FIREPLACES AND FLUE DRAW
Fireplaces are no longer used as the primary source for heating homes. Yet there are few things cozier than a cheery fire, especially on a cold winter night. By the same token, there are few things more distressing than a fireplace which doesn't draw thus belching smoke into the home, chasing people out, setting off smoke detectors, and dirtying everything in sight. Why do some draw perfectly and others so poorly?
There are many factors which affect fireplace performance such as:
Ratio of Fireplace Opening to Chimney Flue Size: The area of the flue should be roughly one-twelfth (1/12) the size of the opening.Chimney Height: The taller the better, but at least 3 feet above the roof and 2 feet higher than anything within 10 feet of it. Damper Size and Location: Full width of firebox and at least 6 inches above the top of the opening. The damper is usually closer to the front of the fireplace than the back. Smoke Chamber Slope and Smoothness: The chamber above the damper should be as smooth as possible, and should slope no more than 45 degrees as it funnels the smoke from the damper opening into the chimney.
Most fireplaces break at least some of the rules of good design and yet many work well despite this. Fireplace design is more of an art than a science. Because there are so many factors which affect the draw, it is impossible to know how "perfect" the unit has to be to work.
What about solving the problem of a fireplace that doesn't draw well? There are several things you can do, and they are presented here in no particular order. The unique situation of your fireplace will dictate which idea(s) to choose. You can reduce the fireplace opening size. This can be achieved by laying an additional row of firebrick on the floor of the firebox. Even before this is done, the solution can be simulated by holding a piece of metal over part of the opening and watching to see if the draft improves.
Second, you can expand the chimney.
This is expensive but often successful.
Less expensive alternatives include a
rain cap or a metal draft hood which
rotates with the wind so that smoke is
always released downwind. Third, you
can move the fire back. Often the fire is
simply too close to the front of the
firebox. Fourth, add air. A fireplace
which is starved for air won't work
properly. Sometimes opening a window
in the room with the fireplace will supply
enough air. Fireplace draw is more
difficult to achieve if the house is under
negative pressure. Don't have exhaust
fans on while trying to start a fire. Most
furnaces also work like exhaust fans. It
is easier to start a fire when the furnace
is in an OFF cycle. Glass doors help to
protect the fireplace from negative
pressure effects in the house, especially
if combustion air can be brought in from
outside. Fifth, try warming the flue. Push
a burning piece of rolled-up newspaper
up past the damper. This will help over-
come the column of cold air in the
chimney and allow a good draft to be
established quickly. Finally, we suggest
expensive damper or smoke chamber
modifications as a last resorts.