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Chimneys that were constructed in homes built before the advent of oil and gas-

red central hea ng systems were invariably

constructed within the walls of the home, typically centrally located, and exited the roof at or near the ridge. The principals
that applied then apply today. A chimney is best located within the envelope of the home to keep it warm, and it should be
installed as close to the peak of the roof as possible. If you have a modern

replace that was constructed with the chimney

outside the home, exposed to the cold, and with a rela vely short chimney, it may not be your best choice for use with a
modern, e

ﬃ

cient wood heater. As central hea ng became prevalent,

replaces and their chimneys were moved outside the

walls of the home to conserve

oor space. If they didn't dra well it was not considered a major problem because the central

hea ng system kept the home warm.

The Environmental Protec on Agency de

nes dra this way: ‘‘Dra is the force which moves air from the appliance

up through the chimney." Simply stated, heated air (wood heater exhaust or any heated gas for that ma er) becomes more
buoyant and tends to rise. The ho er the gasses contained in a chimney

ue are, compared to the temperature of the air

outside, the greater the dra should be. There are many variables and the scien

c explana on can become complicated.

(There are books and scien

c papers wri en for those who would like to know more.) Su

ﬃ

ce to say that you need it. With-

out adequate dra your wood heater cannot burn e

ﬃ

ciently nor vent smoke properly. The dra of the chimney provides the

suc on that pulls air into the wood heater to support combus on. A wood heater provided with adequate dra cannot leak
smoke, as air is being drawn into the heater, connector pipe, and chimney. If a wood heater is leaking smoke or spills a lot of
smoke during refueling, inadequate dra is most likely the culprit.

A complete explana on of all of the elements that may a

ﬀ

ect dra are beyond the scope of this manual but, brie

y, some

factors that contribute to a properly dra ing chimney are these:

• F

It is recommended that the

ue size of the chimney be the same size as the

ue outlet on the heater. Theo-

re cally a larger chimney

ue should have a greater

ow capacity than a smaller

ue. In prac ce however, the heated

ue gases are diluted by the extra air in the larger

ue and cool faster. Dra is decreased therefore and, if cooled too

much, creosote may be deposited in the

ue. The Model 2500 has an eight inch (8")

ue collar and 8" factory-built HT

chimney is the preferred op on and is readily available. An exis ng masonry chimney can be more challenging, de-
pending on the

ue size. If the exis ng masonry chimney has a single eight inch (8") square masonry

ue le, it is only

slightly larger than the 8"

ue of the Model 2500, and can be an acceptable op on, assuming that the masonry chimney

provides adequate dra . (The cross sec onal area of an 8" round

ue is 50¼ sq. in. {Area =

or A =

or A = 3.14

X 16) where Pi = 3.1416} The cross sec onal area of an 8" square masonry

ue le is @ 64 sq. in.) However, masonry

chimneys commonly have 8" x 12" (@96 sq. in.) or even 12" x 12" (@144 sq. in.)

ue les, which would be two (2X) to

three mes (3X) larger than the Model 2500

ue outlet, and may dra poorly. In addi on, masonry construc on tends

to cool more quickly with cold outside temperatures. If a new masonry chimney is being constructed, ask the mason to
source 8" round

ue les. They are available and can be insulated with vermiculite or other approved chimney insula-

on materials. The performance of an exis ng masonry chimney may be improved signi

cantly with the installa on of

an insulated chimney liner that is the same size as the

ue outlet of the wood heater.

• I

A chimney

ue that is kept warm is more likely to dra well than a cold chimney. Keeping the chimney

within the walls of the home is the best solu  on if space allows. If the chimney must be constructed outside the home,
then insula  ng the chimney is strongly recommended. Factory-built HT chimneys are packed with insula  ng mate-
rial and can be further protected by installing them inside an insulated chimney chase (enclosure) outside the home.
Masonry chimneys may require an insulated

ue liner to provide adequate dra in all seasons. Consider that when it

is cold (20 degrees for example) outside the home, the air inside the

ue of an outdoor chimney will be essen ally the

same temperature. When ligh ng the

re it will be necessary to provide enough heat to overcome that column of cold

air ac ng like a plug in the chimney

ue. Even a er the wood heater has been ignited and is burning, a cold chimney

can s ll have a nega ve e

ﬀ

ect on dra . Hot

ue gasses cool more quickly in a chimney exposed to the elements; per-

haps to the point were the dra becomes inadequate to maintain an e

ﬃ

cient

re.

• H

A tall chimney is more likely to dra well than a short chimney. The taller column of heated gas creates a

greater temperature and pressure di

ﬀ

eren al with the outside atmosphere and be er dra is the result. As above, the

ue size and insula on also play signi

cant roles in the process. A minimum chimney height of @ ten feet (10') and

a maximum height of @ thirty feet (30'), measured from the top of the wood heater, is recommended. Loca ng the
chimney where it will project above the ridge of the roof, rather than near the eaves of the home, will net the tallest
chimney height prac cal for your home. External elements like tall trees, hills, tall buildings, and the like can a

ﬀ

ect dra

when the wind is blowing from a par  cular direc  on. Loca  ng a short chimney at the eaves of the home may result in
dra   problems when the wind blows across your own roof ridge. Height alone is not the only factor to consider when
specifying a chimney but, in general, taller is be  er.