vent sizing

[larrycooper]

I see many people have quite large vent openings ( spanning the entire width of opening and a full brick in depth) and others are considerably smaller ( half of width of opening and only a half brick in depth). Is bigger better or overkill?

[brickie in oz]

Re: vent sizing

Bigger is always better in the flue area so the smoke can get up the flue rather than out the door.

[dmun]

Re: vent sizing

A large transition is helpful, as is a smooth one. I think the rule, as much as there is one, is that the mouth of the funnel should be at least twice the area of the flue pipe.

As Al says, the goal is to get the smoke out of your face on startup.

[Ilkay]

Re: vent sizing

I am going to order duravent for my 36 inch oven. It will be 6 inch diameter but dont know how tall chimney should be. Any recommendations?

[GianniFocaccia]

Re: vent sizing

I think the rule, as much as there is one, is that the mouth of the funnel should be at least twice the area of the flue pipe

This is especially true the flatter your inner arch. A flatter arch will create a wider exhaust pattern than a hemispherical arch will. There are many ovens with flatter arches and no funnel to speak of that exhibit the telltale soot on the decorative arch.

Study the entryway arch/funnel geometries and corresponding soot signatures on the ovens in the FB photo gallery and you'll get the idea. Even better, watch the exhaust characteristics of Ken's fine oven below, and you'll see how far down the arch the smoke exits and once it does, how effective the shape of the funnel accelerates the draw.

Pompeii Oven Vent Draw - YouTube

[Pompeii Nate]

Re: vent sizing

John,

That video was great. The question I thought of while watching it was will the draw change when he puts a longer chimney on it? Is one of the variables, as mentioned before, the fact that he has such a short chimney at that point? Do the chimneys still draw that well when there is a long chimney attached? Since the chimney I am planning will be six to eight feet long I REALLY hope they do. Thoughts?

Nate

[GianniFocaccia]

Re: vent sizing

To a point, a longer flue will create more pressure than a shorter one. Look at dmun's super-long flue that draws beautifully. I'm curious whether or not an oven can benefit from a flue damper that tunes a super-effective flue down so it doesn't suck heat out of the top of the dome.

I have a 48" stovepipe on my 30"w x 19"d smoker. A flue of your proposed dimensions will draw like a champ.

[brickie in oz]

Re: vent sizing

TI'm curious whether or not an oven can benefit from a flue damper that tunes a super-effective flue down so it doesn't suck heat out of the top of the dome.

Thats what we do with ours, once the fire is starting to make coals we partially close off the damper and put the insulated door in place slightly ajar.

There is a sweet spot with the damper where no smoke exits the door and you can see the temp rising up on the thermometer.
The damper is made from insulated fire bricks.

Obviously the more heat stopped from going up the flue the more heat will be absorbed in the bricks, and quicker too.