Help understanding air draw and venting?

[cobblerdave]

Re: Help understanding air draw and venting?

Gudday
Most old brick chimneys taper in over the fireplace then expand out over this point to let the smoke expand out and of course suck more smoke in. At the top they again use this method of tapering the end by using a chimney pot to again restrict the smoke and then let it escape into the open air again sucking the smoke behind it.
Used to live in a house with a fire box and single skinned metal flue that exited the wall, behind it. Worst still it was on a southern wall. Try to lite that from cold on a chilly night was interesting. What would happen was the smoke would only get so far up the flue due to it being so cold and of course the fire couldn't breath and the room started to get smokey. If you were really unlucky and the condesation in the cold flue had built up suddenly the smoke would disturb it and you got a stream of black water down the flue that would put you fire out for good. We did't own the house so it was never fixed with a double skinned one ...we just kept a log in it at all times.
Was going to fit a metal chimney to the WFO but decieded to build the widest chimney in brick, have plenty of bushes to cut the wind so I could keep it really low...no soot stains... works for me


Regards Dave

[Amac]

Re: Help understanding air draw and venting?

It's a hollow concrete shell, which supports a flue made of special firebricks.

I'd like to see the "indispensible tool" they used to build that one.

You don't want to suffer claustrophobia or vertigo in that job. Well base jumpimg is not on my list of 100 things to do before I die - but maybe I should add it in as the 101st (and most likely last)

[GianniFocaccia]

Re: Help understanding air draw and venting?

Wotavidone,

The trek to the top via stairs sounds like quite an adventure. Any pics you could share from the outside?

[Amac]

Re: Help understanding air draw and venting?

If you want serious draw over a long chimney, you insulate it to ensure the gas doesn't cool and contract, losing velocity as it travels up the stack. If you can't insulate it, you taper it, so that as the gas contracts from cooling, the velocity is maintained.

Great contribution wotavidone - I wondered what the insulation was for - that explains a lot.

[brickie in oz]

Re: Help understanding air draw and venting?

Im doing a small job for some friends 20kms from home up in the hills and thays all hilbillies up in them thar hills.....

We looked at buying a block of land up there last year with creek frontage, I was going to sit and watch the creek drikin moonshine and playin the banjo.. but my wife didnt like the idea.

[Gulf]

Re: Help understanding air draw and venting?

Man I can just hear the banjos starting up.......

Brickie,
Now that was funny
But, I have been PieAreSquared slam off of this thread! I think I'll just set here on the porch, pick my banjo for a while, listen to the frogs a while and then maybe I'll hear somesome kayakers coming down the crick D

[Faz]

Re: Help understanding air draw and venting?

Draw is powered by a temperature differential that creates a pressure differential. The flue contains a column of air that is hotter then the surrounding air and is therefore less dense. This leads to the air in the flue being at a lower pressure then the surrounding air. This pressure differential is what creates the flow we call draw, higher pressure air will always flow to an area of lower pressure to try and equalize, it's just the nature of things.

A larger volume of air in the flue will lead to more draw. Hotter air in the flue will lead to more draw.

The reason the longer chimney increases the draw is that this pressure differential has an accelerating force on the air in the flue for a longer time/distance - and so the final speed of the gases out of the top of the chimney is greater and therefore the overall flow is greater.

That's part of it anyway!!

[david s]

Re: Help understanding air draw and venting?

More draw equals more oxygen for the fire to feed on. The larger the volume in the flue, the larger the draw. So this can be achieved by making the flue taller or a larger cross section (diam. if flue is round) or both.[/QUOTE]

[sharptailhunter]

Re: Help understanding air draw and venting?

Thanks for the replies. They have been insightful and helpful. That table also has been useful. Thanks again.

[shuboyje]

Re: Help understanding air draw and venting?

Draw is powered by a temperature differential that creates a pressure differential. The flue contains a column of air that is hotter then the surrounding air and is therefore less dense. This leads to the air in the flue being at a lower pressure then the surrounding air. This pressure differential is what creates the flow we call draw, higher pressure air will always flow to an area of lower pressure to try and equalize, it's just the nature of things.

A larger volume of air in the flue will lead to more draw. Hotter air in the flue will lead to more draw.

[Tscarborough]

Re: Help understanding air draw and venting?

Look at the link I provided it, it has the tables. For a given size appliance, a taller chimney allows for a smaller flue, and vice versa. As stated above, however, you do not need ANY flue for the chimney to perform to specifications.

If you choose to use one (which is of course recommended), then you can adjust the size of the flue by adjusting the height of the stack. Or you can simply follow the FB guidelines and be done with it.

[sharptailhunter]

Re: Help understanding air draw and venting?

Ppl tend to over think these things, just build it.

With all due respect, before I drop 2-3k quid on a project that will be sure to absorb a grand amount of my time and will be ever present in my yard, I shall put forth a little thinking and planning so as to maximize my investment and enjoyment.

And correct me if I'm wrong, but I perceive that the Venturi of the oven is in effect related to the inner arch to dome size ratio. As such, I understand the concept of making the inner arch height 63% the height of the dome. What I was trying to understand is how chimney design and size influences airflow through said chimney. It's clear that a round flue is more efficient than a square flue. And it's also clear that a larger diameter of flue will allow for more volume of air flow. What isn't clear to me is how does the height of a chimney effect airflow. An example that I see often in threads on this forum is somebody will write something along the lines of, "with a chimney that tall, it should draw real nice." Yes, I'm paraphrasing but you get the point. Why does a taller chimney draw better?

[sharptailhunter]

Re: Help understanding air draw and venting?

Thanks fornthe help guys. I understand laminar flow, is that one poiseuille's law? Can't remember, been outta school too long already. What I didn't and still don't understand is how a chimney draws air per se. I do understand the logic behind "warming up" the flue with a smaller hot fire before it really gets going. Nonetheless, I'm thinking of using a round 8" stainless flue, length to be determined later, and boxing it off using steel studs and finishing the exterior with hardibacker and rock veneer. Would that be more efficient than using square tera cotta flue?

[Sharkey]

Re: Help understanding air draw and venting?

I most definitely plan on having a vent landing area and chimney. I just don't care for the look of a big chimney flue sticking up from a beautiful oven housing. As such, I just wanna know how small/short I can make it.

I would suggest you make a brick flue and it probably doesn't need to be too high. I have yet to finish my oven but have been cooking in it for some months without any problem. At the moment it only has a flue that is two bricks high and this works pretty well - as you can see in the attached photos. I sometimes get a little bit of smoke out the front on startup when it is windy, but that's about it.

[Tscarborough]

Re: Help understanding air draw and venting?

Chimney And Fireplace Sizing - A Discussion