- mm wrote:

On Sat, 03 Dec 2005 10:55:47 -0500, Mark Cato
wrote:

If your chimney isn't of
sufficient height from the roof-line, this can cause draw issues.

I've read this before, in the first part of a geometry question in
high school**, and Norman raised his hand and asked why.

Mr. Vance didn't give much of an answer, but he seemed annoyed and a
weak later he brought in a book that said what you and the book said,
and made Norman stand in front of the class and read that part of the
book!

But it didn't say why either. So the whole episode just made Mr.
Vance look petty.

So....Why? Why does a tall chimney draw better? Why does going
above the roof line make a difference?


**In the second part, it asked us to calculate the chimney height
based on its shadow, or something.

It's a fair question. There are general guidelines for chimney height,
requiring a chimney to be at least three feet from the roofline, and 2 feet
above anything within 10 feet. While I've largely taken the rule for
granted, I believe the main issue is wind flow and air pressure. If you do
a google search, you can find a variety of sources of information.

The following is from http://www.chimneys.com:

Wind patterns around a house create pressure zones against the roof.
Wind-driven pressure zones are a complicated science, but basically, if the
chimney is not tall enough, a pressure zone created by wind will engulf the
chimney top, forcing wind-driven smoke down the flue and into the house.

This is why current building standards require a minimum chimney height of
three feet above the roof penetration, and two feet higher than anything
within ten feet of the chimney. If your chimney doesn't meet this standard,
have a professional add height to it. (See page 28 for details on the
height rule.)

Sometimes, although the chimney is tall enough compared to the roof, its
overall height isn't adequate to overcome a driving wind. Especially for
short chimneys in one-story homes, adding height above the required minimum
is a good idea if wind is a problem. Ask your chimney professional for
advice.

mark
_____________________________
Mark Cato