Selecting and Locating a Chimney
A good primer on how to ensure good chimney performance
The evil outside chimney
A fireside chat about chimneys for architects and
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Observations on the Causes and Cure of Smoky
Chimneys Benjamin Franklin 1787
are apt to think that smoke is in its nature and of itself specifically
lighter than air, and rises for the same reason that cork rises in
water.” But Franklin
“. . . smoke is really heavier than air; and that it is carried
upwards only when attached to, or acted upon, by air that is heated, and
thereby rarefied and rendered specifically lighter than the air in its
All about chimneys
The chimney is the engine that drives a wood heat system
No woodburning stove, fireplace or furnace can function properly
without a good chimney. A good chimney is:
- the correct type for the appliance because there are a lot of
options, some unsuitable;
- the correct size for the appliance, which is usually the size of the
appliance outlet collar;
- properly located, meaning up through the heated space of the house;
- properly installed following building code or manufacturer's
A good chimney and system design produces desirable performance
- Fires are easy to light and draft builds quickly
- Smoke does not fill the room when you try to light a fire
- No smoke spillage when you open the door to tend the fire
- No foul odors or cold air from the hearth when it is not in use
When planning a woodburning system, the first thing you need is
reliable advice on matching the appliance to the right type and size of
chimney. Most wood heat retailers and chimney sweeps can guide you
and there may be government agencies and publications you could get
locally. Also, unless you have done it before, we strongly
recommend having your chimney professionally installed by someone whose
references you have checked. You never want to lie awake at night
wondering if an incompetent chimney installation is putting your house
and family at risk.
This is good wood stove performance:
(hint: it's really the chimney that's doing the performing!)
When no fire is burning and you open the stove
door, air flows into the stove, not out.
When you light a kindling fire, the smoke
immediately flows up the chimney, not into the room.
A properly built kindling fire burns bright and hot
When you open the stove door to add more wood,
smoke does not spill out.
If you are careful, you can run the stove so that
you never smell wood smoke in the house.
Understanding how chimneys work
Think of the chimney as the engine that drives the
wood heating system. Think
of its fuel as heat. Think
of the power it puts out as draft.
The more fuel (heat) you give this engine (chimney), the more
power (draft) it will deliver. So,
the hotter the exhaust gases, the more draft is produced.
Draft, by the way, is good.
It's the suction that keeps the smoke from coming into the room.
Insulation in the chimney is important because it helps to keep
the exhaust hot until it is expelled outside, and so, increases draft.
The chimney works with the stove or fireplace in a
kind of feedback loop. Heat in chimney makes draft, which pulls in
more combustion air, which makes the fire burn hotter, which delivers
more heat to the chimney which makes more draft and so on. An
insulated chimney makes more draft with less heat.
In winter, a well-designed and properly installed
chimney makes some draft and flows some air upwards, even when no fire
is burning. When you build
a fire in a stove connected to such a chimney, the kindling ignites
easily, draft increases rapidly and you have a nice bright, hot fire
right away—and no smoking. This
is the kind of system you want in your house.
Understanding how houses work
When it is cold outside, the warm air inside the
house wants to rise, producing a pressure difference: low pressure low
in the house and high pressure high in the house.
The pressure difference is called stack effect.
The colder it is outside, the greater is the temperature
difference, so the stronger is the stack effect.
A chimney installed in the middle of a house naturally overcomes
stack effect by being as warm, but taller than the house.
Houses are being built more tightly sealed for
increased comfort and lower energy costs.
This is done by using doors and windows with gaskets and walls
with a continuous air barrier (usually plastic film).
If you turned on a powerful range hood or downdraft kitchen
exhaust in a relatively small, tightly sealed house, it might suck so
much air out of the house that the pressure inside would fall enough to
overcome chimney draft and suck the smoke out of the stove.
It's not that common yet, but it can happen.
Typical problems and ways to avoid them
1. Cold hearths and odors: when no fire is burning,
cold air and/or odors seep from the stove.
The air in a chimney that runs up the outside of
the house gets chilled, so the draft in the chimney is less than the
stack effect of the house, and the chimney backdrafts, making the hearth
cold and causing unpleasant odors
Install the chimney inside the building, keep it as
warm and as tall as the building and it will make draft, even when no
2. Open door smoke spillage: when you go to reload,
smoke rolls out the door.
When you open the stove door, a lot of dilution air
must flow through the opening to keep the smoke inside; if the exhaust
flow is restricted, smoke will roll out into the room
Go straight up, if possible; avoid 90 degree turns
in the flue pipe and offsets in the chimney
3. Sluggish performance: smokey fire, hard to get a
hot fire burning.
Large, cold chimneys, like old brick ones, suck up
the heat from the exhaust, causing slow draft build up.
the flue to match the stove and use an insulated chimney to keep exhaust
hot and moving quickly; never use and air-cooled chimney
Summary of design guidelines
Put the chimney inside the warm building
Go straight up, no elbows or offsets
Insulation around the flue liner
Flue sized to match stove
A word about climate and altitude
If you live in an area that has a real winter –
the ground freezes and you get some snow – or if you live at high
altitude – say more than 4,000 feet – you'll need to follow these
design guidelines exactly in order to get perfect performance.
You people at low altitude with mild winters may not need to be
quite so fussy, but, then again, good design always pays off in better