Plugging the chimney for retained-heat cooking

[Mypsi]

gjbingham,

You're correct about the oven design, but I think the main problem is that our door, and presumably all doors, do not operate with 100% efficiency. When, the flue is closed, we are losing heat out of the door, but it's just drifting out. I think the key to understanding this is to understand how a chimney works. Previously, I had pictured the chimney as serving as just a path to the outside where the smoke would lazily waft up and out of the bakery. What's really happening (any physicists out there, is this an example of Bernoulli's principle?), is that any moving air at the top of the chimney creates a air pressure differential that works to actively suck air up the chimney, out of the oven. So, when the flue is closed, and the door is on, the pressure inside and outside of the oven is more or less equal, so while some heat is leaking out, it is not actually being pulled out. On the other hand, when the door is closed but the flue is open, it's like I'm standing there with a vacuum cleaner hose pressed up against the crack of the door, and the leaks around the door are being actively exploited by the pressure differential.

The biggest difference I've noticed is with steam. With the flue closed, I can give a good long shot, until the oven is full and the steam just starts to drift out the door. Minutes later, I open the door and there is still steam in the oven. When the flue was open, as soon as we started giving it steam, you could see it pouring out and up the chimney. So with the flue open, even if I had a door with a perfectly tight seal, I would need to find a way to give it steam with the door already closed.

[Xabia Jim]

I still cover the chimney the old fashioned way...metal bucket!
Optional wet blanket if I really want a seal.....

[Xabia Jim]

"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."

The picture with this post (from Marcel above) made me look at my damper.

Mine is a simple solid steel damper that seals off the chimney pretty well when closed. I do turn it back 1/8 turn from open to damper the ehaust when heating up the oven and if I want more smoke.

Pics show closed, open and cast chimney with damper handle

XJ

[shvuntz]

can someone tell me if a chimney is absolutely necessary?i am thinking about buying a domed pizza oven on ebay where there is no chimney how does this work? its made from steel re-inforced, cast volcanic pumice stone, clad in traditional stone
its 1000 pounds...so i need to get it right

[gjbingham]

Mypsi,
I understand your ideas. It makes sense to a certain extent that your steam doesn't dissappear up the closed chimney when you steam the oven. I kind of like your idea of the vacuum sucking the steam out of the oven. My own observation is that once the fire is out, the floor has been cleaned and the door in place for awhile, the chimney is no longer sucking hot air, at least to any extent. Sure there's a little leakage around the door, but not enough to keep the chimeny drawing like there's a hot fire in there.

My chimney's 8 feet tall and takes about 5 - 10 minutes of fire to get the thing drawing efficiently. The rest comes out the front of the oven. Every oven is different, but my feeling is that when I steam mine after pulling out the door with the fire out, the excess steam all goes out below the height of the top of the oven opening. Where it goes, up a cooling chimney or out the front is just a matter of easiest exit. The heat will carry the steam out when you open the oven. It will escape by the easiest means possible.


Just my thinking. I've been wrong many times before. I will again - probably tonight!