A Better Chimney, LLC - Safety inspections, chimney sweeps & skilled chimney repair.
Eight Reasons Why Smoke
Enters Your Home:

(The main reason why smoke enters you home is a matter of air-pressure phsyics covered by the section above: "How & Why Chimney's Draft."  Here are eight additional factors to keep in mind):

1)  (The most obvious):   Upon closer inspection, perhaps the damper is actually closed, and not open?  Use a flashlight and poke your into the hearth enough that you can see the damper assembly. 

Damper assemblies are mechanical, and mechanical devices malfunction--this is the only way to tell for certain that a damper is open properly.

2)  Perhaps the damper is only partially openThis could allow some, but not all, of the smoke to exit through the flue.

3)  Is your chimney flue excessively dirty? Creosote deposits can cause a partial obstruction in the flue, just like plaque can clog a human artery.

4)  Do you have a cap? Caps can prevent sudden down-shears that can literally push smoke down a flue and back into the house. 

Caps also serve to interfere with direct wind, creating turbulence that permits smoke to escape the flue during high winds that would otherwise act as a lid at the top of the chimney.

5)  Is your chimney tall enough? In order to draft properly, a chimney flue must extend up at least 10 or 12 feet from the hearth.

It is extremely difficult to reliably control the draft of fireplaces with flues that are too short, since they are so susceptible to wind gusts and sudden pressure changes.

Caps not only block out rain, but also interfere with heavy
winds that otherwise would create a shear effect, acting like a lid and therefore keeping smoke from getting out.

6)  Is your flue large enough for the fireplace opening? The rule of thumb is that the surface area of the fireplace opening (width multiplied by height) can be no more than 10 times the area of the flue opening at the top (the ratio is 12:1 for rounded flues, including metal liners).

Flue opening that are too small will not able to handle all the smoke, which will instead follow the path of least resistance and go back into the house.

Because there is no practical way to make the flue opening at the top larger, the only realistic option is to make the hearth opening smaller by installing metal smoke guards (a strip of metal placed on the top of the hearth that reduces the size of the hearth opening).

7)  Is your chimney on the outside of the house? If you have a masonry chimney that is located on the outside of the house, anytime it is cold outside the air inside of the chimney flue will be as well.

Because cold air is dense, it will tend to sink down the flue, rather than rise up, as warm air does.  In general, the temperature of the air inside the flue should be relatively warm before efficient drafting can begin (ideally 200 degrees Fahrenheit or more).

For this reason, outside-the-house chimneys can make it a challenge to start a fire, and may cause some sporadic back-drafting until the fire heats up, and then once again when the fire cools down.

A time-honored trick to get a fire drafting well quickly is to light some rolled up newspaper, and hold it near the flue opening (or even extend it directly into the flue, perhaps with the thongs that come with your fire place tools).  This will heat up the air in the flue enough to jump-start a draft.

8)  Is your home too tight? Modern homes have increasingly been made airtight (with gasket triple-seal windows, millimeter-precise door fittings, and modern insulation).

That is fabulous for energy conservation, but it makes it much harder for a chimney or wood stove to quickly replace the air that drafts up the chimney during a fire. 

The result? Fireplaces will burn sluggishly, burn-out early, or cause smoke to enter the living area.

Oftentimes the only option to facilitate good drafting during a fire is to open just a crack any window or door that is as close to the fireplace or wood stove as possible (and which, ideally, faces the wind).

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(Copyright 2011. All rights reserved).