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.