I am in the middle of designing/ building a brick built BBQ and am struggling to understand the dynamics of chimney design. The overall design looks like this...
The red areas are all brick/ block/ concrete. The silver areas are stainless steel and the yellow area is the gap in the chimney where the smoke comes out.
What I am trying to do is to optimize the draw to minimize the amount of smoke hitting the cook (i.e. me!) and also to provide good flow of air across the coals. I guess the variables I have with this design are...
1. Changing cross sectional area of the chimney section
2. Varying the width v's depth of the chimney
3. Varying the height of the chimney (although I have a max height based on the surrounding bushes etc.)
4. Changing the angle of the various slopes
Does anyone have any idea how these factors (or others) impact the draw? E.g. if I reduce the cross sectional area of the chimney and keep everything else the same will this increase or decrease (or even have no effect!) the draw?
Any help gratefully appreciated.
thanks in advance
In any fireplace, the draw is most influenced by the aperture. ie keep
the chimney entrance as close to the coals as you can. That's why you
use a sheet of newspaper across the front of the fire when trying to
light it. Fireplace design is not an exact science, there's a lot of
experience required. Copying/scaling an existing design which works can
be a good start. A Google.com search will give some US results.
A solid fuel stove might have a 5-6" diameter liner 7m long which is
what we have (roughly)...
That will draw with very little effort from cold.
So perhaps working on a similar cross section area might be a start?
Thanks very much all. In response to the various questions/ comments..
When you click on the link, if you then click the 3D button you can then sp
in the design around by holding the left button and moving the mouse.
@Capitol - by putting the newspaper in front when lighting, I guess this ef
fectively closes the front of the fire rather than having the effect of mov
ing the chimney nearer the fire. Is this the case? If so, I assume this me
ans that increasing the chimney area would increase the draw?
In terms of measurements, to give you an idea...
Front opening is 1770mm x 770mm
Chimney cross section at minimum (i.e. once the front and side slopes have
reduced it) is 1026mm x 246mm
Angles of slopes is 45 degrees
Height of chimney from where side slopes finish to top is around 1200mm
The only other thing I can think of which may help is that I have knocked i
t up in wood to test it out. Lighting some paper to create the smoke resul
ts in a bit of draw and smoke comes out of the chimney. After a little whi
le (say 10 mins) it now seems to come out of the front as well as the top.
Given it is paper (and not much of it), there was little real heat.
On Tue, 17 Feb 2015 06:01:46 -0800 (PST), firstname.lastname@example.org wrote:
When I opened out an old fireplace in my victorian house I created a
throat at the top of the opening where the fireplace becomes the
chimney, such that there was a smaller opening into the chimney before
it then opened out into the full cross section inside the chimney.
I think that the idea is, that once some heat builds up in the fire,
the hot air/smoke rising up past the throat creates a slightly lower
pressure above it, thus "drawing" the smoke upwards.
More by luck than design it seems to work well once a bit of heat has
been generated in the fire itself.
So with the 2.5 rule, that would imply that I need to double the size of my chimney (if my calcs are correct). This seems very big?
@Davidm - was your throat flat or did it taper up the chimney reducing the hole size and then stopping after a bit so it then went into the full size chimney (if you see what I mean)?
On Tue, 17 Feb 2015 09:32:35 -0800 (PST), email@example.com wrote:
See here for sketch of the cross section of the fireplace:
the throat goes the full width of the fireplace, and is only across
the back. If I was doing it again I'd do something similar on each
side as well. It has to go up into the chimney opening a short way, so
that the "low pressure" area forms inside the chimney, to draw the
smoke up into it.
It's just fashioned out of brick, smoothed over with cement render.
Is this worth it and going to work as it's for a BBQ?
At the point of using the BBQ to cook, there isn't much smoke - except
from a bit of fat burning off maybe. and it's isn't really burning like
a fire does, I doubt it would have enough draft to carry up the smoke
that is produced
And to have enough space to cook, ISTM there will be such a large
opening so far above the BBQ that it's not going to work that
@Chris, you raise an interesting point. You are right, in essence the main
use of the chimney is at the beginning when you fire it up but after that
is limited. In my case I need something to ensure any flames etc. don't se
t fire to the surrounding bushes. I guess the other aspect is ... there is
not really much of an issue with smoke hitting the cook now and again eith
@Michael - on your recommendation, I saw the Argentinian program last night
on iPlayer. It gave me an idea... I could bring the cooking surface up hi
gher and reduce the distance to the bottom of the chimney. The important t
hing for me was the head height so I don't keep hitting my head on the chim
ney. I may try this at the weekend.
I also have found some interesting information
This website seems to say that the rule is 1:8 for chimney area to fire are
Based on this, I would need to reduce the width by about 40% (to 246mm x 66
Also, building regs seems to suggest 15% as the optimal but it does say tha
t for larger fireplaces, you need to consult and expert (or clearly this gr
oup :) )
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