Plugging the chimney for retained-heat cooking

[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]

Re: Plugging the chimney for retained-heat cooking

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]

Re: Plugging the chimney for retained-heat cooking

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

[Mypsi]

Re: Plugging the chimney for retained-heat cooking

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]

Re: Plugging the chimney for retained-heat cooking

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?

[Mypsi]

Re: Plugging the chimney for retained-heat cooking

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]

Re: Plugging the chimney for retained-heat cooking

"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]

Re: Plugging the chimney for retained-heat cooking

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]

Re: Plugging the chimney for retained-heat cooking

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!