Permsteading.com

After playing with batch boxes for a bit I thought I should build a j-tube to compare. This is a 1:2:4 ratio j-tube with a P-channel, tripwire, and kicktail. It is built from clay, sand, wood ash, and straw. It has a 4 inch square feed with a 4 inch round riser. I was 12 minutes into a burn when this happened:

I have had this fear with the J-tube design since I first started researching rocket stoves. Since I am still new to these things I thought I would ask for some opinions on what went wrong.

Looks like the wood is forming its own chimney and therefore reversing the draft. Once the flame creeps higher than the feed tube it's hard to get it working right again. Try shorter wood, or don't use as many tall pieces.

Also, what are the ratios of your mix. If you use a light and well insulated one, the draft should make it work properly. I've used a mix of 1:1 clay to ash that works pretty well. No sand or anything else.

Also, the 4" stoves are a bit more finicky to get working right. I've built a few that small, but now won't build anything less than 6". I just finished an 8" and was surprised at how much better it works than any of my other smaller ones.

Something is wrong there 4, a good J won't do that. My guess is that you still have some internal mold pieces burning out and it curled and obstructed the gas path. If it's not that, then you had wind coming at the thing from the riser side.

The mold was gone by this time, I fished the last bits of wood and screws out just before this burn. It was a little windy so that may have had something to do with it, but something else just occurred to me. I had this thing packed pretty tight and the wood was almost 2 ft long compared to a 3 ft heat riser. When some of the center pieces of wood burned the bottoms off rather than shifting down it might have had a reverse chimney effect and caused the flames to climb. I will give it another try with shorter wood and not pack it as tight.

Wind definitely makes a difference. My (whopping 2) cores have all burned almost crystal clear after about 45 seconds of ramping up, but when the wind kicks up it can pull some smoke out of the feed tube. You can see it happen. No wind, no smoke, then a little gust of wind, voila smoke. If the wind is constant, it starts to pull the fire up. It is very predictable. And yeah once that stuff out of the feed tube starts burning it's going to keep burning.