Plugging the chimney for retained-heat cooking Heat Management Discuss Plugging the chimney for retained-heat cooking in the Oven Management forums; James, I guess it's easy to talk about the need for steam, but it's better to illustrate the result. Here's ...

[CanuckJim]

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]

Originally Posted by 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

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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]

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]

[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 (Pompeii oven - Landing area and vent)

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]

Originally Posted by 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

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

[DrakeRemoray]

Your plan sounds just like mine...

I used 2 pieces of 3/4inch plywood cut to match my opening (it sticks up a little into the vent to get a good seal). I wrapped one piece of plywood (the one that faces into the oven) with aluminum flashing. I attached the two pieces of plywood with a little dead air space in between. I did this just by cutting strips of plywood and making a bit of a frame. Then I screwed the whole thing together and attached some fence handles from home depot. No Cocolumber in Denver Colorado!

Really anything that will keep the air out and will not burn up right away will be fine. You don't need the door when you have a raging fire going, just after it cools down a bit and you are ready for retained heat baking or roasting...

Drake

[Mypsi]

We have a retained heat oven here with a flue that closes by a hand screw crank type thing. We had baked for a couple of years (about one year for me) without ever closing the flue. We had problems keeping enough heat through multiple bakes without burning the first load, so a few months ago, I tried doing a pre-fire the night before, still without closing the flue. I came in the next morning, and it didn't seem like it had made any difference. About a month ago, we had a "duh" moment, and realized how much heat was going up the flue. Now, we give it a fire the night before while we make starters and cold mixes, and just a short fire the day of the bake. We're still getting the feel for it, but on the most successful days, we can get 3 or 4 bakes (haven't tried more than that yet) without the first load getting burned or the last load taking too long or coming out too light.

We bake 3 days a week, so the night before a bake is usually also the night after a bake. Depending on how much granola, cookies, etc, were made that morning, the thermocouples read somewhere around 250-300 F. I open the flue, and burn for 2-3 hours, getting the inside up to 650-800 F, depending on how much time we have. After the fire has burned down to coals, we close the door, let it snuff out for a few minutes, and the close the flue. The next morning, if we're lucky, the heat's leveled out through the whole oven, with most of our readings between 400-500 F. At that point, I open the flue and give it a small fire just to perk it up a little.

Something I'm going to try to do better is scheduling my baking days so that I can get the oven cleaned out, the door sealed, and the flue closed as soon as the fire dies out. Some days, I find myself busy, and it sits for a little while, and I think we lose some valuable heat, and it doesn't even out as well.

Having the flue closed has made a huge difference for steam, as well. The other day, I had sealed up the oven, but the flue was still open, and I saw little ash fragments stuck to the face by the door flapping in the wind. It dawned on me that when the flue is open, rather than steam just escaping out of the door up the chimney instead of into the room, it is being actively pulled out of the oven. With the flue open, we would steam the oven, then rush to close the door before it all disappeared up the chimney. Now, we can give even more steam, and it lingers in the oven before it starts to drift out.

One thing with the flue closed is that any flour dusting that lands on the floor and burns doesn't smoke up the chimney and can burn your eyes when you're peeking in. I've started cracking the flue a bit when I'm unloading the oven, and then giving it a quick sweep after it's empty, then closing it up again.

Any other ideas?

[james]

I'm having trouble visualizing this. Could you post a couple of photos -- that would help me.
James

[Mypsi]

Originally Posted by james

I'm having trouble visualizing this. Could you post a couple of photos -- that would help me.
James

Sure. Anything in particular you're wondering about?

[gjbingham]

I'm perplexed. If your door fits tightly against the oven entryway reveal and your chimney vent is exterior to that, how does a flue help retain heat? The door is already holding the heat inside the oven.

Perhaps your oven(s) are configured differently?