Thermal Store Info

Thermal Store / Accumulator Info

Energy sources

A thermal storage tank, often also called an accumulator, is an essential part of a modern heating system. A standard domestic hot water cylinder is also a thermal store although with limited capabilities. 

Thermal tank is the 'hub' of a heating system where all the available heat goes into, is stored and when needed, the stored heat is transferred to the heating and hot water.

The principle of a thermal storage heating is simple and therefore it is also extremely reliable and energy efficient.

A thermal store does not make heat but is filled with water which then is heated up either directly or indirectly by any available heat source, for example a boiler.

The most common heat sources are wood / woodchip / pellet boilers, heat pumps, solar panels, direct electricity and wind power. Oil and gas boilers are also be used either on their own or in combination with other heat sources.

Being large and the centre of a heating system, the thermal tank is often put in place when the building or boiler room is constructed. Therefore it very important that the store is designed to last for a lifetime of the heating system.

Thermal Store or a hot water cylinder?

The main difference is that a DHW cylinder is often smaller in capacity (<300L) and is designed only to store domestic hot water which is then used to supply the taps.
Coils in these cylinders are designed to transfer heat into the cylinder by a boiler or other heat sources.
Therefore, as a DHW cylinder does not hold water for heating, it's use as a thermal store is very limited.
i.e. if you want to turn on the heating, this heat is taken out of the cylinder via an indirect coil. This coil will use all the available heat fairly quickly and you have no more warm domestic hot water for the taps.

Unvented cylinders are good for domestic hot water but not so for heating.
Generally unvented cylinders are smaller than thermal stores and because of that, they store less heat and need to run hotter, asking more heat from the boiler(s).

Thermal stores are designed to hold more heat by being larger and are generally better insulated.
They work the opposite way to the domestic hot water cylinders; They hold a large amount of water which is the same water that goes around your radiators, UFH and boiler(s).

Domestic hot water is taken via indirect coil(s) so it holds less of the tap water. When the tap is opened, the domestic hot water is heated up by the coil(s) as needed, like in the combi boilers.
All the best thermal stores have one or two high capacity coils, main coil and sometimes also a pre-heating coil. This way, the heat loss is minimised and the thermal store can run cooler for improved energy efficiency.
Thermal stores are also designed to be connected to several heat sources and outputs like radiators, towel rails and underfloor heating.

A Good thermal store

A good quality thermal store is large enough to accept multiple heat sources and has the required connections. 

In normal domestic use, anything under 300 litres of volume cannot be considered as an effective thermal store. This is because the small volume of stored water does not store enough energy to be useful for heating when it is needed.

Future expandability is also a very important factor. As we are getting closer to the time when fossil fuels are becoming scarce and more expensive, our need to find alternative heat sources is already here. Therefore it is hugely important that any heating system fitted now can also accept new heat sources and technologies. A good quality thermal store is designed to do exactly that.


Size of the dwelling, number of rooms, room height, existing insulation, type & size of windows, amount of radiators and number of baths / showers. These all will determine the total heat demand and the thermal store needs to be big enough to cope. If the hot water demand is unusually high this also needs to be taken into account when sizing up the system. Usually underfloor heating requires smaller size thermal tank compared to radiators.

A standard 3 bedroom dwelling should look a thermal store between 500 and 1500 litres, taking on account the variables mentioned above.

Heat sources

As a rule of a thumb, renewable energy sources like solar, heat pumps, pellet or log boilers need larger thermal storage tanks. For example solar panels will produce a good amount of heat throughout the day which then needs to be stored until it is needed, usually in the evenings.

If the thermal store is too small, most of the solar panel output is wasted as the panels cannot put in any more heat and when the heating is turned on, the storage will run out of heat very quickly.
Wood boilers also need larger storage tanks as the heat output is high when the boiler is on but as soon as the fire goes off, there is no heat output at all.
Oil and gas boilers can manage with smaller thermal tanks and on their own, they don't need tanks at all. However, if they are connected together with some other form of heat, for example solar, then the tank will need to be sized on the solar output as too small of a tank will again limit the amount of available solar energy in the system.

The good, bad & the ugly

There is definately some confusion and misinformation on the market regarding the thermal storage tanks.
Good ones are, as mentioned earlier, large and specifically designed for multiple heat sources with the appropriate amount of connections.
Insulation in a good thermal store is fixed onto the tank and is of a high quality, dense material.
All the best thermal stores have two or more indirect coils, for domestic hot water / heating.

The bad - These thermal stores are often delivered with a detachable foam insulation. They look the part but are often made of thinner steel and the detachable insulation will not provide the thermal efficiency required. If installing one of these, a bit of homework is recommended to ascertain the construction material, amount and type of the couplings and the background of the manufacturer.

The ugly - These products are also called as thermal stores or accumulators but are not designed as such.
They are often modified hot water cylinders, smaller in size and in some cases made of copper and therefore cannot withstand the required system pressures or the flow rates of a standard heating system. Short useable working life and generally not fit for purpose. Please avoid.


Thermal storage tanks can be big. The smallest are fridge / freezer size, the largest can be as large as a van.
Therefore it is important that the doors, passageways and other narrow points are wide enough for a tank to pass through. It is common when a new house is built that the thermal tank is put in place before any internal, and sometimes external, walls are built.
Due to the weight of the tank when filled up, it must be placed on a solid floor and only rarely can be safely fitted above the ground floor.

Any pipe couplings, immersion heaters or thermostats fitted to the stores needs to be tightened well and sealed with PTFE tape or hemp. Hemp is particularly good with larger fittings.
It is not recommended to use thread glue and the installer must never use silicone with the thermal store couplings.

If your plan is to have a thermal store but to add solar thermal later on, it is advisable to order the tank with the solar coil already fitted.
Although most of our tanks allow retrofitting of the coils, it will cost more to do this later on as to fit the coil later on requires the tank to be drained and any treatment chemical will be lost. To then refill the system and add new CH additive will easily cost more than the coil in the first place.

Reserve room for the installation and auxiliary items.

Pipes, pumps and expansion vessel(s) require additional installation space. All the connections are on the front, bottom and top of the stores. It is also advisable to leave enough space for the plumber to work in.

Thermal store / accumulator average heat up times

Store size Litres 20 kW 25 kW 30 kW 35 kW 40 kW 45 kW 50 kW 55 kW 60 kW
500 1.0 0.8 0.7 0.6 0.5 0.5 0.4 0.4 0.3
1000 2.0 1.6 1.4 1.2 1.0 0.9 0.8 0.7 0.7
1200 2.4 2.0 1.6 1.4 1.2 1.1 1.0 0.9 0.8
1500 3.1 2.4 2.0 1.7 1.5 1.4 1.2 1.1 1.0
1800 3.7 2.9 2.4 2.1 1.8 1.6 1.5 1.3 1.2
2000 4.1 3.3 2.7 2.3 2.0 1.8 1.6 1.5 1.4
2400 4.9 3.9 3.3 2.8 2.4 2.2 2.0 1.8 1.6
3000 6.1 4.9 4.1 3.5 3.1 2.7 2.4 2.2 2.0