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Thermal stores

Thermal stores are becoming increasingly common in domestic renewables heating technologies as a means of storing excess heat generated.

They are used with an individual renewable heating technology or combine different renewable heating technologies. Thermal stores can also be used as a renewables technology with a conventional boiler or immersion heater.

Thermal stores have proved to work particularly well with wood-fuelled biomass boilers, heat pumps, wind energy and solar water heating systems.

Please note when a solar water heating system is combined with a thermal store the system will not be eligible for domestic Renewable Heat Incentive (RHI) payments. This is because solar panels that are part of a system that also provides heat for a purpose other than domestic hot water are not eligible for the scheme. 

What is a thermal store?

A thermal store is a way of storing and managing renewable heat until it is needed.

In a domestic setting, heated water is usually stored in a large well-insulated cylinder often called a buffer or accumulator tank.

A thermal store may contain one or more heat exchangers, usually in the form of internal coiled pipes or external flat-plate heat exchangers. It may also include an electrical heating element, such as an immersion heater.

A purpose-built thermal store can take inputs from a number of different technologies, provided it has been designed and sized to work with them all. It might store heat from a wood-fuelled boiler, solar water heating or a heat pump.

A thermal store can provide:

  • Space heating and mains pressure hot water.
  • Space heating only (which may be the case with a heat pump system).
  • Hot water only (common in the case of a solar water heating system).

The full potential of a thermal store is realised when it is used to store and manage a number of different heat inputs and outputs. For example, it lets you use your excess solar heat for space heating or to act as a heat pump pre-heat when this is used to supply hot water.

Find out more about how a thermal store works.

The benefits of a thermal store
  • Allows management of the difference in time between when heat is available and when it is needed. For example, hot water produced by a solar water heating system during the day can be stored for use when little or no solar energy is available.
  • Enables water to be heated by a secondary heating source such as a conventional boiler or electric immersion heater.
  • Enables a renewable heating system to work more efficiently. This is particularly relevant to wood-fuelled heating systems such as log boilers that operate much more efficiently if they are used at maximum output rather than kept ticking over.
  • Lets you use a wood burning boiler stove or a stove with a back boiler at maximum efficiency without overheating the room.
  • Reduces the need to buy expensive fossil fuels to meet on-demand hot water or space heating.

Types of thermal store

There are a number of different thermal stores on the market designed with different technologies in mind. Some of the most common are highlighted below.

Thermal stores for wood-fuelled heating

Thermal stores are very important for the efficiency of biomass heating systems, particularly log boilers, which are designed to burn batches of logs at high levels of efficiency rather than in small quantities throughout the day. A log boiler linked to a large thermal store can be used in this way. A thermal store can also reduce the time lag (which could be at least an hour) between lighting the boiler or stove and the demand for hot water by storing water from the last time the stove or boiler was last lit.

As thermal stores used with wood-fuelled heating systems are usually designed to provide hot water for space heating as well as domestic hot water, they tend to be fairly large. Thermal stores linked to wood fuel heating systems are commonly referred to as accumulators or buffer tanks. Typically they will hold between 500 to 5000 litres of water and can store hot water for days if properly insulated.

Smaller thermal stores (300litres) can also work well with boiler stoves and stoves with back boilers. These stoves tend to be in living spaces and are fed with fuel throughout the day. Boiler stoves and stoves with back boilers differ in the proportion of heat they put into the room or water. Boiler stoves will put around 65 per cent of their output into water, whereas stoves with back boilers may only put 20 per cent into water.

The sizing of a thermal store for a wood-fuelled heating system will depend on many factors, in particular the type of wood fuel being used. A pellet boiler will need a relatively small thermal store as it can cope quite quickly with changes in heat demand. A log boiler designed to burn logs in batches will on the other hand need a large thermal store to take all the heat from the batch of logs in one go.

A thermal store working in conjunction with a log burning batch boiler will need to be fairly large, probably no less than 25 litres/kWth and preferably 50 litres/kWth. The sizing of the thermal store connected to a wood fuel boiler is something for the installer to ascertain as part of the total system design.

Thermal stores for solar water heating

Thermal stores work very well with solar water heating systems as they allow solar thermal heat to be used for space heating as well as heating water. On a sunny spring or autumn day, a solar thermal array may harvest far more heat than would be needed for hot tap water alone. Combined with a thermal store also supplying space heating, this harvest can be put to good work.

With a simple design and control strategy a thermal store can also be designed to prioritise solar thermal heat above all other sources. This will mean that if solar heat is available, no other heat source will come on.

Thermal stores for heat pumps

An air source or ground source heat pump will work more efficiently with less wear on the pump and compressor if it does not have to continually cycle on and off (short cycling) when the demand for heat is low. This is more likely to happen if your heat pump is relatively large and less likely if it is relatively small and running continually to meet demand. It is also less likely if you have an air source heat pump with a motor that can modulate its output.

One of the ways to avoid the short cycling of a heat pump is for it to be linked to a thermal store (usually referred in this instance as a buffer tank). However, there are other ways (such as leaving a part of the heating system permanently open). Your installer, in conjunction with the recommendations of the manufacturer, will decide whether a buffer tank linked to your heat pump is appropriate, as well as what size buffer tank you should use.

Combining technologies

A thermal store allows you to link up a number of different heating systems, for example, a wood burning boiler stove and a solar water heating system. This is a particularly beneficial combination as it means that you could have hot water in the summer without having to light the stove.

You can also connect the following to a thermal store:

  • One or more renewable energy technologies (solar, biomass, heat pump).
  • A conventional boiler.
  • An Aga-type range with a boiler.
  • Electrical heating elements such as an immersion heater.

Please note when a solar water heating system is combined with a thermal store the system will not be eligible for domestic Renewable Heat Incentive payments. This is because solar panels that are part of a system that also provides heat for a purpose other than domestic hot water are not eligible for the scheme. 

This content has been developed by the Energy Saving Trust in partnership with the OCTES project, with funding from the Northern Periphery Programme (NPP) and the Scottish Government.

More information

UK Government’s Domestic Building Services Compliance Guide 2010

Guidance for installers on how to comply with the Building Regulations made available by the Planning Portal.


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