Osburn 900 Installation and Operation Manual
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8.4.2
WHY THE FLUE SYSTEM SHOULD PENETRATE THE HIGHEST HEATED SPACE
When it is cold outside, the warm air in the house is buoyant so it tends to rise. This tendency of
warm air to rise creates a slight pressure difference in the house. Called ‘stack effect’, it produces
a slightly negative pressure low in the house (relative to outside) and a slightly positive pressure
zone high in the house. If there is no fire burning in a heater connected to a flue system that is
shorter than the warm space inside the house, the slight negative pressure low in the house will
compete against the desired upward flow in the flue system.
There are two reasons why the flue
system in the house at right will cold
downdraught when it is cold outside
and there is no fire burning in the
wood fire. First, the flue system runs
up the outside of the house, so the
air in it is colder and denser than the
warm air in the house. And second,
the flue system is shorter than the
heated space of the house, meaning
the negative pressure low in the
house will pull outside air down the
flue system, through the wood fire
and into the room. Even the finest
wood fire will not work well when
connected to this flue system.
8.5
INSTALLING THE FLUE CONNECTOR
The flue has been called ‘the weak link’ in the safety of wood heating systems because failure to
install the flue connector properly (which has been common in the past) can result in house fires.
The best flue is one that rises straight up from the wood fire to the base of the flue system with no
elbows. Straight assemblies are less likely to cause problems like smoke roll-out when the door is
opened for loading. They are also more stable and easier to maintain than assemblies with elbows.
Horizontal runs of flue should be avoided where possible because they reduce flue system draft.
