Vermont Casting 1450 Benutzerhandbuch

26

A stove is part of a system which includes the chim-
ney, the operator, the fuel and the home. Each part of 
the system affects how well the stove operates. 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 smoke 
is hotter (and therefore lighter) than the outdoor air 
at the top of the chimney. The larger the temperature 
difference, the stronger the draft. As the smoke rises 
from the chimney it provides 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 depend entirely on the draft 
of the chimney. 
Depending on the features of your installation - (i.e. 
metal or masonry chimney installed, 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 or 
insert, one which restricts the amount of air getting into 
the firebox, the chimney must keep the smoke warm all 
the way to the outdoors. Some chimneys do this better 
than others. Here is a list of features and their effects.

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 smoke 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. 

Most factory-made metal chimneys have a layer of 
insulation around the inner flue. This insulation keeps 
the smoke warm. The insulation is less dense than ma-
sonry, so a steel  chimney warms up more quickly than 
a masonry chimney. Metal doesn’t have the good looks 
of masonry, but it often performs much better. 

Because the chimney must keep the smoke warm, it is 
best to locate it inside the house. The relatively warm 
house then surrounds the chimney keeping it warm. 
This also means that heat from the chimney walls will 
transfer to the house and not be wasted outdoors. An 
indoor chimney will not lose its heat to the outdoors, so 

 

it takes less heat from the stove to heat it up and keep 
it warm.

The interior 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. Exhaust gases move 
more slowly through larger chimneys and can lose 
more heat to the chimney walls. This weakens the draft 
strength. If an oversized flue is also outside the house, 
the heat it absorbs gets transferred to the outdoor air 
and the flue is further cooled.
It’s 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 diam-
eter as the stove or insert’s flue outlet; the liner keeps 
the exhaust warm, and the result is a stronger draft. An 
uninsulated liner is a second choice - the liner keeps 
the exhaust path restricted to its original size, but the 
air around the liner must still be heated. This makes the 
warm-up process take longer.

Every turn the exhaust must take as it travels to the 
chimney top will slow it down. The ideal pipe and chim-
ney layout is straight up from the stove, and into a verti-
cal chimney. If you are starting from scratch, use this 
layout if possible. If the stovepipe must turn to enter a 
chimney, locate the thimble about midway between the 
stove top and the ceiling. This achieves several goals: 
it allows the exhaust gases to speed up before turning, 
it leaves some pipe in the room for heat transfer, and 
gives you long-term flexibility to install a future stove 
without relocating the thimble. 
There should be no more than 8 feet (2.4m) of single-
wall stove pipe between the stove and a chimney; lon-
ger runs can cool the exhaust gases enough to cause 
draft and creosote problems. Use double-wall stove 
pipe for long runs. 

Each ‘airtight’ stove requires its own chimney. If an air-
tight stove is vented to a flue that also serves an open 
fireplace, it is easier for the chimney draft to pull air 
in through those channels than it is to pull air through 
the stove, and performance suffers. Imagine a vacuum 
cleaner with a hole in the hose to see the effect here. 
In some cases the other appliance can even cause a 
negative draft through the airtight, and result in a dan-
gerous draft reversal.