HEAT STORAGE

STOVES WITH HEAT STORAGE ELEMENTS

In recent years heat storage stoves have experienced a renaissance, partly due to the rising costs of alternative forms of heating a home – but that‘s not the only reason. Modern houses are much better insulated and therefore need less heating energy to keep them warm. The captured heat which is gently released from a stove with ceramic or stone heat storage elements can be, for many people, sufficient to meet a lot of their heating needs as well as being a very pleasant form of heat.
However, there is a misleading impression given today that the use of heat storage elements make a stove more efficient – which we‘re sad to say is not the case. While there is no doubt they do provide a practical heating solution, they do not increase the heat efficiency, merely delay the release of heat that would have been delivered anyway.
Efficiency is determined by the energy yield from the fuel burned in the fire chamber – whether it‘s radiated, convected or released through a heat storage mass.

STEEL BODIED STOVES

BA typical convection stove with a steel body and without any heat storage elements will deliver direct heat quickly into the room.
With the recommended full fuel load burning the stove will transmit the highest heat output but, as the fire burns down, then the heat transmission is also reduced because the stove does not have a heat sink only the thermal mass of its bodywork. During the stove‘s operation the heat output develops the following way:

  • Large fire = high heat output
  • Small fire = lower heat output

The heat output curve during a normal operation period will therefore have its ups and downs as fuel is reloaded. However, this curve can be significantly altered by heat storage.

HEAT STORAGE STOVES

Convection stoves are normally not designed to include a space for a heat storage mass – there simply isn‘t the space inside the traditional format, and until recently there just hasn‘t been the demand. The heat storage capacity of a stove, say with with stone or ceramic heat storage elements, is determined by the mass and specific weight of the heat storage material. The greater the mass then the greater the heat storage potential and the longer the heat will last. The big advantage of a stove with additional heat mass is that the stored heat ‘irons out’ the peaks and troughs associated with stoves without heat storage, discussed above. So when the fire is large the heat is transferred to the heat mass and when the fire is small, even with only the burning embers left, the saved heat is released into the room. Stoves with heat storage elements thus have a linear, consistent heat output, making them very comfortable to live with.

HEAT TRANSFER

With or without heat storage elements

HEAT STORAGE MATERIALS

Thermal mass is the ability of a material to absorb heat energy. A lot of heat energy is required to change the temperature of high density materials like Soapstone (traditionally used for heat storage) and so these materials are described as having a high thermal mass. Conversely lightweight materials, such as timber, have a low thermal mass. Thermal mass acts like a thermal battery which stores and re-radiates heat.
However, heat storage elements should also demonstrate good thermal conductivity by allowing heat to flow through them. For example Stone is a good conductor of heat but if conductivity of a material is too high (for example steel) then the energy which was quickly absorbed will also be given off too quickly to create the beneficial time delay essential for heat storage stoves.
As the heat storage material absorbs the heat from the stove,
especially at the peak of the burn cycle, energy waste is eliminated and later, with the time-delayed heat release, all of the heat produced is delivered to the room. Ceramic, Sandstone and similar materials, with their lower thermal conductivity, need longer to get fully ‘charged’ with energy, but as can be seen from the table, they have the advantage that they release their energy slowly too and
make an excellent and attractive alternative to traditional soapstone heat sinks.

Material Gross Density kg/m3 Thermal Conductivity λ
Basalt-Lava 3000 3,0
Soapstone 2980 6,4
Granite 2800 3,5
Sandstone 2400 2.1
Ceramics (dense) 2400 2.1

TECHNICAL INFORMATION

Different heat requirements can be fulfilled by choosing materials with different densities and thermal conductivities. The more dense a material is then the greater is its specific weight. The time based conductivity of the material is determined by the composition of the material. The higher the -value is then the better the conductivity. So, fast thermal conductivity, for example high density steel, produces a shorter thermal wave. It is materials such as soapstone with longer thermal waves that show a greater time-delay.

HEAT RADIATION

Critical for healthy heating is heat which is equally distributed throughout the living space and not the intensely high heat which is radiated from a traditional stove. The intensity and the high thermal conductivity speed (high -value) is consequently not the best solution for effective and comfortable stored heat. Even if the convection channels of a stove were closed up you would not get a higher heat output either. Heat which would therefore not get to the room through the convection channels is only time delayed through the heat storage value of the heat storage material.

STORAGE CAPACITY

As a general rule one can say that the more mass and surface storage a stove body has, then the more heat can be absorbed and then released later and it is the thermal conductivity of the bodywork and heat sinks that determine the time delay of released heat. Be wary of unsubstantiated claims of ‘post heating‘ periods of 15 hours or more which are probably misleading – simply compare the overall weight of the stoves and the materials used for the heat storage. Heat storage stoves can only release to the room the heat that was originally absorbed. The more mass and the longer this mass is heated then the longer the time span this heat can be released to the room. 30 to 40kg of heat storage mass would typically require around 1 hour of pre-heating.

SPOILT FOR CHOICE

DAN SKAN now offer you an attractive choice of highly efficient heat storage stoves with a number of alternative heat mass materials and finishes, not only designed to fit in with your interior decor but your lifestyle too.
We can be happy that nowadays we live in better insulated houses, in which heating with wood is a realistic as well as comfortable choice, that will save money general on heating costs. The good news is that heat storage has made the choice to heat with wood even more attractive.