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-   -   What am I doing wrong?? Chimney won't draw. (http://www.fornobravo.com/forum/f9/what-am-i-doing-wrong-chimney-18646.html)

Phxdt 11-24-2012 08:56 AM

What am I doing wrong?? Chimney won't draw.
 
2 Attachment(s)
First of all, it is a high of 81 degrees in Phoenix today. Not sure if I should brag or complain!

I installed a vent that starts out 12.5 x 4.5 inches wide at the bottom, then going to 9.5 x 4.5 at the transition to the flue. The flue is 12x 8 interior, and then transitions to a smaller and smaller diameter- ending up in a 4 inch diameter vent 42" tall in total (trying to get the smoke up and out).

About 50% of the smoke is still coming out of the front (instead of up the flue). I see the potential problems as:

1. Going from a 9.5x4.5 opening vent to a 12x8 flue creates a high pressure (Bernoulli's law), no chance for it to suck up/out through vent

2. Ending vent of 4 diameter is too small

3. 42" tall stack is too high

4. My entire accomodation for the vent is one brick length (wanted a very shallow transition for easy access), and that is too shallow

5. My exit is 1" higher than the exit from the oven, and it makes it too easy for smoke to exit there.

Pictures attached (I put the wood form infront to force the smoke out when taking this pic). One one is correct? How do I fix??
:confused:

Phxdt 11-24-2012 09:03 AM

Re: What am I doing wrong?? Chimney won't draw.
 
Or, am I just too green here? The flue was cold, and its not a cured oven?

Faith In Virginia 11-24-2012 09:20 AM

Re: What am I doing wrong?? Chimney won't draw.
 
Hay Phxdt,

From what you have stated I would think you have two issues. The first and foremost is the necking down of the pipe to 4". If you can I would go 8" all the way up.

The second and not that big of a deal is the short transition. Any over flow will look for the path of least resistance and will come out the front. So I'm thinking once you increase the size of your pipe that may greatly reduce the issue with the short transition.

So start with the pipe and go from there.

deejayoh 11-24-2012 09:52 AM

Re: What am I doing wrong?? Chimney won't draw.
 
I'm with Faith's guidance. You don't say how big your oven is, but even the smallest ovens typically need at least a 6" chimney to draw well. I have a 39" oven and use an 8" chimney. If you do some searching, I think there is a formula in one of the threads.

On your height hypothesis - your chimney would need to be taller, not shorter in order to get it to draw with such a small diameter pipe. Taller chimney can compensate for smaller pipe.

As an experiment, pull the pipe off above where you have the blue tape. I bet it will draw just fine once the flue heats up.

Edit: checked your build thread - see that you have a 42" oven. You will need an 8" chimney

Phxdt 11-24-2012 10:17 AM

Re: What am I doing wrong?? Chimney won't draw.
 
Thx for the advice. My math must have been wrong when I did the calcs. I pulled off the spot above the blue tape, and now it is about 80%/20% up through the flue vs. out of front.

I assume that once it cures, that ratio should improve. Thx for the quick response! 10 months of work and near the finished line.

deejayoh 11-24-2012 11:27 AM

Re: What am I doing wrong?? Chimney won't draw.
 
Quote:

Originally Posted by Phxdt (Post 142149)
I assume that once it cures, that ratio should improve. Thx for the quick response! 10 months of work and near the finished line.

What I find is that the oven really starts to draw well once the flue gets up to temperature. Some people "prime" the flue by starting the fire in their landing, or holding lit newspaper under it.

I don't know that curing should make that much of a difference. I guess having a hotter fire matters.

Phxdt 11-24-2012 12:04 PM

Re: What am I doing wrong?? Chimney won't draw.
 
perfect, thanks.

david s 11-26-2012 04:02 AM

Re: What am I doing wrong?? Chimney won't draw.
 
What is your ovens internal diam? A 42" oven requires an 8" diam. flue. My oven is only 21" diam. and has a 5" diam flue so whatever is in between could be a guide for you. The cross sectional area of the flue needs to be about 15% of the oven door entry area,
Also all new or wet or moist ovens will be more smoky than a well dried one so now you've increased the pipe diam try keeping firing it and you should notice an improvement. And ovens with shallow entries need flues with good draw.

jab49 12-03-2012 03:01 PM

Re: What am I doing wrong?? Chimney won't draw.
 
I'm just starting the curing process in my 42" oven, which has 8" pipe all the way, and the chimney is 6ft long, and when the fire is hot, she draws like a beauty, but when the fire is going out, and the oven, not being cured is still not very hot, it starts to come out the front. I hope to see more consistent behaviour once it's cured, but the fact that it draws well when it's at its hottest is an indication of what you can expect. My oven opening is quite wide too, so when it's windy, wind blows straight in there, upsetting the flow and cooling things down when I only have a small curing fire going. By the end of the week, I hope to report a nicely venting oven. Good luck!


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