Справочник Пользователя для Vermont Casting 1990

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A stove is part of a system, which includes the chimney, 
the operator, the fuel, and the home. The other parts of 
the system will affect how well the stove works. When 
there is a good match between all the parts, the system 
works well. 
Wood stove 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 bigger 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 inlet 
fully open indicates a weak draft. A brisk fire, supported 
only by air entering the stove through the normal inlet, 
indicates a good draft. The stove’s air inlet is passive; 
it regulates how much air can enter the stove, but it 
doesn’t move air into it. 
Depending on the features of your installation - steel or 
masonry chimney, 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, 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’s 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 steel chimneys have a layer of in-
sulation around the inner flue. This insulation keeps the 
smoke warm. The insulation is less dense than mason-
ry, so the inner steel liner warms up more quickly than a 
masonry chimney. Steel doesn’t have the good looks of 
masonry, but it performs much better. 

Because the chimney must keep the smoke warm, it’s 
best to locate it inside the house. This uses the house 
as insulation for the flue and allows a little heat release 
into the home. An indoor chimney won’t lose its heat to 
the outdoors, so it takes less heat from the stove to get 
it warm and keep it warm.

The inside size of a chimney for an ‘airtight’ stove 
should match the size of the stove’s flue outlet. When 
a chimney serves an airtight, more is not better; in fact, 
it can be a disadvantage. Hot gases cool off through 
expansion; if we vent a stove with a six-inch flue collar 
(28 square inch area) into a 10 x 10” flue, the gases 
expand to over three times their original size. This cools 
the gases, which weakens draft strength. If an over-
sized flue is also outside the house, the heat it absorbs 
gets transferred to the outdoor air and the flue usually 
stays cool. 
It’s common for a masonry flue, especially one built for 
a fireplace, to be oversized for an airtight stove. It can 
take quite a while to warm up such a flue, and the re-
sults can be disappointing. The best solution to an over-
sized flue is an insulated steel chimney liner, the same 
diameter as the stove or insert’s flue outlet; the liner 
keeps the smoke at its original volume, and the result is 
a stronger draft. An uninsulated liner is a second choice 
- the liner keeps the smoke restricted to its original size, 
but the smoke still must warm up the air around the 
liner. This makes the warm-up process take longer.

Every turn the smoke must take in its travel from the 
stove to the chimney top will slow it down. The ideal 
pipe and chimney layout is straight up from the stove, 
to a completely straight chimney. If you’re starting 
from scratch, use this layout if possible. If the stove-
pipe must elbow to enter a chimney, locate the thimble 
about midway between the stove top and the ceiling. 
This achieves several goals: it lets the smoke speed up 
before it must turn, it leaves some pipe in the room for 
heat transfer, and it gives you long-term flexibility for 
installing a taller stove without relocating the thimble. 
There should be no more than eight feet of single-wall 
stove pipe between the stove and a chimney; longer 
runs can cool the smoke enough to cause draft and 
creosote problems. Use double-wall stove pipe for long 
runs. 

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