As they heatup, the gases formed during combustion undergo an increase in volume and, as a result, have a lower density than the cooler surrounding air.
This difference in temperature between the inside and outside of the flue results in a negative pressure which increases proportionally to the flue pipe length and
the temperature.
The draught must be stronger than the fume circulation resistance so that all the exhaust fumes generated during combustion inside the stove are drawn upwards
through the outlet and the flue pipe. There are many meteorological factors that influence the operation of a flue pipe, rain, fog, snow, altitude, but the most important
one is the wind, which can cause negative thermal pressure as well as dynamic negative pressure.
The wind actionvaries depending on whether it is ascending, descending or horizontal.
•Ascending wind always results in an increased negative pressure and draught.
•Horizontal wind results in an increased negative pressure as long as the chimney cowl was properly installed.
•Descending wind always diminishes the negative pressure, sometimes inverting it.
Excess draught causes an increase in the combustion temperature and consequently a loss in stove efficiency.
A part of the combustion fumes is drawn up through the flue pipe together with small wood particles before combustion, reducing stove efficiency, increasing fuel
consumption and resulting in the emission of polluting fumes.
At the same time the high fuel temperature, due to an excess amount of oxygen, wears down thecombustion chamber sooner than expected.
Instead, poor draught slows down combustion resulting in a decrease in the stove temperature, fume spillage inside the room, a loss of stove efficiency and a
dangerous build-up in the flue pipe.
In order to avoid excessive draught it is appropriate to use:
DAMPERDRAUGHT REGULATOR
04.5 STOVE EFFICIENCY
Paradoxically, highly efficient stoves may pose difficulties for fume extraction.
The correct functioning of a fireplace depends on the increase in temperature inside caused by
combustion fumes.
The efficiency of a stove is determined by its ability to transfer most of the heat
produced to the room to be heated. Consequently, the greater the efficiency of the stove, the
the "colder" are residual fumes from combustion and, as a result, the lower the "draught".
A traditional fireplace, of approximate design and insulation, works much better when used with a
traditional open fireplace, or with a poor quality stove, where most of the heat is
lost with the fumes.
Buying a quality stove therefore often means having to intervene on the chimney, even if it already
exists and is functioning with old systems, to better isolate it.
Poor draught results in the stove not operating when hot or in smoke spillage.
•A common mistake is to connect the stove pipe to an existing fireplace, leaving this
to also work with the old system. In this way, two solid fuel systems are
joined by the same flue pipe, which is incorrect and dangerous.
•If the two appliances are used simultaneously, the fume load could exceed the existing chimney flue capacity resulting in downdraught. If only one appliance
is used, the fume heat will facilitate draught but the cold air coming from the other appliance not in use will cool down the exhaust fume temperature again,
blocking the draught.
•Besides the problems described so far, if the two appliances are placed on different levels the communicating vessel principle might be interfered with, causing
combustion fumes to be drawn in an irregular and unforeseeable way.
05. FUEL
Dry wood produces more usable heat than wet wood as it produces less smoke, less soot and less carbon dioxide. Dry wood is the correct choice to maximise use
of your stove. Always remember to split the wood so that it loses moisture: the higher its water content, the greater the quantity of heat needed to ignite.
Based on the weight in Hg of a cubic metre of material, the wood can be classified as either “softwood” or “hardwood”.
- "Softwood" (300--350 kg/m³ such as spruce, pine, poplar, alder, chestnut, willow) burns rapidly, producing a fiercer heat; it is therefore ideal for starting
a fire but to use it as fuel would require frequent refilling of the stove. “Softwood” can also produce more creosote, meaning that the chimney will require
more frequent cleaning.
- "Hardwood" (350-400 kg/m³ e.g. elm, oak, holm oak, beech, ash), instead, has a slower combustion thus producing a longer-lasting heat. It is due to
these characteristics that it is preferred for domestic heating.
In terms of heating output, firewood can be divided into:
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