About Thermal Stores

Facts and figures about our thermal storage tanks.
A good quality thermal storage tank will last for decades with room for new connections and without loosing it's thermal efficiency. All our thermal stores are designed to do this.


Information about our thermal storage tanks


How do they work ?

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 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 from where the heat is transferred to the heating and hot water when needed. 

The principle of a thermal storage heating is simple and therefore it is also extremely reliable. A standard thermal storage tank is filled with water which then is heated up either directly or indirectly by any available heat source.

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

A modern thermal storage tank enables multiple heat sources to be connected into one point making the heating and hot water system flexible and future proof.

For this, the thermal store must have enough coils and as importantly, plenty of couplings to where the heat sources and heating flows / returns are connected.

Being large and the centre of a heating system, the thermal tank is usually put in place when the building is constructed.

Which one ?

A good quality thermal tank 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 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 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 simple thermal calculation formula can be found here.

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 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 instance solar, then the tank will need to be sized on the solar output as too small a tank will limit the amount of available solar energy in the system. 


A good thermal tank can be connected to several heat sources. This should be taken into account when sizing up the system as again, too small a thermal tank will limit it's usability in the future.

For example, 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.

Does it fit ?

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.

The lifetime of a quality thermal tank is decades and it does not need changing. Therefore it can be integrated into the building at an early stage. Due to the weight of the tank when filled up, it must be placed on a solid floor. If there is no room available in the main house, the tanks are often put into the garage or other outbuilding nearby.

Preserve 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.

  Heat up time / Hours at kW boiler output
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

Blue areas indicate the recommended store size v. boiler output.
Thermal store must always be sized to be large enough to handle one full log / solid fuel boiler burn. This can vary between 1 and 4 hours, depending on the boiler size, air settings and the material being burned.

Jaspi Thermal Stores
All our stores listed with dimensions

Model  Litres  Dimensions mm
H x W x D
Elegant 270s Direct  265  1996 x 597 x 598 
Elegant 270s Indirect  260  1996 x 597 x 598 
GTV 270  270  1406 x 593 x 593 
GTV 500  500  1665 x 743 x 743 
GTV 700  700  2000 x 780 x 820
Ultrastore 500 500 1825 x 667 x 675
Oval 1.0 EPK  1000  1720 x 1230 x 780 
Oval 1.2 EPK   1200  2020 x 1230 x 780 
Oval 1.5 EPK   1500  1720 x 1770 x 780 
Oval 1.8 EPK   1800  2020 x 1770 x 780 
Oval 2.4 EPK    2400  2020 x 2310 x 780 
ThermoStore 500K 500  1980 x 790 x 790 
ThermoStore 700K  700  2130 x 870 x 870 
ThermoStore 1500K 1500  2290 x 1260 (Round) 
ThermoStore 2000K 2000  2300 x 1410 (Round) 
ThermoStore 3000K 3000  2440 x 1610 (Round) 
J-Immersion heaters  3 - 6kw Single and triple phase, 50mm flange

A rough guide for selecting a Jaspi Thermal tank:

GTV 270 - 700L

Smaller dwellings with radiators or mid-size houses with underfloor heating and normal hot water demand. Excellent buffer tanks for wood and multi energy boilers.

Ultrastore 500L

New generation high efficiency thermal store with 667mm narrow design. Very good expandability with three indirect coils as standard. Multiple couplings with 'Leak Detect' sleeves and an internal baffle plate.

Oval 1000 - 2400L

Mid-size and larger houses with narrow access. Radiator or underfloor heating. Possibility to connect several heat sources into one unit. Normal or high hot water demand. Delivered with 2 coils as standard.

ThermoStore 500 - 3000L

From mid-size to large and very large dwellings and office / industrial units. High or very high hot water demand. Needs wide access to fit in place. Excellent expandability. One coil included as standard.

Elegant 270L

The first in it's class modular domestic hot water cylinder. Ready connected safety valves & expansion vessel with no visible pipework outside. Classy, clean appearance for easy siting. Stainless Steel, mains pressure with a 3.3kW ultra long life alloy immersion heater. Direct & indirect models.  

Installation of Oval 1200 with solar




All of our large products like the thermal stores and boilers are delivered directly from the factory to the customers.
Most of them will also need a forklift or similar for unloading.
More information can be found from here:

How to purchase

All our standard products can be ordered through this website. Please find out how to order by following this link:


NORTHMAN JASPI UK  01392 247 343