Plugging the chimney for retained-heat cooking

[Fio]

I've been experimenting with retained heat cooking in the oven, and I've noticed the temp drops off pretty fast (I have a dome-style pompeii oven). It seems to me that, in addition to sealing off the door, I could save a lot of heat by blocking off the top of the flue liner used as my chimney. Yet I don't hear of anyone doing that, even among Alan Scott/bread oven users.

Is this done? Any drawbacks (other than trapped smoke, which is a definite plus when cooking meats)?

Thanks.

- Fio

[DrakeRemoray]

When I finally make my door I plan to have it fit the inner opening of the oven, past the vent/flue...When I stacked up bricks as a door, I put them inside the oven opening all the way up so that they hit the bottom lip of the vent.
Drake

[james]

I think Drake has it right. The door should fit snugly across the opening into the oven chamber itself -- not the arch that forms the opening into the oven landing. That way you can cut off the air supply to put out your fire, and completely hold in your heat. That should help. Leaving your door off the oven for retained heat cooking is like leaving your oven open -- we'll it's not that bad, but you get the idea.

I cooked once in an oven where the door closed at the front of the vent area, and when I tried to put the fire out, it kept breathing by drawing air down the chimney. Not a good design.

James

[Fio]

I think Drake has it right. The door should fit snugly across the opening into the oven chamber itself -- not the arch that forms the opening into the oven landing. That way you can cut off the air supply to put out your fire, and completely hold in your heat. That should help. Leaving your door off the oven for retained heat cooking is like leaving your oven open -- we'll it's not that bad, but you get the idea.

I cooked once in an oven where the door closed at the front of the vent area, and when I tried to put the fire out, it kept breathing by drawing air down the chimney. Not a good design.

James

A-HA! I've got to completely re-think my door design. My door currently goes on the outside. I'm going to check to see what can be done to block it off BEFORE the chimney.

I cannot believe I never thought of that. Amazing!

[maver]

you could still plug the chimney, no need to make a new door.

Probably easier to make a new door (or modify) than install a damper after the chimney is complete. I think I've read here that some oven builders have used a damper, but considering the insulation efforts are focused around the dome in the conventional design I wonder how well heat would be retained with a damper and outer door.

[CanuckJim]

Door

Fio,

If you plan to bake bread, you definitely need a door that seals the oven opening. This is both to retain heat and so the steam you add (garden sprayer, etc.) will stay in the chamber and caramelize the crust. Your breads won't be properly baked without it.

Cheers,
Jim

[james]

Steam!!!

Excellent point Jim. Thanks for that -- no one had mentioned that yet.

Would you making a quick posting on how you do steam in your oven? I will make it a sticky to keep at the the top of the Bread section.
James

[Alf]

At the Museumsdorf museum in Cloppenburg, Germany they have several preserved and rebuilt wood-fired ovens. Apparently in the old days they baked the hard German black bread over several hours in a cool oven and steam was an essential part of the baking process. To obtain a good airtight seal round the wood doors and seal up the cracks in the wood of the door they used cow dung. I am assured this true and the bread was wonderful.

Alf

[arevalo53anos]

Nowadays, in constructing clay ovens, the cow/horse dung is used mixed with clay and grass or sedge.

The grass helps the structure to do not have crashes, the dung and clay (pretty well mixed) makes the dome hard and impermeable.

When dry the oven is a strong construction, heating too well and with no smell more than the smoke and bread aroma.



Luis

[CanuckJim]

Steam

James,

I have used two methods for generating steam in my oven. When the ash has been raked out, I leave my high mass oven to moderate for two hours, about the time I take my slow rise doughs out of the fridge. Then I brush and mop the hearth. At this point, I either put an old sheet pan loaded with damp rags into the oven, or give it a long spray with a cheapo garden sprayer. When the breads go in, there should be visible steam in the chamber. Once they're loaded, I give the oven another long spray, pointing the nozzle upwards, not at the breads. Again, there should be visible steam in the chamber. Once more, too, without steam, the crust will not develop properly and caramelization will be underdeveloped.

To retain the steam, it's important that the door seals the oven opening. Although I made my door quite carefully to ensure this, I still have to prop a brick against it to retain the steam, mostly because the metal part of the door has warped a bit from heat.

Simple, but effective.

Jim

[CanuckJim]

More on Steam

James,

I guess it's easy to talk about the need for steam, but it's better to illustrate the result. Here's a close up of one of my breads that shows the grigne that opened so well because of oven spring, the caramelization of the interior and a bit of the crust. These just won't happen without a wood fired oven and steam.

Hope that's of some help. Members might want to visit my web site, www.marygbread.com, for additional examples of what I mean.

Jim

[Marcel]

Closing the chimney flue with a built in damper

I've been experimenting with retained heat cooking in the oven, and I've noticed the temp drops off pretty fast (I have a dome-style pompeii oven). It seems to me that, in addition to sealing off the door, I could save a lot of heat by blocking off the top of the flue liner used as my chimney. Yet I don't hear of anyone doing that, even among Alan Scott/bread oven users.

Is this done? Any drawbacks (other than trapped smoke, which is a definite plus when cooking meats)?

Thanks.

- Fio

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(M) I used a commercial metal vent flap in my chimney but I can't unequivocally endorse it since it allows what seems to be about 25% seepage of hot air around the "slop" between the flap and the encasing cylinder.

(M) I used DMUN's angled entry for my door stop but found it difficult to construct, though now I know what building technique would have simplified it.

(M) My door is made of 2 pieces of cement board that have a perimeter strip, also of cement board, to provide an insulating airspace. The door parts have been secured to each other with furnace cement that cures with heat.

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This should show the external rod that opens and closes (by rotation) the metal plate in the flue:

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This should show the "S" shaped flex elbows I used to avoid the flue intersecting the ridge beam.

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A top view of the damper in the closed position shows the sloppy fit but which should still restrict a significant loss of convected hot air. The flue vent and the door are of course only used for retained heat cooking, such as bread baking; never for pizza.

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The prototype of my cement board door before I attached handles.

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To see more images, go to

http://s14.photobucket.com/albums/a318/marceld/

Ciao,

Marcel

[Alf]

Steam and Dampers

Oven design and the use / need of steam in a baking / cooking process is always an interesting conundrum to building a wood-fired oven. The simplest oven and a lot of folks will argue ?the best oven? is the Roman configuration that the Pompeii, Alan Scot, Italian, Spanish, Portuguese, Lebanese, North African etc ovens follow. That is, oven chamber, relatively small door that controls the combustible air in and the burnt gasses out of the oven and some sort of flue / chimney arrangement outside the oven chamber to disperse the burnt gasses and smoke.

It?s a great oven design, however, for producing and utilising steam the above ovens have one small (though not insurmountable, as Canuck Jim has demonstrated) problem, that is the distance between the cooking / baking product and the ovens ?roof? requires that a lot of steam has to be generated to fill the space and thus be able to influence the crust / top of the baking product. One way to avoid this is to build a flat oven arch / roof similar to the one in the photo. However, for combustion to take place correctly so the wood will burn ok in this type of oven the oven has to have some sort of flueing system within the oven chamber. I think this may be where Fio may have become a little side tracked.

Now to dampers.

For baking and the retention of steam with an oven that has flues within the ovens chamber the flues have to be able to closed tightly so heat and steam cannot escape. The way to achieve this is to have metal dampers that slide open and shut on a fixed metal guide. This needs to be of heavy cast iron or steel construction so that the weight of the damper will create a seal between the damper and the guide and thus keep heat and steam within the oven.

As Marcel pointed out the damper in his chimney is a sloppy fit and is for use in the flue / chimney of a wood burning stove. The reason for the sloppy fit is that if you have some sort of restrictor / damper in the flue of a burning appliance, for safety it should only close the flue by 80% thus allowing gasses from the burning wood to escape the appliance up the flue and not in to say, living accommodation and affecting folks. With retained heat baking / cooking this is not a problem.

Hope this helps

Alf

[Marcel]

Luis Arevalo's solution is elegant http://www.fornobravo.com/forum/showthread.php?t=2

[img]file:///C:/DOCUME%7E1/Marcel/LOCALS%7E1/Temp/moz-screenshot-181.jpg[/img](M) If you want to see an elegant solution to a full closure damper, go to

http://www.fornobravo.com/forum/showthread.php?t=203

and look at Luis Arevalo's post #07.

(M) I don't know if you will be able to open the image with a click below:

http://www.fornobravo.com/forum/atta...&stc=1&thumb=1

(M) Alf wrote, in part:

"Now to dampers.

For baking and the retention of steam with an oven that has flues within the ovens chamber the flues have to be able to closed tightly so heat and steam cannot escape. The way to achieve this is to have metal dampers that slide open and shut on a fixed metal guide. This needs to be of heavy cast iron or steel construction so that the weight of the damper will create a seal between the damper and the guide and thus keep heat and steam within the oven.

As Marcel pointed out the damper in his chimney is a sloppy fit and is for use in the flue / chimney of a wood burning stove. The reason for the sloppy fit is that if you have some sort of restrictor / damper in the flue of a burning appliance, for safety it should only close the flue by 80% thus allowing gasses from the burning wood to escape the appliance up the flue and not in to say, living accommodation and affecting folks. With retained heat baking / cooking this is not a problem.

Hope this helps

Alf[/QUOTE]

(M) I wish now that I had used Luis' horizontal chimney vent slider :-(

(M) If you haven't built your chimney yet, this is definitely worth a visit.

Ciao, & Obrigado, Luis.

[jayjay]

Making a door

When I finally make my door I plan to have it fit the inner opening of the oven, past the vent/flue...When I stacked up bricks as a door, I put them inside the oven opening all the way up so that they hit the bottom lip of the vent.
Drake

I am making a door today, but can make another one if you will
tell me how to make one. The one we are making will be the width
of the door and the height of the arch of the door. We will put two
peices of plywood in the door, and seperate the plywood with two
pieces of cocolumber. Then we will put a handle on the plywood that
faces the outside of the door. We cut the cocolumber from our own
trees and will use two peices about 2 to 3 inches wide to seperate the
two pieces of plywood.

Any suggestions for a door will really help.

JJ
Philippines