Vermont Castings • Defiant Flexburn 1975 Installation Manual_R1 • 2021 - ___ • 08/21
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8390-035i
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Chimneys & Venting
A. Venting & Draft Management
A stove is part of a system, which includes the chimney, the
operator, the fuel, and the home. The other parts of the system
will affect how well the stove works. When there is a good
match between all the parts, the system works well.
Wood stove or insert operation depends on natural (unforced)
draft. Natural draft occurs when the exhaust is hotter (and
therefore lighter) than the outdoor air at the top of the chimney.
The bigger the temperature difference, the stronger the draft.
As the hot gases rise through the chimney they provide suction
or ‘draw’ that pulls air into the stove for combustion. A slow, lazy
fire with the stove’s air inlets fully open indicates a weak draft.
A brisk fire, supported only by air entering the stove through
the normal inlets, indicates a good draft. The stove’s air inlets
are passive; they regulate how much air can enter the stove,
but they don’t move air into it.
Depending on the features of your installation - steel or
masonry chimney, inside or outside the house, matched to the
stove’s outlet or oversized - your system may warm up quickly,
or it may take a while to warm up and operate well. With an
‘airtight’ stove, one which restricts the amount of air getting
into the firebox, the chimney must keep the stove’s exhaust
warm all the way to the outdoors in order for the stove to work
well. Some chimneys do this better than others. Here’s a list
of features and their effects.
Masonry Chimney
Masonry is a traditional material for chimneys, but it can
perform poorly when it serves an ‘airtight’ stove. Masonry is a
very effective ‘heat sink’ - it absorbs a lot of heat. It can cool
the chimney gases enough to diminish draft. The bigger the
chimney, the longer it takes to warm up. It’s often very difficult to
warm up an outdoor masonry chimney, especially an oversized
one, and keep it warm enough to maintain an adequate draft.
Steel Chimney
Most factory-made steel chimneys have a layer of insulation
around the inner flue. This insulation keeps the chimney warm.
The insulation is less dense than masonry, so a steel chimney
warms up more quickly than a masonry chimney. Steel doesn’t
have the good looks of masonry, but it performs much better.
Indoor/Outdoor Location
Because the chimney must keep the smoke warm, it’s best to
locate it inside the house. This uses the house as insulation
for the flue and allows some heat release into the home. An
indoor chimney won’t lose its heat to the outdoors, so it takes
less heat from the stove to heat it up and keep it warm.
Single Venting
Each ‘airtight’ stove requires its own flue. If an airtight stove is
vented to a flue that also serves an open fireplace, or a leakier
stove, it’s easier for the chimney draft to pull air in through
those channels and performance of the stove suffers. Imagine
a vacuum cleaner with a hole in the hose to understand the
effect here. In some cases the other appliance can even cause
a negative draft through the stove, and result in a dangerous
draft reversal.
Chimney Height
The common wisdom tells us that a taller flue draws better
than a short one. A rule of thumb for minimum height states
that the total system height (from the floor the appliance is
mounted on to the top of the chimney) should never be less
than 4.8 m (16 ft.). Most normal installations exceed this
height, but installations in cottages with shallow-pitch roofs
may not. Don’t make a chimney taller unless you must in
order to meet the safety rules, or unless there’s some nearby
feature causing a downdraft. Even then, there are downdraft-
preventing chimney caps available, which are probably the
smarter choice.
Flue Sizing
The inside size of a chimney for an ‘airtight’ stove should
match the size of the stove’s flue outlet. When a chimney
serves an airtight stove, more is not better; in fact, it can be a
disadvantage. Hot gases lose heat faster as they travel slower
through a chimney; if we vent a stove with a six-inch flue collar
(28 square inch area) into a 10 x 10" flue, the gases slow to
one third their original speed. This allows the gases to cool
more rapids, which weakens draft strength. If an oversized flue
is also outside the house, the heat it absorbs gets transferred
to the outdoor air and the flue usually stays cool.
It is common for a masonry flue, especially one serving a
fireplace, to be oversized for the stove. It can take quite a while
to warm up such a flue, and the results can be disappointing.
The best solution to an oversized flue is an insulated steel
chimney liner, the same diameter as the stove or inserts flue
outlet; the liner keeps the exhaust warm, and the result is a
stronger draft. An non-insulated liner is a second choice - the
liner keeps the exhaust restricted to its original size, but the
hot gases still must warm up the air around the liner. This
makes the warm-up process take longer.
Pipe & Chimney Layout
Every turn the exhaust must take as it travels to the chimney
top will slow it down. The ideal pipe and chimney layout is to
vent vertically into a completely straight and vertical chimney.
If you are starting from scratch, use this layout if possible.
If the stovepipe must elbow to enter a chimney, locate the
thimble about midway between the stove top and the ceiling.
This achieves several goals: it allows the gases to speed up
before they must turn, it leaves some pipe in the room for
heat transfer, and it gives you long-term flexibility for installing
a different stove without relocating the thimble.
There should be no more than eight feet of single-wall stove
pipe between the stove and a chimney; longer runs can cool
the exhaust enough to cause draft and creosote problems.
With prefabricated chimney, bring it down to six to eight feet
from the stove. With a masonry chimney, arrange the pipe
so that it turns into the chimney within eight feet of the stove.
