an Unvented Cylinder is considered a "Controlled Works", it therefore requires certification and submission upon completion.

Good Pressure and Flow from an Unvented Cylinder Two Showers from an Unvented Cylinder

Two reasons why unvented cylinders can be so effective, luxurious Water Pressure and Flow to quickly fill large baths or run multiple showers.

If the water pressure in your property is insufficient, there are methods that can be adopted so almost any requirement can be accomodated.

This technology can be applied to almost all systems, including most renewable energy heat sources. If connecting to some low temperature heat sources, specialist cylinders with larger heating coils must be specified and installed.

If you are located within a hard water area, a suitable form of water treatment is required to ensure the manufacturers warranty remains valid.

When considering an Unvented Cylinder, it is important to consider that they require annual servicing and we suggest that this is completed when the boiler is serviced.

Tel. 01206 432 999