Most folks think a chimney simply exhausts smoke from your wood stove to the outside. However it not only removes the smoke, it’s responsible for supplying the combustion air which starts and sustains your fire.
The old adage “nature abhors a vacuum” is true, as the chimney drafts out smoke it creates a vacuum in the firebox that pulls in the needed combustion air. We remember back to science class that nature seeks equilibrium. Higher pressures will flow to areas of lower pressure, in this case the firebox and the heat from the fire will sustain draft because heat (buoyant air) rises.
Typically a taller flue will perform better than a shorter one. Chimney’s for wood burning must be a minimum of 3 feet taller than the roof and 2 feet taller than any part of the building within 10 feet. However, keep in mind that more is not necessarily better. At some point excess height means flues gases spend more time in the flue. More time in the flue means they lose heat reducing draft. Conversely excess chimney height can cause over drafting; over drafting can cause over firing, catalytic element failure and appliance damage.
Getting the chimney right is paramount to wood stove performance
A good venting system can make a bad stove perform well. A bad system can make the best stove worse than worthless, it can make it down right dangerous. Combustion products can spill into the living area including carbon monoxide (CO) – at best wood smoke is a health hazard, at worst CO is a killer. A properly drafting chimney will expel all smoke to the outside atmosphere. Even when you open the stove door, you shouldn’t get any smoke in the home. A properly drafting chimney will keep combustion air going in and smoke going out.