HearthStone Nestor Martin S31 Owner's Manual page 4

THE FLUE
There is often confusion as to the terms "flue" and "chimney" and for the purposes of this manual we define
whatever duct conveys the products of combustion as the flue, and the term chimney to mean any masonry
structure within which the flue may be contained. The overall efficiency and reliability of the stove will depend
upon the flue's ability to provide a consistent negative pressure or draft, and it is therefore important to
understand what can affect the flue's performance and how to ensure the flue installation provides your stove
with the optimum operating conditions.
However well the fuel metering valve is calibrated, proper combustion depends on the correct amount of air
being supplied to the stove at all times. This is ultimately dependent on a properly adjusted and stable negative
flue pressure. The initial flue draft is created by the gas confined within the flue being hotter and therefore
lighter than the air outside the flue. The tendency for the hot gas to move up the flue is proportional to the
height of the flue since the difference in weight of equivalent columns of air and flue gas is greater the higher
the column. While this may be theorectically true, in practice, because the temperature of the flue gas is cooled
through the wall of the flue and the flow is slowed by the friction of the internal surface of the flue, the benefits of
extreme flue heights are negated.
The need to minimise the fluctuating effects of wind by having very hot flue gas temperatures inducing the
greatest possible constant negative pressure within the flue conflicts with the ideal of utilising all the heat
generated within the stove for heating. The compromise is to minimize whatever heat loss is necessary to
create a gas flow within the flue by constructing the flue itself with an internal surface as smooth as possible
and by being thermally insulating the system. Both these requirements can be met in an existing chimney by
lining it with a stainless steel oil liner insulated with vermiculite or mineral wool. Where no chimney exists,
double walled insulated stainless steel flue systems are available.
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