JANUARY 2014 page 9www.fisherstoves.co.nz
Correctly installed and operated, Fisher solid fuel heaters do not spill smoke into the house. There are three
main reasons why some woodstoves smoke:
Bad installation design: Incorrect flue installation, resulting in low flue temperatures and low draught, can
make a wood heater more likely to spill smoke. For example, flue systems that run up the outside wall of the
house and are not correctly insulated can rob the heat from the flue and produce very little draught. Each
elbow in the flue pipe assembly slows down the flow of gases and causes a small restriction to flow. When a
flue system includes more than one elbow, the restriction can be enough to cause spillage.
Extreme negative pressure in the house: Energy efficiency practices and new building code rules are
making our houses more and more airtight. This makes the houses energy efficient, but also makes them
more sensitive to negative pressure when air is exhausted from the house. Large, fan-forced exhaust
ventilators, like down-draught-type kitchen stove exhausts, can cause extreme negative pressure in the
house when they are operating. Because new houses are tightly sealed, there are few holes to allow
replacement air to enter, and the house pressure becomes negative. This negative pressure works against
flue system draught. In severe cases, the negative pressure in the house overcomes the flue system draught
and the appliance begins to spill smoke, especially when a fire is started or when it dies down to coals.
To prevent this extreme negative pressure, one option is to open a window slightly to allow combustion air
into the room.
Improper heater firing technique: When a solid fuel heater is starved for air it smoulders, producing a
relatively cool, smoky fire. The temperatures throughout the system are low. During a smouldering fire,
the flue system will not be receiving the hot gas it needs to produce strong draught. When the heater
door is opened, smoke will spill into the room. A smouldering fire is the single most common reason for
smoke spillage and totally unnecessary. By using the suggestions on proper firing technique earlier in this
document, you will be able to avoid these smouldering fires.
PURCHASING THE FIREWOOD
The quality of the firewood you burn can have a dramatic effect on the efficiency and operation of the heater.
The main factors that affect the burning characteristics of firewood are moisture content, tree species and
piece size.
The moisture content of the wood affects the rate at which it burns and the efficiency of combustion. When
trees are cut, the wood moisture content ranges between 35 and 60 percent by weight. If you attempt to burn
wood this wet it will be hard to ignite, slow to burn and will hiss and sizzle in the firebox. So much energy will
be consumed in boiling off the excess water that the efficiency of combustion and the heat to your home will
be low, condensation and corrosion may be occurring in the flue and smoke may be causing problems to
your neighbours. Properly seasoned wood ignites readily and burns efficiently.
PLEASE NOTE: WOOD WITH A MOISTURE CONTENT OF 25% OR LESS IS THE ONLY APPROVED
WOOD TO USE IN CLEAN AIR ZONES.
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