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  #11  
Old 05-25-2012, 06:15 AM
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Default Re: Help needed! can't get a good draw

Thanks again everyone - great to know I haven't built a dud!

More follow ups,

Lburou

Couldn't see you pic of a blast door - can you repost? just to double check this sits on the very front of the door - not in between the cooking area and the chimney like a cooking door? Do people have 2 doors ?

Gulf

How do I preheat a chimney?

Thank you guys
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  #12  
Old 05-25-2012, 07:42 AM
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Thumbs up Re: Help needed! can't get a good draw

This is my blast door. This is my insulating door.
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Old 05-25-2012, 02:43 PM
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Default Re: Help needed! can't get a good draw

Quote:
Originally Posted by MarcMunier View Post
pic of a blast door -
Here is mine.
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Old 05-25-2012, 06:06 PM
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Default Re: Help needed! can't get a good draw

The use of a so called "blast door" creates extremely efficient combustion, which leads to a very rapid rise in temp. This rise is likely to exceed 500 C/ hr which is really way to rapid for the safety of any refractory, especially from start up and is likely to cause premature (usually unseen) damage. Refractory materials are not particularly good heat conductors relative to materials like steel for instance. It takes around an hour for the heat to penetrate right through the brick so uneven expansion leads to stress on the refractory. To be kind to your oven it would be prudent to allow the temp to rise at a more reasonable rate, which means not using the blast door.
However in practice we usually want to get the oven up to operating temp as quick as possible. Just be aware that it is probably not doing your refractory much good pushing it this fast.

Last edited by david s; 05-25-2012 at 06:09 PM.
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Old 05-25-2012, 07:36 PM
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Default Re: Help needed! can't get a good draw

Quote:
Originally Posted by MarcMunier View Post

Gulf

How do I preheat a chimney?
I am not disagreeing with any of the info which has been given on this thread. This technique just helps get the most out of some bad situations. IE: windy days, days when smoke seems to lay close to the ground, days when your chimney is colder than the ambient temperature. If the ambient temperature of the outside air is higher than your oven or chimney then the air inside is cooler (heavier). Cooler air in your chimney will fall creating a natural down draft. If you light a fire under some of these conditions the smoke will take the least path of resistance, straight out your door opening which it is already trained to do. That is probably why blast doors work so well.

As far as preheating your chimney goes, it is a pretty straight forward approach and real cheap to prove or disprove. Think of it as training the smoke to go where you want it to. This is assuming that you have dry wood and kindling which are properly stoked in the oven/fireplace. (It definitely works for fireplaces)

In the conditions which I mentioned above, if you light a match and hold it up high in the flu you will notice the flame/smoke being pushed back down. It may take a second match, a lit piece of kindling, or a rolled up news paper to get this draw headed up your chimney. Once you see the smoke going the right way slowly lower your flame to the kindling set in you oven/fireplace. I like to have a little kindling set on top of my wood bundle and once that is going, light the kindling under the stack.
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  #16  
Old 05-25-2012, 08:20 PM
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You are probably getting more info than you want on these issues, but here is some more for you. I think there are some important variables involved in designing and using a blast door. You control the air charge in the oven with the size of the openings in the door, so you can be conservative with the air charge transiting the blast door by design. Secondly, a smaller fire with a blast door isn't as hard on the refractory as a larger fire and a blast door, so you can slow the heat charge that way. It is all under your control. I've controlled these variables with smaller openings in the blast door for the air charge and smaller fires that take about 70 to 90 minutes before getting the heat charge I'm looking for. A fairly conservative management protocol.

David s is correct in saying you can go too far with a big fire and ram air to feed a hot, hot fire. I have a lot of wind directly into the oven entry and do not have much choice unless I want to breathe smoke, which I don not. You pay your money and take your choice.
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Last edited by Lburou; 05-25-2012 at 08:29 PM.
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  #17  
Old 05-28-2012, 04:28 AM
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Default Re: Help needed! can't get a good draw

Quote:
Originally Posted by Lburou View Post
This is my blast door. This is my insulating door.
Can't seem to see the links - can you re-share?
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