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What Makes A Good Chimney
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When you think about installing
a wood stove, your next thought should be about the chimney.
The best, most well built stove, can only perform as well
as the chimney that it’s connected to. They work together
as a system. The chimney drives the system by exhausting
flue gases from the stove and simultaneously pulling fresh
combustion air into the stove. A continuous supply of air
is crucial to maintaining a steady, hot fire. That supply
of air is dependent on the ability of the chimney to exhaust
flue gases as they are created by the combustion occurring
in the stove. Air supply, combustion, and exhaust are all
part of the same balanced process in a well-designed system.
A lazy, smoldering fire, back puffing, sooting, and down
drafting are all symptoms of a poorly designed chimney, or
one that wasn’t intended for the stove it is being used with.
So what makes the system work? And how can you feel confident
that your installation is going to perform well? The good
news is by understanding a few basic principles of draft
and flow, not to mention safety, you will be well on your
way to understanding good chimney design. There are excellent
products on the market to install a pre-fabricated chimney
from scratch, or to adapt an existing chimney or fireplace
to a new wood stove. The design principles are essentially
the same for each. |
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A. Draft and Flow
| The most basic
principle of chimney design is one that we are all familiar
with: hot air rises. In this case we actually mean hot gases.
The greater the temperature difference between the gases
in the chimney and the outside air, the faster the gases
rise. This natural movement of gases up the chimney is draft.
For there to be adequate draft to maintain proper combustion,
a certain volume of gases has to move through the chimney.
This volume of gases is the flow. The stronger the draft,
the greater the flow. The other important design principle
is the suction effect: air (or gas) always moves from a zone
of higher pressure to a zone of lower pressure. As the warm,
buoyant gases exit the stove and move up the chimney (draft),
they create a low-pressure zone, pulling the lower temperature
air near the opening of the stove in behind it. Fresh air
for combustion is drawn into the stove at the same rate that
exhaust flows out of the stove and up the chimney. This is
what makes for a balanced system. |
Hot air rises up the chimney pulling fresh
air into the stove in its wake. |
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| The amount of fresh air being pulled
into the stove depends on how quickly smoke is being pulled
up the chimney. |
B. The Perfect Chimney
| The ideal
layout for a woodstove is one in which the rooms are all
connected to each other so that air can circulate. In this
respect, traditional colonials are ideal for woodstoves.
They often have a central staircase surrounded by four connected
rooms – living
room, dining room, kitchen and sitting room. A ranch house,
on the other hand, presents more of a challenge. The open
area is easy to heat, but the bedrooms can be difficult.
The bedrooms in a ranch-style layout are difficult to heat
because they are usually not connected to each other, and
circulation is poor. |
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| An inside chimney stays
warm and smoke rises quickly out of the stove and up the
chimney. An outside chimney is a cold chimney. The smoke
cools quickly and slows down. This slows down the amount
of air brought into the firebox and makes it harder for the
stove to produce heat in the home. |
| 1. Keep the chimney
inside the house |
| The difference
in temperature (and therefore pressure) between the flue
gases and the outdoors determines draft. By locating the
stove and chimney inside the house you ensure warm exhaust,
resulting in better draft. A cold exterior chimney will not
draw as well and will be subject to down drafting caused
by cold, heavy air working against the warm exhaust. It will
also be subject to heavier creosote build up than an interior
chimney. Creosote is the tarry substance created when warm
smoke condenses on a cold surface. When an exterior chimney
cannot be avoided, it should be located on the gable end
of the home and insulated, either by using insulated chimney
pipe or building an insulated chase (simply, a framed box,
sided to match the house siding) around an existing masonry
chimney. |
| 2. Match the
flue size of the stove |
| The flue is the
opening in the chimney that allows for the passage of exhaust.
The size of the flue is mainly determined by the size of
the flue collar on the stove. Wood stoves are designed and
tested for the flue size that maximizes combustion. A flue
that is too small will constrict the flow. A flue that is
too large will cause a drop in pressure, and therefore a
decrease of flow. Picture water flowing in a stream. When
the stream bed is narrow, the water flows quickly. If the
streambed becomes wider, the water slows down. The same thing
happens to smoke as it flows through a chimney. An oversized
flue allows the smoke to slow down and condense inside the
chimney - resulting in water, creosote, and sluggish draft.
A six or seven inch flue is ideal for our stoves. A chimney
that is either 8” in diameter if round, or 8” x 8” square,
will still provide good draft for our stoves, as well as
for most wood stoves available today. If you are designing
a new chimney, it’s better to go with a round flue. They
create less resistance to flow and are easier to clean. Creosote
tends to build up in corners of rectangular flues. |
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| Venting into a chimney that
is too large allows smoke to cool quickly resulting in creosote,
condensation, and sluggish draft. |
| 3. Give yourself
enough height |
| A tall chimney
performs better than a short one. The taller the column of
warm gases the greater the difference between its pressure
and that of the outdoor air. We recommend a minimum chimney
height of fourteen feet for our stoves. Additionally, all
chimneys must conform to the “3 foot, 2 foot, 10 foot rule”.
This means it must be a minimum of three feet above the roof
on the uphill side of the chimney, and at least two feet
higher than any part of the roof within 10 feet (measured
horizontally). Where possible, the chimney should be located
as close as possible to the highest point in the house. In
some cases, a chimney can be too tall, possibly resulting
in over drafting which, in turn, can cause a fire to burn
too hot. Over drafting can usually be controlled with the
stove damper or a pipe damper, or a combination of the two. |
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| The chimney must extend
a minimum of 3’ above the roofline, and a minimum of 2’ above
anything within a 10’ radius. The minimum height for good
draft is 14’ above the flue exit of the stove. |
| 4. Limit the
bends |
| Resistance can
be caused by elbows, tees, offsets, obstructions, or long
horizontal runs in the chimney. Many chimneys will require
some elbows, tees, or other restrictions, but the best performing
chimneys will have a limit to how many. Generally, a rear-vented
stove should have no more than three elbows, and a top-vented
stove should have no more than two. |
| 5. One per flue,
please |
| There is a lot
of contradictory information regarding using a chimney flue
for more than one appliance, especially if they use different
types of fuel. State and local codes differ on the subject
from one place to another and are always changing and being
updated. It is our recommendation that a wood stove have
its own flue, both for safety and to ensure good draft. |
C.
How Will Your Chimney Measure Up? |
| If by now you’ve
gotten the impression that the ideal chimney is one that
runs straight up from the stove through the center of the
house and out the roof, with no elbows or bends, you’d be
right. However, your house layout or other factors simply
may not allow for “the perfect chimney”. This does not mean
you can’t install a wood stove with a chimney that performs
well. There is latitude in most of the guidelines above.
In fact, there are very few installations that meet all of
the “perfect” characteristics. Our hope is that by understanding
the principles of what makes a good chimney work you can
avoid some obvious mistakes right from the start. For more
information on venting your stove into an existing brick
or stone chimney, check out our article “Masonry Chimneys”.
If you are planning to install a prefabricated metal chimney
you may want to read our article “Prefabricated Chimneys”.
Or give us a call at 1-800-866-4344. We would be happy to
help you plan a safe and effective chimney system. |
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