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Hot Water Cylinder Guide

A hot water cylinder can be found in most homes across the UK. Hot water cylinders are commonly used to ensure copious amounts of stored hot water is available on demand for baths, showers and kitchen use. The cylinder most suitable for your home will depend largely on how much water you intend to use throughout the course of a single day and the type of boiler system you have. Cylinders are divided into two main types – Unvented Cylinders & Open Vented Cylinders. See our Hot Water Cylinder Guide below for a full in depth view of Cylinders and hot water systems.

Unvented Cylinder

Suitable for use with System Boilers.

An Unvented Cylinder works off the mains pressure water and therefore eliminates the need for a header tank in the loft.

Unvented Cylinders offer higher hot water flow rates and are therefore able to provide higher water pressure for showers and baths etc. An Unvented Cylinder does not require a feeder tank so this frees up valuable space in the loft area and also gives you the option to site the Cylinder anywhere you want, freeing up further space in the airing cupboard.

Unvented Cylinders are generally manufactured out of top quality stainless steel; this ensures long life and durability. Most Unvented Cylinders can also carry a manufacturer’s guarantee of up to 25 years. 

PlumbNation Heating Supplies offer a wide range of Unvented Cylinders from some of the UK’s most popular manufacturers.

Open Vented Cylinder

Suitable for use with Regular Heat Only Boilers.

An Open Vented Cylinder (or Cistern Fed Cylinder) has a header tank that is usually situated within the loft area. This header tank feeds through the cold water from the mains feed and relies on gravity to pass the water through the pipes to your heating/hot water system. A vented hot water cylinder is normally associated with a conventional heating system.

Vented Cylinders are usually manufactured out of copper, with the copper varying in thickness measured in grades. The better the grade, the better the resistance to copper corrosion, and therefore advances the longevity of the cylinder. A domestic vented copper cylinder is usually Grade 3.

All Cylinders supplied by us at PlumbNation Heating Supplies are compliant with the new Building Regulations, Part L.

Direct Cylinder

A Direct Cylinder is heated via an internal element, usually an immersion heater. Immersion heaters heat the water in the cylinder directly, hence why the cylinder is called “direct”. The water in the cylinder is not heated via any other heat source such as a boiler, making the cylinder an Electric Only option. Once the water has been heated, it is then fed out the top of the cylinder and supplied to the required outlet i.e. a bath or a tap. Larger capacity Direct Cylinders can have up to three internal immersion heaters to ensure the maximum amount of hot water is available. Direct Cylinders are available in Vented Cylinder or Unvented Cylinder Models. Direct cylinders are normally used in a situation where no central heating is installed, mainly flats and apartments and therefore the immersion heater is connected to the electricity.

Indirect Cylinder

Indirect Cylinders are heated via an external heat source, such as a boiler or solar panel. The water inside the cylinder is then heated using an internal coil/heat exchanger, which transfers the heat obtained via the external heat source, to the water inside the cylinder. Although the cylinder is heated via an internal heat exchanger, the original heat is obtained externally, hence why it is commonly referred to as “indirect”. Indirect cylinders may be fitted with immersion heaters acting as a supplementary heat source, but they will still be considered to be Indirect Cylinders. Some manufacturers supply an immersion heater as standard, any additional heaters will need to be purchased separately. Indirect Cylinders have a faster heat up and recovery time than that of a Direct Cylinder model. Indirect Cylinders are available as Vented Cylinder or Unvented Cylinder models.

Solar Cylinder

Solar Cylinders have been specially designed for Solar Heating installations. A mixture of water and a special Glycol liquid (specially designed to absorb maximum heat from the sun’s rays) is circulated through pipe work in the solar panels. This liquid is then returned to the cylinder where the heat is transferred to the stored water via an internal coil/heat exchanger, the liquid is then pumped back to the solar panels to be reheated.

An additional coil is also required to obtain heat from an alternative source, if the heat from the Solar Energy is insufficient. This is most commonly referred to as an Indirect Twin Coil Cylinder. Direct Cylinders can be used for Solar Power with the added heat source being supplied via the internal immersion heaters. It is advised however to install an Indirect Cylinder, as this provides faster heat up and recovery time.

Thermal Store Cylinder

A Thermal Store Cylinder works in the opposite way to a standard cylinder. The water held in the cylinder is heated via an internal or external heat source, cold water is then passed through the hot cylinder via a coil/heat exchanger. As the cold water passes through the heated cylinder the temperature is raised and the cold water exits the cylinder as hot water.

With a Thermal Store Cylinder there is no requirement for a pressure and temperature relief valve, meaning that design safe discharge facilities are unnecessary. A thermal store is cheaper than an unvented cylinder during “on peak” hours when heat pumps are used as the main energy input. The full output of the heat pump is passed to the store without losses through a heat exchanger. A thermal store cylinder is safe to use with a wood burning stove, in fact a thermal store cylinder can accept input from a number of energy sources.

When an alternative unvented cylinder is used with a heat pump, it is necessary to raise the temperature, often with an electric immersion heater, to thermally disinfect the storage vessel, which uses more energy. If a wood burning stove is used with a thermal store there is often no need to boost the temperature for hot water production. If however the heat pump will not sufficiently raise the temperature to give an adequate supply of hot water, an immersion heater and a thermostat pocket may be required.