JANUARY 2014
page 9
www.fisherstoves.co.nz
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 fire 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 firing technique:
When a wood fire 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 fire 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 FireWood
The quality of the firewood you burn can have a dramatic effect on the efficiency and operation of the fire.
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 per cent 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.
Firewood should be cut and split in the early spring and stacked under cover, with good ventilation,
to be ready for burning when required.
Look for cracks in the end grain as a sign of dry wood. Firewood should be stacked in an open area so that
air can circulate through it. During the summer, as warm breezes flow through the stacks, carrying away the
evaporating water, the moisture content of the wood will fall to around 20 per cent. At this moisture content
the wood is ready for burning.
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