# Outdoor fireplace sizing



## Fine Edge (Jul 13, 2013)

We just finished a nice outdoor fireplace for a client. We've built approx 15 with no problems but now the client is being a little picky at the end.

Dimensions are:
Firebox width: 5'
Firebox height: 42"
Depth: 28"
Flue liner size: 13x18
We opened up the flue liner at the top to accommodate the 3 chimney pots he bought and they are all functioning properly. 
The problem is that there is smoke that's rolling out the firebox as the fire starts to burn down. When it's a fully engulfed fire, there's no problem.
I tend to believe that with a firebox that wide, it's almost inevitable to get smoke roll as the fire subsides. 
He thinks the dimensions are wrong so he wants to have us shrink the firebox size a few inches. Would this help?
Got a lot of money tied up in this project so I want some professional opinions. 
Will post pics soon.
Thanks 
Dave
Fine Edge Design


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## Fine Edge (Jul 13, 2013)

*Fireplace pic*

Here's a decent picture showing the firebox in question.


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## artisanstone (Nov 27, 2007)

Flue size is wrong, should be 18x18 or bigger. That chimney is also extremely short, I have a hard time imagining that there is room for a flue at all. 

Take a picture up the throat so we can see what's going on in there.

Just as a general rule, flue size should be 1/10 of fireplace opening. This is measured at I.D. of flue. Also outdoor fireplaces are subject to more/different forces than indoor ones. I would suggest a general disclaimer about this when selling.


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## bytor (Jan 23, 2010)

I would agree with artisanstone on all counts. 

I would add that splitting the flow into the three chimney pots will be causing a heck of a lot of turbulence which will dramatically affact draw. 

This idea was explored somewhere I can't put my
finger on. Essentially the conclusion was that for a given size of flue, the size of each of the split flues would have to be about as big as the single flue for the 'draw' to overcome the added turbulence.


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## Tscarborough (Feb 25, 2006)

All the dimensions are good, except for the flue size. I see no remedy short of rebuilding it.


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## JBM (Mar 31, 2011)

How much of that arch is below the top of the firebox? If its less then 8" you will get smoke rolling out. 

The chimney is a bit short and while technically the flue is too small people built fireplaces for 30 years with an 8x12 and even some of them worked ok. If you have a tall throat that throttles the smoke up the chimney it will work ok. 

I suspect you have a few issues. The throat is short, the firebox doesnt go up beyond the face enough, the chimney is a tad short and the slant on the back wall might be slightly too much. Although a narrow opening on the top of the firebox is perfectly fine if all the other elements are there to support it. I would also rather a straight back firebox on outdoor chimneys and have the taper behind the face to narrow the opening down. But its tit for tat and either way they work fine.

Also for what its worth a couple slabs of bluestone would go a long way on those slopes. Could even put a bluestone band at the transistion and another one for the cap stone, all rock edged of coarse.


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## JBM (Mar 31, 2011)

bytor said:


> I would agree with artisanstone on all counts.
> 
> I would add that splitting the flow into the three chimney pots will be causing a heck of a lot of turbulence which will dramatically affact draw.
> 
> ...


Yes I would agree with that. Good point.


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## brickhook (May 8, 2012)

I'm not trying to kick you while you're down, but I agree with artisanstone. The chimney looks to short to do everything you need to do. There's not enough room for a properly built throat 'and' liners.


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## JBM (Mar 31, 2011)

Does anyone have a picture of the graph that shows the height of the chimney in relation to the size of the flue size?


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## bytor (Jan 23, 2010)

Page six of this article;

http://www.whysanity.net/richardstone/CHIMSIZING.PDF


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## Fine Edge (Jul 13, 2013)

Thanks for the quick replies.
I'm borderline on the top of arch to damper dimension so I was wondering if I could lower the arch and bring the sides in a couple inches. 
Not trying to get out on the cheap here but before I rip this thing down, I want to make sure I try everything else that would be easier to rule out.

It really doesn't roll out that bad, especially when there's a good fire. We lit it yesterday to test and couldn't get it to roll at all except when there was barely any flame and the logs were smoldering. 
Of course today when the client was home, it was a little windier and he had raised up the grate to simulate a smaller firebox height ( I didn't question him, just went with the flow). Now any coals that formed were 5" below the grate so I'm sure it didn't help to keep the logs burning. 

Thanks for the help!


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## artisanstone (Nov 27, 2007)

You could probably make it work by lowering the arch. Try mocking it up by temporarily putting some metal flashing in the opening.


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## dbrons (Apr 12, 2010)

Excellent advice from artisanstone

Dave

Ps I would drystack some bricks or 4" blocks on the sides as well. If that works OK, if it doesn't then you haven't wasted your time. You never know those pots up top might be the main issue.


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## Fine Edge (Jul 13, 2013)

That's what I was thinking. 
Great advice all, I appreciate it.


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## fjn (Aug 17, 2011)

Just my .02 cents,when sizing a flue you must draw a circle into a square or rectangular flue. The corners are dead air space and do not figure into the size. If you are close with the square or rectangle in reality you will fall short with the dead corners. 


As other said,three flues into one = problems + chimney is too short.


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## brickhook (May 8, 2012)

If I was going to do anything to the chimney, it would be easier to add height to the chimney than to change your arch and size of the firebox. But I'm still courious to how you built the throat.

And another thing, you can't control the outside elements like you can the inside.


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## fjn (Aug 17, 2011)

brickhook said:


> If I was going to do anything to the chimney, it would be easier to add height to the chimney than to change your arch and size of the firebox. But I'm still courious to how you built the throat.
> 
> And another thing, you can't control the outside elements like you can the inside.





This last statement is something I did not consider however there may be much value in it. Depending on atmospheric conditions I can not help but think the draw of the chimney will vary.

Before much is changed in design look at the following; is fireplace surrounded by trees,taller buildings,in a low lying area,on a hill,does the draw diminish as evening approaches,how does it draw on sunny days etc. I think John raised some valid concerns.


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## dbrons (Apr 12, 2010)

Yes, an outside fireplace is susceptible to problems an indoor one never is. The biggest is the wind. Anytime a breeze passes by the firebox opening a negative pressure is placed on the fireplace. It's kinda like how if you drive down the road with your window open, papers can get sucked out.

Dave


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## fjn (Aug 17, 2011)

Just a thought / kinda a question. I understand the logic and sales pitch along with the dynamics of chimney pots. In theory they act much as the venturi of a carburetor,speed up and or increase air flow. My question,is there a min. height a chimney needs to be to receive the benefit they claim to add,also by their very nature they decrease flue size,does anyone know to what extent they are allowed to diminish the 10 % firebox opening to flue size? Thank you.


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## stonecutter (May 13, 2010)

Welcome to CT FE.... you won't find a better group online for collective masonry advice.


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