Types of Hot Water Storage

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This article is a summary of all the types of hot water cylinder, including thermal stores.

Vented Tank Fed Cylinders

British Standard Copper (and stainless) Vented Cylinders

Grades 1, 2 and 3, with grade 3 the most common and suitable for 10m head. Grades 2 and 1 take higher pressures.

Cylinders are also available with a built in water storage tank, fitted with ball float valve, for a compact installation. Such systems are rarely used to day as they generate very low water pressures and are prone to the cold tank warming up.

Primatic Cylinders

Rarely found these days, the primatic cylinder makes use of an internal arrangement that enables the primary system to be filled from the secondary (domestic water) system. A trapped air bubble keeps the two systems separate and stops back contamination, however incorrect sizing can result in a failure of the system to maintain separation and over time they have dropped out of use.

A more advanced variant was the DuoFlow Cylinder patented by DPS Ltd.

The Spartan ET50 and ET100 were systems that created a primatic system using a vessel external to the cylinder to provide the same functionality as a primatic system.

Unvented Cylinders

The most common form of storage, 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.

Unvented cylinders were illegal in the UK up to 1986 due the inherent dangers of a change-of-state explosion caused by high pressure water heated to over 100C suddenly been dropped to atmospheric pressure due to a failure and instantly turning to steam. The following video on YouTube provides an idea of the dangers... https://www.youtube.com/watch?v=61uDmQF5I2E

This is the reason all unvented cylinders require 3 levels of safety controls, and must be installed and regularly maintained by an unvented registered installer with certification. You must also inform the local building control of any installation.

They shouldn't be used with uncontrollable heat sources such as wood burners without proper separation within the system so heat can be removed from the store and directed elsewhere when needed.

Please read our full article on Unvented Hot Water Storage for further information.

The Amazon HXIN Unvented

Unvented cylinders, like most domestic water stores, can be heated by either electric immersion elements, or from a boiler by use of an internal coil. The HXIN system is a variant manufactured by Thermal Integration whereby a plate heat exchanger is used instead of a coil to improve the reheat performance and allow variable quantities of heat to be heated.

The following schematic shows a typical HXIN system. Cold water is pumped from the base of the cylinder through the plate heat exchanger (where it is heated) and returned back to the top of the store heated to full temperature. It is synonomous to haveing a combination boiler bolted to the side of the cylinder.

A more recent advance on the HXIN design makes use of HIUs to provide the same functionality, and can be retro-fitted to existing cylinders to improve their performance. Retro-fitting also frees up the existing coil for connection to an alternative heat source such as solar panels.

More information on the HXIN can be found here... http://www.systemdesigner.co.uk/documents/Amazon_HXIN.pdf

Thermal Stores

Coil Type Thermal Store

Heat Banks




Originally the Stowaway, the Pandora Heat Bank is a patented type of thermal store that is manually filled with a hose pipe so the stored water is a stand-alone system not fed from any water source. The system is vented to atmosphere above the water level in the store, and makes use of an expansion vessel that is also vented to atmosphere above the diaphragm. As a result, there is no need for a discharge pipe, and as the system is unpressurised the unvented G3 regulations do not apply and the unit can be installed by anyone 'competent' without a need for annual maintenance.

There are thousands of Pandoras installed across London (and elsewhere) as they were the store of choice for many house builders such as Laings, Galliards and Barrats, as well as some local authorities.