Help with vent floor size

[gjbingham]

I'm stumped with the amount of space I need to leave for the floor of the vent. I read (and reread) James' sticky posting about size of the hearth.

I'm planning a 36 inch oven. The formula James posted translates to :
36"(floor)+9 (walls)+6 (vent floor)+5 (rear insulation)+.5 (one wall)+12 (landing)= 68.5".

After looking at all photos I could find of the vents and floors, it looks like the majority of the floors are 12" to 18" under their brick arches vs the 6 inches used in the formula. Am I misinterpreting the 12 inch oven landing portion?

I'm getting ready to pour the vermiculite concrete portion of the hearth on a corner installation and want to make sure that I get enough coverage under this area., but not necessarily cover unnecessary areas as Vermiculte is hard to find around here.

Can somebody please turn on the lights for me?

Many thanks

[biondoli]

George,
according to James plans the vent should be only 4" not 6"...here my plan for a corner oven with a dome of 37.5" internal...hope the picture would be visible...any input/suggestion from anybody? I am planning to do the foundation this week end if the weather will allow it...thanks!
Carlo

[dmun]

Did you do a cross-section, which would be diagonal in this case? Remember that the dome is a hemisphere, and will be smaller at the level of the vent, giving you more room. A four inch vent slot seems really small to me.

[james]

As David notes, you have vertical space, where the vent can go up and a little back if you need the space. If you go 4"x8" with the vent opening, I think it will be OK. Double check my high school math, but if 4"X8" is 32 sq in, and 4"x6" is 24 sq in, and a 6" pipe is 3.14*9=28 sq inches, you should be OK.

Also, do any of our engineers have a good description of how the physics of the vent work. My layman understanding is that the heat of the vent creates a low pressure area that effectively sucks air up. Does the idea of a more narrow vent opening in front of the chimnney stack create a more powerful draw?
James

[asudavew]

Originally Posted by james

My layman understanding is that the heat of the vent creates a low pressure area that effectively sucks air up.
James

I wasn't sure how this worked ... so I looked it up.

Here is a quote from this link:
The Chimney


"Chimneys operate on the principle that hot air rises above cold air – thus, the hot gas in a chimney rises because it is less dense than the air outside the house. The rising gas creates a pressure difference called draft, which draws combustion air into the appliance and expels the exhaust gas outside through the chimney. The hotter the gas compared with the air outside, the stronger the draft."

That helped me some.


Dave

[james]

OK. High school science. :-)

The electrons get excited and spread out, making hot air lighter by volume than cold air, so it rises. It's the rising air that creates the pressure difference.

Cool. Thanks Dave.

James

[Dannyboyblue]

That's very interesting and useful, Dave.
Since the draft of the chimney pulls in combustion air it would seem that the size of the arch and door relative to the chimney would also be important and again in relation to the oven size.

would it be fair to say that the vent/chimney works as a pump?

[asudavew]

I guess so.

The relationship is very important.

To big a chimney... to much draft.. hard-to-heat oven.

To small.. I guess not enough draft... hard-to-heat oven.

etc. etc. door size... 63%.etc.

Thank goodness lots of others have already built and shed some light on the subject for the good of all of us following in their footsteps.

[asudavew]

Originally Posted by james

The electrons get excited
James

Electrons are cool..... or hot I guess in this case!

[wlively]

Originally Posted by james

As David notes, you have vertical space, where the vent can go up and a little back if you need the space. If you go 4"x8" with the vent opening, I think it will be OK. Double check my high school math, but if 4"X8" is 32 sq in, and 4"x6" is 24 sq in, and a 6" pipe is 3.14*9=28 sq inches, you should be OK.

James

James's math is correct. If you are going with 6" pipe then you should go with the 4"x8". You want the chimney entrance to be larger than the area of the flue. This is to take advantage of Bernoulli's law, increasing speed of gas decreases pressure, this will increase the pressure drop of your vent creating a better draw.