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Oven temperature vs Chimney Height

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  • 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?

  • #2
    Re: Oven temperature vs Chimney Height

    Originally posted by mikku View Post
    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 .
    I don't care what folks say behind my back........They are either braggin' or.......lyin'


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    • #3
      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".

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      • #4
        Re: Oven temperature vs Chimney Height

        Originally posted by mikku View Post
        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.
        I don't care what folks say behind my back........They are either braggin' or.......lyin'


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        • #5
          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.

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          • #6
            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.
            Last edited by david s; 02-13-2014, 03:49 AM.
            Kindled with zeal and fired with passion.

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            • #7
              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!

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              • #8
                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.
                Old World Stone & Garden

                Current WFO build - Dry Stone Base & Gothic Vault

                When we build, let us think that we build for ever.
                John Ruskin

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                • #9
                  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.

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                  • #10
                    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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                    • #11
                      Re: Oven temperature vs Chimney Height

                      Originally posted by mikku View Post
                      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?
                      Yes, I agree. I think that's a correct analogy to what's going on, so stopping the airflow is the answer. Being winter and the entering air being very cold may also explain part of the greater heat loss.

                      You should also fix the air leaks in the cab of your truck.
                      Last edited by david s; 02-13-2014, 02:07 PM.
                      Kindled with zeal and fired with passion.

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                      • #12
                        Re: Oven temperature vs Chimney Height

                        Originally posted by mikku View Post
                        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?
                        Consider this....

                        The heat is going to be lost somewhere, unless it is contained. In your illustration, the most likely place will be through uninsulated surfaces, like the windows and windshield. When you are driving, air flow is increased over the most conductive surface, which would be your windshield ( like your door panel ) so, your heat loss is greater because it is being removed from the conductive surface faster. The problem isn't the forward motion, it's the uninsulated surface bleeding the heat out. I don't endorse covering your windshield to stay warmer.

                        The greater vacuum created by your flue isn't the problem, it's your door. I do think a damper may marginally improve the heat loss, simply by restricting the air flowing over the door, but at least to me, why bother when a good door will do the job. Heat that isn't flowing up the flue will be absorbed into the vent masonry, and or lost through the outer opening. You may see improvement with a damper, but not as much as with a very good door.
                        Old World Stone & Garden

                        Current WFO build - Dry Stone Base & Gothic Vault

                        When we build, let us think that we build for ever.
                        John Ruskin

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                        • #13
                          Re: Oven temperature vs Chimney Height

                          If your inner door is uninsulated then the cold draft coming in the entry hitting the very conductive stainless inner door will accelerate the heat loss from it and hence the oven.
                          Kindled with zeal and fired with passion.

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                          • #14
                            Re: Oven temperature vs Chimney Height

                            Originally posted by stonecutter View Post
                            Consider this....

                            The heat is going to be lost somewhere, unless it is contained. In your illustration, the most likely place will be through uninsulated surfaces, like the windows and windshield. When you are driving, air flow is increased over the most conductive surface, which would be your windshield ( like your door panel ) so, your heat loss is greater because it is being removed from the conductive surface faster. The problem isn't the forward motion, it's the uninsulated surface bleeding the heat out. I don't endorse covering your windshield to stay warmer.

                            The greater vacuum created by your flue isn't the problem, it's your door. I do think a damper may marginally improve the heat loss, simply by restricting the air flowing over the door, but at least to me, why bother when a good door will do the job. Heat that isn't flowing up the flue will be absorbed into the vent masonry, and or lost through the outer opening. You may see improvement with a damper, but not as much as with a very good door.
                            Stonecutter, you are missing my point.
                            If the cab is warm in a static position, but cold in a moving position it has all to do with the movement of air.

                            Both conditions have heat loss occurring, however the movement makes the loss more dramatic.

                            I agree about improving the door--Point taken, my door however is insulated with ceramic fiber BUT there are still points where conduction can occur. However, when my oven is hot and the door is in place, I can put my hand onto the stainless steel handle and open and close the door without burning myself... it is warm but not burning hot! So loss is occurring, also the perimeter of the door has woodstove gasket material that forms a seal. There is no lock like a stove might have, but it can be forced tight with a wedge under the tapered handle.

                            Right now, I am in the process of constructing an enclosure that will add from 6 to 15" of loose fill perlite/vermiculite around the oven. Now with the oven supported by beams, I can insulate beneath the oven with up to 8" of additional insulation (anything is OK), finally where the structural slab is exposed, I am adding Styrofoam insulation. Choice of conventional insulation materials are in areas that will never be damaged by oven temperatures.
                            When all is complete, the entire thing will be like a large thermos bottle--only area not super-insulated will be the door.

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                            • #15
                              Re: Oven temperature vs Chimney Height

                              Originally posted by david s View Post
                              Yes, I agree. I think that's a correct analogy to what's going on, so stopping the airflow is the answer. Being winter and the entering air being very cold may also explain part of the greater heat loss.

                              You should also fix the air leaks in the cab of your truck.
                              I'm sure that Stonecutters big Chevy doesn't have any leaks in its' cab! It is too well maintained for that! But the big engine with performance improvements might be the reason for it being cooler when he zooms along the highway--but for the next few days, the ice is going to slow forward motion down a lot. Really hope this crazy weather worldwide changes for the better OR we somehow can reverse all the bad things we have done for the earth to heal itself.

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