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Chimney Entrance Reduction


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I have a question regarding some theory I read in the text The Kiln Book. On page 78 of the 4th Edition, it states that: 
"At the point where the exit flues enter the chimney, they should be restricted so that the chimney cross section is larger than this flue area"
In the diagram (3-11) beneath this, it shows decreasing the point of entry to the chimney by 25% (from 4 bricks to 3), coming from the kiln exit flue.
What's the thought behind this? I'm happy to trust my elders, but I'd also like to know what the theory is. I'm curious how this decrease affects the firing, vs keeping the same area of flow in the inlet flue>exit flue>chimney entry>chimney.  
As I'm building a chimney coming up in the next couple weeks, I'd love to figure this out. I can taper in the walls of the exit flue to the entry point of the chimney, which would accelerate flow heading into the chimney (while also creating slight backpressure?), which would then open back up to the full area of the chimney as it rises. Thoughts?
Thanks in advance!

2018-02-18 10.54.07.jpg

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Interesting question that I don't really know the answer to. Read through the pages a few times and it still doesn't make much sense. He says "the inlet-flue areas must be equal to the exit-flue areas" then below tells you to restrict the exit flue. The only thing I can think is he also mentions "If the exit flues are too restricted, this will slow down the flow and retard combustion efficiency" I guess maybe he is restricting it a bit but not too much to slow down the exiting of gasses without destroying combustion efficiency. 

Edited by High Bridge Pottery
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Flue exits work best if they mimic a Venturi  for the same effects-that is they speed up the flue gas.

In Lou Nils Books the art of firing he outlines this really well and I have used his double Venturi in my salt kiln stack-it has the flue Venturi and another one part way up the stack.

Lots books are more modern thinking than the older Olsen . Both will make your stack draw better but Lots will make it even more so.


The down side is do not make the flue to small-if its to big its easy to add bricks to make it smaller later.

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Yes it is for the Venturi effect, which Nils Lou does spend a lot of time promoting in his book.  However, even after a few readings (of both Olsen and Lou) I was still hazy on the exact benefits of the Venturi effect.  

And, yes, I also found that Olsen’s book has an inconsistency on this issue as mentioned.  Irritating given how much time he spends setting out ratios, etc.  but maybe that is exactly why it is easy to make a mistake?....

Venturi or no, do as advised and make the exit flue as large as the chimney.  THEN,  play with brick set ups in the flue to restrict air flow, create Venturi effects, etc. to your hearts desire.  

May be convenient to build in an access point in the chimney so you can get to the flue area easily, without having to come in thru the main kiln chamber itself. I have found it convenient to be able to look straight into the flue area (and even under the kiln floor) from the back of the chimney during the firing to watch the flame path.

My own experience with an Olsen Fast Fire is that draught is a make or break issue.  Not enough draught and you may well never get to temperature.  make sure the chimney is built plenty tall from the outset.  Too much draught is a high class problem to have.  It is way easier to damp a chimney down during a firing than to try to increase the height of a chimney structure later.  

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  • 2 weeks later...

Thanks for the follow-up. I've done a lot of reading since posting (Fred Olsen's The Kiln Book,  Nils Lou's The Art of Firing , Mel Jacobsen's 21st Century Kilns, and even Kiln Construction by Joe Finch), and have also plugged in numbers from different kilns in the books and those I have access to locally. Unfortunately, there are no consistent ratios here.  I love the idea of the double venturi as described by Nils Lou, but after running the numbers on his MTF kiln (that he uses as an example of a double venturi), I found the area increases after the entrance to the exit flue as follows: 31.5"² @ exit flue >35"² @ base of chimney >41.5"² @ damper >81"²@ chimney >31.64"²@ the reduction at top of chimney/base of stack. The second venturi at the top of the chimney/base of the stack is really clear, but the exit flue into the chimney increases in area, which would decrease pressure and velocity, not demonstrating a venturi effect. 

Here are my numbers. 4 MR-100 burners firing into 3" diameter intake ports give me an intake area of 28.28" sq. I'm designing the exit flue to match that and will be 4.5" tall x 6.28" wide (28.28"sq). From here, Fred Olsen suggested a 25% reduction at the back of the exit flue, which would be consistent with a double venturi design. I'm creating a spot in the back of the exit flue/base of the chimney to insert brick and decrease the flow area. A 25% reduction is about 4.71" wide by 4.5" tall (21.2"sq), opening to my chimney base which will be 9x4.5" wide (40.5"sq). I can decrease again at the top of the chimney heading into the base of the stack to create a second venturi, but I'll figure that out once I get an exit flue and chimney size I feel good about. 

Two examples from research.

A similarly designed downdraft at FireArts fires with 6 MR-750's, and has a combined intake size of 29.46"sq. The exit flue entrance is 42.19, opening to 64.12"sq, and opening again to 76"sq going through the damper. It then opens again to 108"sq above the damper. Not sure on the size of the decrease at the base of the stack.  That means the exit flue is 43% larger than the intake, opening an additional 74% through the exit flue, and even more still through the damper and chimney. Next, we have Vince Pitella's Downdraft Soda Kiln. 3 MR-100's fire through 60"sq of intake. That feeds through a flue  9x7.5" (67.5"sq) and into a chimney 9x9" (81"sq). 

So, despite lots of research, I'm not all that clearer about the science. Thoughts?

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I think that what you're seeing here is that there can be a lot of variability within designs that work. I think that as long as nothing is too small, you'll be fine. The damper cuts it all down anyway. Personally, I've always just made everything about the same, while making the chimney with as few brick cuts as possible. The height of the chimney will have a greater affect on the performance than whether or not the chimney is 10% vs 20% larger than the intake openings.

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Agreed.  It's hard for me to do things like this without clearer understanding. And because I don't build kilns very often, I'm definitely overthinking this. I'm sure it will be fine, I'll do the exit flue in a way that can be reduced, and the chimney such that the damper can control flow. I just get nervous thinking about a 9x4.5" chimney. It seems really small, but at the same time it's a small kiln.  

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I just figured out from the last few posts that you are building/converting a gas kiln.  My experience with regard to these issues (and my comments in this thread) is in the context of wood kilns, so will leave it up to you to judge the relevance.

Here are some actual measurements from an Olsen Fast Fire wood kiln I have worked on and fired many times.  The kiln was originally built with an arched exit flue which was 79% of the area of the firebox entries.  When the flue arch started to sag, a vertical support brick was put in place which reduced the exit flue area to 69% of the firebox entries.  Over time, as the flue sagged all the way down to rest on this brick, the exit flue area was reduced to 55% of the firebox entries. 

However, aside from the sagging arch issues, in the beginning the kiln was perceived to have too much draught.  So, a decision was made to insert more bricks behind the exit flue to further block it.  These bricks had the effect of reducing the effective exit flue to around 25% of the firebox entries.   I am not sure how much calculation or research was done around this modification. ,

For a while (many years), the kiln still fired OK.   (surprises me even now given how small the exit flue was).  However, over time, with each successive firing, the kiln struggled more and more to reach temp (cone 10/11).  It now appears that there were several reasons for this, but the most important in my view was the rusting away and eventual removal of the top 3 feet of the chimney, which was a piece of thick metal stove pipe.

In a recent major overhaul of this kiln, we added back the missing stovepipe at the top of the chimney, adding back about 15% to the height of the chimney.  We also removed the "draught-restricting" bricks behind the exit-flue, increasing the effective size of the exit flue from 25% to around 55% of the firebox entries.  Put another way, we increased the size of the exit flue to just over Olsen's 2:1 ratio for firebox entry area to exit flue area.

This kiln now works a lot better.  Get's comfortably to cone 11 (and cone 13 near the exit flue). And fires faster if desired.  Can't really tell you what the most important reason was, but there you go.  Just some live experience gained the hard way.


What Neil said.  Try not to build anything permanent that you may later find out is too small.   Make it generous, and come back and restrict later if needed.  

Maybe as Mark says the Venturi structures increase gas flow (which I would call draught?).  However, I agree with Neil that extra chimney height from the beginning is a good idea.   Having more than adequate draught on tap if you need it may be useful to compensate for any constriction issues, venturi problems, etc..  You can always knock it off later if you don't want it!  Even if you decide not to make the chimney overly tall for the initial build, design it so that you can come back later if needed and add a couple of feet.

Like you I am worried that your chimney square area is too small.  I would be looking to make it bigger.  Something in the back of my mind about flow friction if a fluid (gas in this case) flows through a pipe too small.    

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I tend to agree with a bigger size chimney interior size  . Also since it was a working updraft the size of your exit should be at least that same size if not a tad bigger that whats on it now as an updraft. Chimney height will make this thing work well so keep that as tall as you can.


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Chimneys can definitely be too short. I have never seen one that is too tall. All the outdoor kilns we had in grad school were required to have chimneys that were taller than the neighboring building, which put them well beyond the height needed when making 'proper' calculations. But through the use of the damper, adjusting flue sizes, and the use of passive dampers in the case of the wood kilns, they always worked just fine.

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Please forgive me for my English. I'm using Google translation. I am engaged in development and introduction of wood and gas furnaces in 2003. I began to build my approach on the fact that in the furnace the chimney has a third meaning in importance. First of all, the heat generation device itself is important. That is, the furnace is a horse. A chimney is a rut from the wheels of the cart or is the ground itself, along which the horse is dragging the cart.
If we start from this position in the design of the furnace, the correctly designed furnace or the "horse" burner will carry the "cart" through any dirt and any rut.
In the world practice of designing and creating furnaces for heating and firing, I observe such an approach that the "horse" is not a furnace, that "horse" is a chimney. This approach is obsolete.
In the video shown, a furnace operating with an overlapped chimney. At the same time, the processes are not quenched. At this point in the chamber 900C. Later in the same cell I achieve 1300C. Using fresh birch firewood. The exit from the combustion chamber into the chimney has an area of 75 cm2.  power 10-20 kWh


Edited by KirillRudde
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  • 2 years later...

Building a downdraft kiln right now.


I think the Venturi effect at the exit flue serves to speed up gas leaving the chimney, creating efficiency, without necessarily removing the kiln heat fast.


If you can imagine, an Eddy of heat convecting it's energy, slowly, like a paper cup circling an Eddy in a river, then when it hits the river, or the Venturi flue, it speeds up and zips away, allowing more energy to enter.

Allows more even heat I reckon.


If you ask me, while there may be new information, some older information is better.

There are more people now who don't care how much gas you use, well we are going back the efficient way aren't we?


Anyway...test test I guess.


Hey @neilestrick Mike from CSC.

Can you get me properly signed up here?

I never got an email back about the problem I encountered, I think I am doing something wrong!



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I built my fiber downdraft using this exact theory.  There are 2 venturis built into the flue box.  One out of the main load chamber and one into the chimney, which is only as tall as the kiln.  The kiln pulls well from about 200F and never shows flame out the top of the chimney.  I use post bricks to fine tune the draw, but the original design works great.

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