NOVEMBER 2012
page 9
www.jayline.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 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 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 checks or cracks in the end grain as a sign of dry wood. The stacks of firewood should be in an
open area so that air can circulate through them. 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.
