Gbp Art Stn Thermal Stores 07 04 10 V1

Thermal Stores – Questions & Answers

This document has been written to answer the most common questions regarding thermal stores. Extensive technical detail is provided in the HWA Performance Specification for Thermal Stores.

What is a thermal store? Thermal stores are hot water cylinders that store primary water as opposed to secondary (drinking) water and satisfy the requirements for mains pressure hot water and the integration of alternative energy. Alternative energy fuel sources are typically low grade, inherently unpredictable and often plentiful in supply but not always when the energy is needed. A thermal store provides the means to harness the energy when it is available for later conversion into both hot water and heating.

What energy sources can be used with a thermal store? Thermal stores are able to use heat from a variety of different sources at the same time. Of course they also work well with conventional gas or oil boilers but additional input can be taken, when available, from a wide range of devices including solar panels, solid fuel wood-burning stoves, ground or air source heat pumps and electricity from renewable or low carbon sources, and harnessed for later conversion into both hot water and heating.

How is hot water produced? The principles of a thermal store are different to a conventional open vented or unvented cylinder. The water in a thermal store remains constantly hot, heated by one or many sources of energy. This energy is used to instantaneously heat mains cold water as it passes through a special high efficiency heat exchanger before being delivered to the taps as mains pressure hot water.

Will the alternative energy be able to make a contribution to the heating in the property? Yes, the cylinder essentially stores heat energy that can be used either to produce domestic mains pressure hot water or for distribution around the heating circuit. This allows a wood burning stove or solar panels, for example, to provide heat which is initially stored and is then available for distribution around a wet central heating system. This can be especially beneficial on cold autumn or spring days when there is a useful solar gain available, or where the householder would rather use a renewable energy resource such as wood burning rather than a gas or oil boiler. Careful design of the thermal store to the HWA Specification ensures that the heating circuit cannot extract all the thermal energy and that there is always enough energy retained to provide hot water which is deemed a priority.

Can a thermal store be used with wood burning or other ‘uncontrolled’ heat sources? Thermal stores are particularly suitable for use with wood burning, solid fuel stoves or other ‘uncontrolled’ heat sources as the thermal store can be open vented.

Does a thermal store system need controls? Building Regulations require heating and hot water systems to have a level of control to prevent waste of fuel resources.

For a thermal store this will consist of control and overheat thermostats on any immersion elements that are fitted. The boiler and other alternative energy circuits will require control and overheat temperature thermostats of their own. There will need to be an overall control system methodology suitable for the particular property and anticipated usage. Detailed examples can be found in the ‘HWA Performance Specification for Thermal Stores’ If there are multiple energy sources, is a neutraliser needed? The thermal store can act as the neutral point of the system so there is no requirement for an additional ‘neutraliser’.

Are all thermal stores insulated? Building Regulations require that thermal stores are insulated to meet the demanding requirements of both new build Part L1A and refurbishment Part L1B. This applies even if the product is to be installed in a refurbishment situation and provides a high standard of insulation consistent with an energy efficient cylinder.

What are the other benefits of a thermal store?

Upgrade potential A thermal store fitted with all the required tappings offers ease of installation for the plumber. It also means that installers can offer ‘solar or wood-burner ready’ options to householders who may want to add additional alternative energies in the future as an upgrade to their system.

What else do I need to know? Water mains As thermal stores are mains pressure hot water systems it is important to check and ensure that the performance of the incoming mains cold water is adequate before choosing to fit them. To achieve the best performance it is recommended that a dynamic pressure of between 2 and 4 bar is available at the appliance although they should work adequately as pressure as low as 1.5 bar. A minimum flow rate of 20 litres/minute is recommended although in properties with more than one bathroom the required flow rate should be calculated using the method set out in British Standard BS6700 or other suitable design guide.

As with any other hot water appliance or combi-boiler Building Regulations require a suitable scale inhibitor to be fitted on the cold supply to the appliance if the hardness levels exceed 200ppm (mg/l). The engineer will check this as part of the installation.

Which thermal store is the right one? Whilst the principles and benefits of a thermal store are straightforward to understand, creating a functioning system requires expertise. To provide guidance to the industry the Hot Water Association (HWA) has invested significant time and effort to develop the ‘Performance Specification for Thermal Stores’ which sets out the characteristics necessary for an effective system to be constructed. Installers should check that any product meets this standard which is named in Building Regulations. The document can be downloaded on the Specification page of this site.