Does an outdoor fireplace need a flue?

[Jason New Mexico]

I started building my outdoor fireplace before I knew what I was doing. I can't say I know a lot more now, but here's my question. I am building a cement block fireplace with a 27" x 21" opening, with a planned chimney height of 10 feet above the opening of the fireplace. The internal dimensions above the opening (where the firebrick currently stops) are about 32" x 16". Do I need to narrow that chimney down and put in a flue to get proper draw, or given the height of the chimney, am I okay just to build it straight up and not worry about a flue or smoke shelf? I am planning on putting a crown/spark arrestor on the top.

We live in Montana, and this is not going to be a major source of warmth for our back patio. Our plan is to stucco or use some other colored cement plaster to finish the outside (not rock). It is intentionally going to look somewhat monolithic and modern, so I have no aesthetic reason to narrow the chimney. I'd like to keep it simple if possible.

Thank you for your expertise and suggestions.

 

[begreen]

Recent discussion.

Outdoor fireplace - flue height

Hi all, Im about halfway done building an outdoor brick fireplace with chimney. I have 6 feet of 13x13 clay flue liner that i am planning to use to get some draft. The opening is roughly 26x32. Is that enough height and a big enough flue to get enought draft so the smoke wont billow out of the...

www.hearth.com

 

[bholler]

I started building my outdoor fireplace before I knew what I was doing. I can't say I know a lot more now, but here's my question. I am building a cement block fireplace with a 27" x 21" opening, with a planned chimney height of 10 feet above the opening of the fireplace. The internal dimensions above the opening (where the firebrick currently stops) are about 32" x 16". Do I need to narrow that chimney down and put in a flue to get proper draw, or given the height of the chimney, am I okay just to build it straight up and not worry about a flue or smoke shelf? I am planning on putting a crown/spark arrestor on the top.

We live in Montana, and this is not going to be a major source of warmth for our back patio. Our plan is to stucco or use some other colored cement plaster to finish the outside (not rock). It is intentionally going to look somewhat monolithic and modern, so I have no aesthetic reason to narrow the chimney. I'd like to keep it simple if possible.

Thank you for your expertise and suggestions.

Yes you need a flue. No you don't need a damper or smoke shelf. The flue is the passage that the exhaust travels through from the smoke chamber out the top of the chimney

 

[begreen]

Do you mean, are flue tiles needed? Possibly not, but they will assist draft. However, too large a flue (that big 32" x 24") is not going to draw well, potentially causing smoke spillage out the front instead of going always up the chimney. Just a slight wind can cause this. Think of how hard it would be to drink water from a bucket with a 2" straw vs a 1/4" one. This is where the ratio of opening to the flue area comes into play. A smoke shelf isn't a necessity, but it will also help when it's windier.

 

[bholler]

Do you mean, are flue tiles needed? Possibly not, but they will assist draft. However, too large a flue (that big 32" x 24") is not going to draw well, potentially causing smoke spillage out the front instead of going always up the chimney. Just a slight wind can cause this. Think of how hard it would be to drink water from a bucket with a 2" straw vs a 1/4" one. This is where the ratio of opening to the flue area comes into play. A smoke shelf isn't a necessity, but it will also help when it's windier.

Smoke shelves serve no functional purpose anymore. If anything they hurt performance by creating excess turbulence

 

[begreen]

Smoke shelves serve no functional purpose anymore. If anything they hurt performance by creating excess turbulence

I've read the opposite, maybe that could be due to the narrowing there? It's my understanding that a properly designed one helps create a venturi to increase flow, but I am not a mason. Sounds like if there is no smoke shelf then a narrowing is still recommended.

 

[bholler]

I've read the opposite, maybe that could be due to the narrowing there? It's my understanding that a properly designed one helps create a venturi to increase flow, but I am not a mason. Sounds like if there is no smoke shelf then a narrowing is still recommended.

Yes narrowing is good, a flat shelf after that narrowing creates tons of turbulence. The shelf is a hold over from the days chimneys were cleaned by hand off of ladders that would be set there.

 

[alivegas99]

Not sure I can add too much more than these guys have but love to see a fellow outdoor fireplace DIY’er.

Mine has a 32x26 opening and using a 12” ID clay flue will be going up to 10 feet high (7.5 feet from top of opening to top of flue). it looks like it should probably work on paper but will test it before I top it off.... may add another foot.

I’d be wary of not placing a clay flue for the reason begreen said. I put my hands in dry mortar and clapped in the firebox with the top piece of clay flue attached and could see the airflow pulling the particles upward. I doubt this test is sufficient to tell you the draft is good enough but it should probably tell you if it isn’t good enough without having to light a fire in your unfinished fireplace.