28-09-2012, 11:37 AM
What Makes A Good Chimney
[attachment=34675]
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.
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.
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.
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.
[attachment=34675]
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.
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.
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.
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.