# sizing chimney for outdoor fireplace.



## dom-mas

I'm building my first out door fireplace. I've got most of the smoke chamber built (the gather for stuart45) and I'm wondering about how to size the chimney. I'm presently at the size required for an indoor fireplace so I threw in a couple cement bags just to see how it would draw. It wasn't great. Not terrible but a fair bit of smoke was coming from the face. i know I don't have the opening closed in at all, and I have to go up another 2' or so of chimney and it was windy so all those things effect draw, but I'm wondering if I'm making a mistake in using the same size chimney outdoors as indoors. 

Any help appreciated

The other thing is that I didn't roll the back forward at all, hopefully that isn't a problem either. i did tell the homeowners that because of wind it won't draw as well as an indoor FP and to expect some smoke. They're fine with that, but I would still like it to be as pleasant as possible


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## JBM

Ive built a couple 36" opening outdoor fireplaces, and I build them just like I build an indoor one. Nice tall throat, smoke shelf, 12x12 flue. they draw fine. I put a lemance damper on the top, would make sense to put a slab of bluestone and maybe no damper at all really. Both of these were 30ish feet tall though.

Rolling the back forward doesnt enhance the draw of the chimney in my experience. It is designed like that to give room for a shelf, which in turn might be a performance increase, other then that it has no other function. Build a 28" chimney and leave the back straight, it would work fine ill bet.


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## dom-mas

JBM said:


> . Build a 28" chimney and leave the back straight, it would work fine ill bet.



Whoa 30'!!!! Mine will be 8' +/- I'm kind of building it on the fly. I didn't roll the back so no smoke shelf. No damper either, I was worried about rusting. But I'm going to put a raised cap on it. Are you saying 28" sq or high. The FP is 36" x +/- 30" the chimney as it stands now is 16"x12", it looks pretty big to me.


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## Tscarborough

Roughly 10% of the opening size, err on bigger for a short stack.


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## dom-mas

Tscarborough said:


> Roughly 10% of the opening size, err on bigger for a short stack.


yeah that makes sense, i think I'll bring it down to 12"x12", maybe 11"x11" I'm just corbeling so i have a lot of flexiblity


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## stonecutter

dom-mas said:


> Whoa 30'!!!!


I was thinking the same thing but I'm betting it was attached to the house Dom.


If not SuperSeal would have been anxious to see the scaffolding set-up


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## JBM

lol not there was a fireplace on the outside of the chimney going up the side of the house.


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## stonecutter

JBM said:


> Nice tall throat, smoke shelf, 12x12 flue.
> 
> Build a 28" chimney and leave the back straight, it would work fine ill bet.


Me too....outdoor fireplaces function differently in my experience. I have built them a couple ways, a few like indoor fireplaces and a few that were more of a straight chase. The only time a shelf seemed to help was when it was really windy but it didnt really matter because the smoke exiting the chimney was getting blown down into the seating area anyway.


How about some pics Dom?:thumbsup:


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## JBM

dom-mas said:


> Whoa 30'!!!! Mine will be 8' +/- I'm kind of building it on the fly. I didn't roll the back so no smoke shelf. No damper either, I was worried about rusting. But I'm going to put a raised cap on it. Are you saying 28" sq or high. The FP is 36" x +/- 30" the chimney as it stands now is 16"x12", it looks pretty big to me.


Well no, I mention 24" or 28" chimney as the size of the chimney on the outside of the house. Add the fireplace and it is larger then that.

The chimney is 16x12? with a flue inside?


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## dom-mas

Oh i see, 24 or 28 being the outside of the chimney including facing

no flue liner . I'm not going to use one. I'm corbelling brick then i'll parge the inside with a heatstop50 mix. The 12x 16 looked really big. i don't know where i got the idea for that size. I may have just been measuring as i went up and that # sounded good.

I'll take some pics tomorrow and post them. I should be able to finish the chimney, get the hearth laid and start the facing tomorrow.


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## greg24k

Keep in mind flue diameter has a certain usable area, which is measured in SQ IN on some drawings that area will be referred to as cross section. Take 8 1/2 X 13" flu it has only 70 SQ IN, 13 X 13 has 127 and 13 X 18 has 173. etc The reason for the cross section is when smoke rises it moves in a circular motion and only certain area of a square flue will be used to its full capacity. This is why in most masonry fireplaces they use square flue because it takes longer to create a buildup and you don't have to clean the chimney very often. 
If you use a larger diameter flu you can't go wrong, and you can always control the intake with a damper.

Here is from the code book, just so you get an idea how to size them in the future. Or you can look it up 
2006 IRC 

R1003.15.1Square chimney flues shall have a minimum net cross-sectional area of 1/10 of the fireplace opening. Rectangular chimney flues with an aspect ratio less than 2 to 1 shall have a minimum net cross-sectional area of 1/10 of the fireplace opening. Rectangular chimney flues with an aspect ratio of 2 to 1 or more shall have a minimum net cross-sectional area of 1/8 of the fireplace opening.


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## dom-mas

greg24k said:


> Keep in mind flue diameter has a certain usable area, which is measured in SQ IN on some drawings that area will be referred to as cross section. Take 8 1/2 X 13" flu it has only 70 SQ IN, 13 X 13 has 127 and 13 X 18 has 173. etc The reason for the cross section is when smoke rises it moves in a circular motion and only certain area of a square flue will be used to its full capacity. This is why in most masonry fireplaces they use square flue because it takes longer to create a buildup and you don't have to clean the chimney very often.
> If you use a larger diameter flu you can't go wrong, and you can always control the intake with a damper.
> 
> Here is from the code book, just so you get an idea how to size them in the future. Or you can look it up
> 2006 IRC
> 
> 
> 
> 
> 
> R1003.15.1Square chimney flues shall have a minimum net cross-sectional area of 1/10 of the fireplace opening. Rectangular chimney flues with an aspect ratio less than 2 to 1 shall have a minimum net cross-sectional area of 1/10 of the fireplace opening. Rectangular chimney flues with an aspect ratio of 2 to 1 or more shall have a minimum net cross-sectional area of 1/8 of the fireplace opening.


I do understand the code,(why i had a different # in mind i don't know) but I'm fairly certain that it applies to indoor fireplaces only. An outdoor fireplace has totally different circumstances, the main ones being height of the chimney and the face being exposed to wind.

Also while the code seems to be giving minimum sizes, there are maximum sizes as well. too large a flue does not draw well at all, and this is what I'm probably experiencing


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## dom-mas

Looking at my code book, 12x12 is the min for a FP of this size and 12x16 so I guess I did read that right. Again for interior. Maybe i should stick with what I have and just go up. Or maybe I'll just stick with hearth and facing tomorrow, get the opening figured out and see if it draws any better after that.


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## Tscarborough

If you are in the 13x13 OD range, you should be good. Note that square and rectangular flues are normally measured OD, while round flues are measured ID. 

What I have found from troubleshooting outdoor fireplaces is that the main problem is the depth of the fireplace. Rumfords are not good for outdoor fireplaces. Deep fireplaces with low openings and wide shoulders (masonry to each side of the opening) seem to work best. Next is the stack height which is usually problematical, and should be dealt with at the design stage. Normal box height of 5' plus a 3 or 4 foot stack is not going to draw well unless is A) deep B) has wide shoulders and is oriented for the prevailing wind C) has a good cap/spark arrestor.

I have seen no correlation between damper/no damper or smoke shelve/no smoke shelve.


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## Captainsls

I do same as JBM and build them as an interior, but I make the firebox deeper than usual. A 30x30 would be 16" or so deep... but ill make it 20".

_edit- Tscar beat me too it!_


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## dom-mas

yeah, this ones pretty deep, 5 courses of firebrick @ 4.5" so 22.5" deep, maybe I'm wrong though and it's only 4 courses deep. the firebox is 4' high or so with a 2' smoke chamber then I was planning on a 2' chimney. heightening the chimney is possible but i don't want it to be taller than the pool house. And orientation was decided by HO, it's incorporated into the fencing for the pool.

when I was doing some napkin drawings to design it those were the #'s that looked good to me. I later searched the net for plans and came across Forno bravo's outdoor FP which had almost the exact same dimensions as i came up with which is why i stuck with it


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## JBM

A 4' tall firebox is pretty tall, with a 12x16 flue I would keep the lintle height around 32-36".


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## dom-mas

JBM said:


> A 4' tall firebox is pretty tall, with a 12x16 flue I would keep the lintle height around 32-36".


31.5 will work with the (manufactured) stone i'm using, which is about where I was thinking of putting it. I could go 3.5" higher or lower. Not sure if I'll go with a lintel or arch yet.


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## dom-mas

Here you go stonecutter. 










Closed it in and it draws much better. And I was wrong, the chimney size is 10x 16, I think I'll leave it as is.


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## stonecutter

Better late than never Dom! Looks cool..like a modified Rumford.:thumbsup:


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## dom-mas

thanks! It's been a bit different. Not having a damper or rolled area really threw me a curve ball in how to make the smoke chamber. I'm on the home strectch now. Hopefully the material holds out. It's leftover from the last house I did, luckily it was the same product that the HO has on his pool house. However it's running out and my guesstimate is that I'll have 0.0001 stones leftover (meaning i may have to dig into the scrap pile to finish)


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## stonecutter

Trow and Holden should make a stone stretcher for occasions like this.:laughing:


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## JBM

dom-mas said:


> Here you go stonecutter.
> 
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> 
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> 
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> 
> 
> Closed it in and it draws much better. And I was wrong, the chimney size is 10x 16, I think I'll leave it as is.


The flue size, the chimney is the entire chimney. Looks good though. Would look good if you did a 5' tall taper above the mantle :thumbsup:


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## dom-mas

Got mostly rained out today, but i did manage to get the mantle on and a few stones on top. 

Definitly not enough material to taper up 5', it will taper +/- 2' with a 2' chimney above that. The chimney outside dimensions will be 32x32. It will be a bit bulky i think but it is what it is. it should look pretty solid anyway.


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## dom-mas

HA, guy showed up from the city today to look at the HO's pool enclosure. Looked at the fireplace and said "whoa, that things epic". I hope so.

PS, anyone looking for a used T&H dallet air hammer with an assortment of chisels, shoot me a PM


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## stonecutter

more pics, more pics!


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## dom-mas

I should finish tomorrow. i'll post some then. It's not that great, I just really liked the "epic" comment. Definitely turned into the word of the day. "how was your sandwich?" "Oh man it was EPIC" and on it went


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## fjn

*flues*

Not that any concerns apply to this particular fireplace,the flue is plenty large enough. One thing i did want to mention,the 10%rule for flue to box size is correct.The one thing that may need a bit more imput is as follows. The size of a square or rectangle flue can not be counted as it computes. One must draw a circle in those flues and only count the area in that circle or oval. The rest (corners ) are dead air space and can not be counted.


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## dom-mas

Thanks FJN, I think Greg24 already touched on that point. Anyone know where the max size comes from (ratio)? I remember hearing that 10% rule but I've always checked my charts for flue sizes in the past. I also remember being told that if there are different sized flues available that both meet code, choose the smaller one as it will perform better. Large flues are lazy, smaller flues add to the venturi effect that really helps to create draught. in this case though I see Tscarborough's view that because of the short chimney that volume is desireable.


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## Tinstaafl

dom-mas said:


> I should finish tomorrow. i'll post some then.


While I love seeing pics of these creations, could you also snap a couple to show how it fits into the whole back yard layout?

I'm an avid camper, as are most of my friends--and all of our fire "places" are set up so people can enjoy them from any direction. I need a better grasp of how the other crust lives. :jester:


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## dom-mas

Tinstaafl said:


> While I love seeing pics of these creations, could you also snap a couple to show how it fits into the whole back yard layout?
> 
> I'm an avid camper, as are most of my friends--and all of our fire "places" are set up so people can enjoy them from any direction. I need a better grasp of how the other crust lives. :jester:


Sure, but it's no great shakes. Not how I would have designed it anyway. But it makes me no never mind, i don't have to live there


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## raskolnikov

d m,
My experience has been that a rectangular flue versus a square flue is always hands down the winner. The more the rectangular the better. Give me an 8 1/2" by 18" flue instead of a 13" by 13" and I know that box will hum. Maybe I should say HUMMMMM!

D.


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## stonecutter

dom-mas said:


> I should finish tomorrow. i'll post some then.


Thanks, that would be epic.


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## dom-mas

one for stonecutter









and one for tinstafaal









and a few of a little guy who really seemed to enjoy listening to the oldies with us

























He didn't mind the mixer but he took off once the saw got started

Ring necked Pheasants aren't really native around here. Climate is just a little too harsh, but about 20 years ago there was a wild game farm that had a break in the fence and about 100 of the critters got away. There's still a few around the area.


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## stonecutter

Really nice work there Dom. I think that chimney would look even better with a nice chimney pot on top.


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## dom-mas

Yeah I need to put one on. I'm not sure what to do though. The chimney is 31.5x31.5. I can't find a stone big enough to cover the whole area. I'm thinking concrete, but I'm not the best former out there, but maybe this is the place for the hand pressed technique. Whatever I use it's going to be heavy. And my labia popped a rib playing soccer last week so he's not 100%


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## dom-mas

Oh and thanks for the compliment


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## stonecutter

What about a cast in place cap and then stain it? Or get some sheet copper and get creative.


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## dom-mas

cast in place is what I'm thinking. Like I say, I'm not the best at forming but the few chimney caps I've done have turned out OK. Sheet cooper? I've already been at this job 2 days too long. I'm not going to put any real money into this cap.


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## Tinstaafl

dom-mas said:


> He didn't mind the mixer but he took off once the saw got started


That's funny. Last fall, I was banging out the mixer at the end of the day, and a buck and two does showed up, close enough to throw a rock at. It was almost like they came to see what all the commotion was about.

Fireplace is looking good. :thumbsup: I agree it needs a cap.

And yeah, a good old fire ring wouldn't work too well in that space.


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## dom-mas

Tinstaafl said:


> That's funny. Last fall, I was banging out the mixer at the end of the day, and a buck and two does showed up, close enough to throw a rock at. It was almost like they came to see what all the commotion was about.


This guy liked the oldies, i swear. The beachboys came on 2x while he was hanging out and he looked over and made a call both times. What a weird call, it sounds like someone with emphysema blowing across the top of a bottle.


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## dom-mas

I've decided tom pour a cap. It will be 2.5" thick with a slight crown. Any of you concrete guys have a good mix for something that thin. I'm thinking 2 bags premix concrete to 1 bag sand mix to make it less likely to honeycomb. Some mesh in there for sure

Yea? Nay? Any other ideas


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## Tscarborough

Lath and Maximiser if it is available.


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## dom-mas

Maximiser?

googled it. Never seen those bags before. We get Sakrete but not maximiser. Sounds like good stuff.


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## superseal

Glad to see your finally posting some pics Dom- told ya it was easy...looks great, really does and I like the stonework. You should put some "epic" raised ribbon work on that puppy and you'd really make my day. 

Pour your cap a little thicker and through in some poly fiber, hang it over two inches min. - don't forget to form in a drip relief and isolate the flue. Caulk it upon cure to seal the deal.

Keep up with the pics :clap: I'm lovin' it


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## dom-mas

superseal said:


> Glad to see your finally posting some pics Dom- told ya it was easy...looks great, really does and I like the stonework. You should put some "epic" raised ribbon work on that puppy and you'd really make my day.
> 
> Pour your cap a little thicker and through in some poly fiber, hang it over two inches min. - don't forget to form in a drip relief and isolate the flue. Caulk it upon cure to seal the deal.
> 
> Keep up with the pics :clap: I'm lovin' it


Thanks SS. Bead joint is not in the works for this one. I;ve spent more than enough time on it already and with the speed that joint goes at I;d be quite a while longer. I did do some bead joint on the building that i was doing some brick cleaning at. I;ll be there to finish that job tomorrow (try pressure washing but they said they really don't care that much) so I'll take some pics if i rmemeber

The cap is going to be raised above the chimney 8" or so so no flue going through it. Because it won't be directly attached to the chimney i don't think i'll put a drip on either. Wish me luck, I;m no concete guy.


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## stonecutter

dom-mas said:


> The cap is going to be raised above the chimney 8" or so so no flue going through it. Because it won't be directly attached to the chimney i don't think i'll put a drip on either. Wish me luck, I;m no concete guy.


Neither am I. But I made these last year and it works. The best part is they are inexpensive to make.

I used acrylic additive to as a binder and water replacement in a high strength bagged concrete mix from QuickCrete. I also used galvanized mesh because these caps are thin and big....and they had to be set 60ft up. I used a colored stain (3 coats) then 2 coats sealer. I did put a drip line on them too.


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## dom-mas

Nice. How did you get them up there Genie lift? 
So you stained the cap after it was poured? then sealed it? What type of stain do you use? I'm only familiar with adding colour to the mix

About the drip. i just don't see the purpose if the cap is above the chimney. When the water hits the drip and falls it will still hit the chimney.


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## stonecutter

dom-mas said:


> Nice. How did you get them up there Genie lift?
> So you stained the cap after it was poured? then sealed it? What type of stain do you use? I'm only familiar with adding colour to the mix
> 
> About the drip. i just don't see the purpose if the cap is above the chimney. When the water hits the drip and falls it will still hit the chimney.


The drip line is past the face of the chimney. Granted, it will not be beneficial if there is a lot of wind. The main reason I did it is a drip line will stop any water from traveling back under the cap and dripping into the flue. It certainly doesn't hurt to put one on there..and it's easy.

Yes, I stained the cap after it was poured and cured for a month. I had the time on this job. I dont remember the brand name. It was a pretty good product...I almost could get away without sealing. But since I dont want to go up there anytime soon, I sealed them. On your job, I would incorporate color into the mix. 

6oft straight boom for this one...and it was maxed out.


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