STOVE GUIDE - DanSkan

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
A 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
without… without… without…
with… with… with…
large... small... large... small... large... etc.
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/m 3
Basalt Lava 3000 3,0
Soapstone 2980 6,4
Granite 2800 3,5
Sandstone 2400 2,1
Ceramics (dense) 2400 2,1
Thermal
Conductivity λ
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 λ-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 λ-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 heating costs. The good news
is that heat storage has made the choice to heat with wood even
more attractive.
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