Dear Michal, (and Stovers)
I am sending this reply and your message (below) to the 500+ readers of
the Stoves Listserv. Any replies from them need to be sent to you also
at Michal Usowicz <[email protected]> because you are not
subscribed to the Stoves Listserv.
About chimneys, I am not qualified to respond. My only comment is to
question the size (and probable expense) of what you are discussing.
Dr TLUD
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 3/23/2013 7:23 AM, Michal Usowicz wrote:
Dr Anderson,
I hope that everything is going wonderfully with you and that your
TLUDs are still burning!I recently got to Madagascar for Peace Corps
service and things have been progressing well.My host family for
pre-service training here cooks with wood inside but they are planning
to build a chimney and I would like to help them construct a good
one.Large diameter (about 3-4 ft) brick chimneys similar to what you
may expect to see in US homes are common in the community that my host
family is in and I think a brick chimney will be the best solution to
ventilating the home in this context.
A large diameter, natural draft, brick chimney seems like a straight
forward problem but I wanted to make sure that I was not making any
mistakes.I thought you may be a good person to ask even though I know
this isn’t your exact specialty.I expect the chimney will be about
15-20 ft tall and at least the hearth will have a rectangular
cross-sectional area of 3.5x4 ft^2 since that it the current size of
my host mother’s fireplace (although I am unsure if my host family
will construct the chimney with a taper).It will primarily be made out
of clay bricks and mortar.Could you please give me some guidance on
the following questions?
1.Is there a good rule of thumb for the minimum chimney height to have
good draw from the hearth?The only way that I have to estimate this is
using the chimney effect equation (ie flow is proportional to Area,
height^0.5) and I found that for practical chimney heights for my
situation (ie 15-20 ft) there was not a significant change in the flow
rate of the chimney.However, for the equation to apply the chimney
needs to be adiabatic, which I think is a bad assumption in this
case.Do you think 15 ft will be tall enough or does it need to be taller?
2.I do not understand the effects of geometry on chimney draw and was
curious if there is a problem with necking down the chimney
significantly after the hearth?I suspect that my host family may copy
the design of a neighbor’s chimney (since they are hiring the same
mason to build theirs) which tapers down asymmetrically (ie the left
wall is slanted and the right is straight) after the hearth from about
2x3.5 ft^2 at the base to about 2x1 ft^2 over about 6 ft.
3.Along the lines of the previous question, I am concerned that the
rain cap for the chimneys may choke the flow.The rain caps of the
chimneys here is a thin slab of concrete set at the top of the chimney
with 1-3 bricks at each corner.Do I need to pay attention to the size
of the gaps under the rain cap?
4.I know if you build the chimney shorter than the apex of the house
the chimney effect of the house may overwhelm the effect of the
chimney.Do you have any intuition or advice about the height
difference between the chimney and the roof apex that is needed to
avoid this?
5.I feel intuitively that it would be a good idea to minimize the
height of the hearth entrance to capture the smoke plume before it
spreads but I am not sure if this matters if there is good draw in the
chimney.Do you have an opinion on this?
6.Have you ever heard of Rumsford chimneys?I had a construction
engineer friend of mine build one in his house and he said that the
draw from the chimney was excellent.I was just curious if you had
heard of it and if you had an opinion of it.
Again I hope that everything is going well with you.I would really
appreciate any advice you may be able to give.
Sincerely,
Michal Usowicz
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