Cob Oven questions, tips, advice...

Hello! I am going to build my first cob oven in a couple of months and I would like to start planning exactly what I want. However, I have a few questions, so I decided to post here as I am sure many of you are cob oven masters

So my main question is: chimney or no chimney? I thought there was no need for a chimney, as you light the fire, let it get hot, remove the fire, and the oven is ready to use.
But I have seen many models of cob ovens with a chimney in it. What would be the reason to have/not have a chimney in it?

My other question is concerning the door, I have also seen cob ovens without a door... Is it always needed?

Also I would like to know if you have any main tips concerning the building of a cob oven, what did you learn building yours? Would you do anything differently?

Thank you so much, I appreciate your knowledge!

Ernie's design is the best.

I retro-fit an outer chimney on mine and it makes a HUGE difference in how clean it burns. I didn't change anything inside, just added a cob vent hood/scoop and a couple pieces of chimney pipe.

The only downside is trying to cover it with a tarp. I need to make a tin roof over it.

I see..., that's a good video, thanks! A cleaner burn is definately desirable...I will look at those plans!!

Judit,

I'm certainly no expert at cob oven building, but I can give you a bit of feedback pertinent to your questions. If I were to build mine again, I would probably build it with a chimney. I first decided not to build it with a chimney because I wanted to keep it simpler on my first attempt. Overall, I have really had satisfactory burns in the oven. It does smoke in the beginning of the burn, but mine almost always clears as the fire gets hotter and hotter. It is typical to have the flames licking out of the front of the oven when it is burning really well, even when all the wood is pushed to the very back of the chamber. Having said that, I think I could get a much more efficient and more steadily hot fire with a chimney (as in the double chamber design). I am planning to retrofit a chimney into mine at some point for this very reason. Because I made mine slightly thicker than expected, I find that I have to fire the oven for much longer to heat it all the way through (c. 3.5 hrs). I'd like to make better use of my fuel by implementing some of the rocket stove techniques as with the double chambered oven.

About the door...I don't ever use one when I bake pizzas, as I like to have a live fire around the periphery of the oven. Yet, when I bake or cook other things like breads, casseroles, soups, roasts, etc., I think, the thicker the door, the better! In fact, though my door is not cob, a door of cob would probably by the most efficient in terms of holding in the heat during baking. My door is adequate (1.5" thick kiln dried hardwood with a sheet of tin spaced about 1" off of the inside of the door), but I sometimes find myself stuffing bricks into the door space in addition to it so as to preserve more heat.

Here's a link to a blog post about our oven if you're interested: http://www.theyrenotourgoats.com/homestead/making-our-cob-oven/

I hope yours turns out well! It will be a valuable experience however it turns out.

Timothy, thank you so much for your feedback, it was really helpful!
Your post arrived as I am about to start the oven construction...a daunting but exciting experience!

Your oven looks great Good luck with any modifications you undertake.

My son built a cob oven last month. He let me post pictures and a video here...
http://solarbeez.com/2014/10/22/brian-builds-a-cob-oven/
I brought some aged cherry wood for him to use.
We cooked pizzas in about two minutes. It really works well once it's heated up and it doesn't take much wood.

Pat Redd wrote:My son built a cob oven last month. He let me post pictures and a video here...
http://solarbeez.com/2014/10/22/brian-builds-a-cob-oven/
I brought some aged cherry wood for him to use.
We cooked pizzas in about two minutes. It really works well once it's heated up and it doesn't take much wood.

I liked your son's oven a lot! Do you know how long it takes to warm up enough to cook the pizzas?

My son says, "it takes about 2 to 3 hours of firing to heat this particular oven up that much. That is because I put a rather large thermal mass in there, for longer baking sessions. If you just made the ovens for pizza you could get away with an hour and a half or so. If you use live fire when cooking probably even less."
Hope that helps.
Pat

Hey!

I found this thread when looking for the exact issue - chimney or no? - I'm about to rebuild my first cob oven, that has baked for us every week for about 8 years. it's finally cracked enough and the door frame area has taken enough abuse that I'm worried the thing will collapse mid-bake.

what everyone else has said here fits with my experience as well. I didn't do a chimney in part because I'd heard that there's a lot of heat-loss during the burn, but I started to really want to control smoke and cinders more, so I put up a metal chimney outside my door (just pull it down when feeding fuel) and with a screen over the top, I felt that I was both having less smoke and letting less sparks fly around.

I generally do about 3.5 to 4 hour fires, as I've added plaster to my standard Kiko suggested thickness over the years. The great part of that is that I then have LONGER heat retention. I now generally do a roast or something overnight after the rest of the baking is done. it's still 150-200f in there the next day at 11am.

MY QUESTION: what's this 2 chamber business (from the video)? is that really key to clean burns? does the fire sit way in the back of the 2nd chamber? what sort of heat sink is a cob front chamber? All these photos of artful frog ovens with chimneys don't seem to have 2 chambers... do they work as well/better than a no chimney?

MY ANSWER/Addition: when thinking about building an oven, something I will take into consideration is the SIZE of my pans and pots. I can't get all my cookie sheets into my current door (I do granola and crackers most weeks too) and while my cast iron pot goes in, it's really tight, and I know my big turkey-roaster won't fit in my current door, so I'm planning for that on my next one.

I've only used one door my whole time, and it's a big chunk of wood that I soak in a bucket while the fire is going. it's about 4 inches thick at the bottom and maybe 2in at the top. Imagine a slice off a firewood log, smoothed down with a handle. I'm actualy planning on making a lighter one of 2in board and covering it with metal. will probably still soak it.

if anyone else can weigh in on the chimney *design* debate, I'd love to hear about it.
thanks,
Tys

There is another approach to a "two chamber" cob oven design, that places a burn chamber below the baking chamber and runs the exhaust through the baking chamber. I have not built one using this idea yet, but I believe that it would permit you to build a rocket stove, with all the clean burning and high heat that implies, which would vent into the oven space. Effectively a rocket mass heater using the oven as the mass element.

It would also eliminate the need to clean ash off of your oven floor (or nearly so - depends on how clean your burn is from the chamber below) and definitely makes it much easier to keep a fire going while you are baking, since it is not in the same chamber with your food.

I think in this design it would make sense to run a duct through the mass of the cob oven, so that you have an exhaust chimney, but you capture as much of the heat in the exhaust as possible. Doing this would probably call for a thicker cob dome than regular ovens, as you need to keep large thermal mass while introducing the flue space into the shell.

So many things to do

Check out this thread on Donkey's forum - it includes descriptions of at least a couple of rocket-fired cob ovens.
http://donkey32.proboards.com/thread/1301/all-adobe-mud-cookstoves-ovens
You would have the same oven as usual on top of the rocket system, with the exhaust out the door or front chamber chimney. It would require no special considerations in that regard.

The two-chamber oven Ernie built (and possibly other people's, I don't know for sure) uses the front chamber to ensure a clean burn and little smoke to annoy the neighbors. The front chamber would not be a heat sink but a heat reservoir, as it only sees the hot gases after they exit the baking chamber.