Rumford Technical Discussion
How Tall Should the Chimney Be?
Code requires that a chimney be three feet above the roof where it exits and two feet higher than any part of the roof within ten feet. Rumfords draw at least as well as any other naturally drafting appliance so there are no special rules for Rumfords.

Obviously, taller is better because a taller chimney provides a greater accumulated pressure differential due to the taller column of warm air inside the flue, each cubic foot of which is lighter than a cubic foot of cooler ambient air. Add them up. A 20 foot tall chimney draws twice as well as a ten foot tall chimney.

That implies that what we're really talking about is a pressure differential, or, in other words, relative differences in pressure which can be affected by many factors. For example, a tall chimney, if it's not as tall as the tallest part of the house, may have trouble competing with the house - a leak or an open skylight in the tall part of the house may make the house draw better than the chimney and may induce a downdraft in the chimney. Sometimes you can compensate for a short chimney by overcompensating with make-up air. Blow enough outside air in the house to positively pressurize the house.

A related problem is that many new homes are very tightly constructed with inadequate provision for ventilation make-up air. A home with radiant heat, a large commercial kitchen fan, a couple of bath fans and a fireplace may be so de-pressurized that none of the appliances work very well.

So, some practical "rule of thumb" answers:

1) Build the chimney as tall as you can. Use a chimney pot to make it even taller without adding to a top-heavy looking mass. Most fireplace books assume 15 feet from hearth to top of chimney is sort of a minimum. Old houses tended to have taller chimneys (because they worked better) than modern houses. See the Historical Chimney Gallery

2) Build the chimney taller than the tallest part of the house. Even if the chimney is on a one story section of the house and meets code in that it is ten feet away from the tallest part of the house, build the chimmney taller than the tallest part of the roof.

3) Balance the ventilation system - bring as much tempered air into the house as is being leaked or pumped out. We have many more comments on balancing the ventilation system on our website at http://www.rumford.com/ventilation.html and linked to http://www.rumford.com/technical.html

Bottom line, the height of the chimney is only one factor. A tall chimney can overcome or draw even in a fairly de-pressurized house while a short chimney may be made to draw by pressurizing the house.

Discusion
To ask technical questions or contribute to the discussion click here

Mr Buckley,

I'm trying to find out if there are rules of thumb for how high a chimney needs to be to provide adequate draw for a Rumford fireplace. I am working on a single story building that has quite low roofs, and a couple fireplaces with 3x3 firebox openings. I would appreciate your thoughts if you have a moment.

Thank you very much,

Jeff Boone
Prentiss Architects

See also examples of smoky fireplaces because the chimney was too short.

[Back to Technical Discussion]
Buckley Rumford Fireplace Home Page
Copyright 1996 - 2010 Jim Buckley
All rights reserved.
webmaster