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-   -   Oven temperature vs Chimney Height (http://www.fornobravo.com/forum/f9/oven-temperature-vs-chimney-height-20388.html)

mikku 02-12-2014 04:59 PM

Oven temperature vs Chimney Height
 
I recently extended the height of my chimney and have noticed that the "day after" temperatures are much lower than before...all other things being equal.

I'm thinking that with the higher stack, cool air gets drawn across the front exposed surface of the oven and causes it to cool more quickly.

Am thinking about installing some type of stack plug to prevent this flow. A little too late for installing a flue damper.

Maybe this should be SOP for all WFO's to prevent this from happening?

Any other thoughts on this topic?

Gulf 02-12-2014 05:49 PM

Re: Oven temperature vs Chimney Height
 
Quote:

Originally Posted by mikku (Post 169442)
I recently extended the height of my chimney and have noticed that the "day after" temperatures are much lower than before...all other things being equal.

I'm thinking that with the higher stack, cool air gets drawn across the front exposed surface of the oven and causes it to cool more quickly.

Am thinking about installing some type of stack plug to prevent this flow. A little too late for installing a flue damper.

Maybe this should be SOP for all WFO's to prevent this from happening?

Any other thoughts on this topic?

I'm really into heat breaks which totally separate the chimney from the oven. But, If you can come up with some type of control to "easily" raise and lower, say a hinged type of cap: I think that would be just about the same thing as a damper :).

mikku 02-12-2014 06:03 PM

Re: Oven temperature vs Chimney Height
 
Haven't come up with a good solution yet. Just noticed the difference in the last couple of firings.

Even with the chimney thermally separated from the oven itself, I'm thinking about the amount of air that passes the oven door as it exits up the stack--something like "wind chill".

Gulf 02-12-2014 06:25 PM

Re: Oven temperature vs Chimney Height
 
Quote:

Originally Posted by mikku (Post 169452)
Haven't come up with a good solution yet. Just noticed the difference in the last couple of firings.

Even with the chimney thermally separated from the oven itself, I'm thinking about the amount of air that passes the oven door as it exits up the stack--something like "wind chill".

Maybe so,
I know that my door (which is insulated) does not have a perfect seal, even when it is placed in tightly to the entry arch.

mikku 02-12-2014 08:01 PM

Re: Oven temperature vs Chimney Height
 
I still have the form section that was used to make the transition to my chimney opening. I'll probably just make another matching plug that I can place into that opening.

The important time to conserve heat is when the oven is left unattended like over night or for a few days. It would be easy enough to just place the plug into the opening and put a prop below it to hold it in place--it would be more effective than a stack damper because it would totally close the opening.

Final steps in insulating the support slab from beneath, building the enclosure for the entire oven and insulating that with loose fill perlite/vermiculite will have a lot to do with the heat retention of the finished oven.

At least those are my thoughts for now.

david s 02-13-2014 02:45 AM

Re: Oven temperature vs Chimney Height
 
I think you are right. You can stop the airflow up the flue pipe either by blocking it at the top or the bottom. A second door on the outside to prevent airflow would be a better option IMO, because it will help to keep the heat that's stored in the entry.

mikku 02-13-2014 03:42 AM

Re: Oven temperature vs Chimney Height
 
This is true, but it all depends upon the seal you can achieve with the outer door. My chimney creates a very strong draft now that it extended above the roofline of the WFO and wood storage structure AND it is not yet and the proper height according to good practices--this would mean another 6 feet of stack!

stonecutter 02-13-2014 11:15 AM

Re: Oven temperature vs Chimney Height
 
A damper wouldn't hurt, but dont you think the real culprit of heat loss is the door design? If heat is leaching through or passing through the door, and then prevented from traveling up the flue by some sort of damper, it is still going to be lost to the oven by being absorbed by the outer masonry or blown out the vent opening.

A damper may slow that heat loss down by reducing air flow across your door, but if the stored heat gets out of the chamber, it is still not really beneficial to the oven....and all a damper will do is prevent the heat from going up into the flue.

mikku 02-13-2014 12:11 PM

Re: Oven temperature vs Chimney Height
 
Stonecutter, Thank you for your input--as always.

I started this little thread because I noticed a significant difference in "morning after" temperature after extending the chimney. There are several things that I am doing now to better insulate the entire oven now that it is perched on its' final resting place. Those things should improve the heat retention of the oven. But the door has remained the same during this period.

I want to solve the initial problem before getting into redesign of the oven door. My oven door design can be greatly improved with the addition of an insulation layer--like a wood panel that would prevent the conductive heat loss through the stainless skin.

Another option would be an entirely different door to be used when the oven is left unattended. But that is "another rat to be skinned" at another time.

mikku 02-13-2014 12:24 PM

Re: Oven temperature vs Chimney Height
 
Another way to look at this is: For example:

You are sitting in your truck with the heater on low on a very cold day but the truck feels quite warm and comfortable.

Now you put your truck in gear and go onto an express highway at 70 mph with the heater set the same...

Immediately you feel chilly because the speed you are driving is cooling your cab more rapidly.

Also, the wind passing over your cab makes all the little air leaks more significant.

??That is the way I think of my chimney stack in this situation?? No?


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