Sometimes I get liftoff, sometimes I don't

Hello everyone, I'm trying to understand why my heater sometimes gets a draft and sometimes just duds out. I've been trying to toy with the measurements but nothing seems to be giving me a consistent draft, and I can never actually burn wood consistently with it (The only draft/rocket sound has been with paper) Could my measurements be wrong? My system is made out of firebricks. Here are my dimensions:

The opening of the feed box and heatriser are both 7.5"
The feedbox is 13" high
The heatriser is 44" high

Following is where I think the problem might be:

The bridge (the row of bricks that connects the heat riser and feedbox) Mine is 10" long and the burn tunnel below has 6.5" of space heightwise until of course it reaches the heat riser.

I keep messing with the length of the bridge, but I'm still not getting consistent results. I am making this outdoors so the whole thing is usually very cold which I know can be a problem.

Frustration is consuming me.

I know there are a lot of variables involved when making these and I need to keep tinkering, but any help is greatly appreciated!

S Jones wrote: Here are my dimensions:

The opening of the feed box and heatriser are both 7.5"
The feedbox is 13" high
The heatriser is 44" high

Following is where I think the problem might be:

The bridge (the row of bricks that connects the heat riser and feedbox) Mine is 10" long and the burn tunnel below has 6.5" of space heightwise until of course it reaches the heat riser.

I think your dimensions are off by a little bit. The preferred ratios for a J-tube system are 1:2:4 (feed, burn tunnel, heat riser) all measured from the longest dimension. So if your feed tube is 13" deep, your burn tunnel should be 26" long, and your riser should be 52" tall. Although you did say "feed box", so I'm wondering if you built a batch box?

If you are testing this outdoors, what kind of exit path do you have? Do you have a barrel over the riser? Do you have a vertical chimney leading from the bottom of the barrel, and if so, how tall is it? What is the configuration of the transition from barrel base to chimney? Do you have your firebrick joints sealed with clay or mortar?

Ordinary hard firebricks are dense and conductive, so it would be more difficult to establish a draft in cold outdoor conditions. Even wrapping some fiberglass insulation around the core will help you for testing.

The burn tunnel length in the formula is a maximum, not a minimum, and 7.5 x 2 + 10 = 25 is very close to the recommended length. If any part of the system is smaller than the rest, it is recommended to be the burn tunnel, so 6.5" high is probably okay.

Other ratios that have been recommended are 1:2:3 (older, and generally superseded by 1:2:4), and 1:1.5:3.

Generally, I think it is important that the riser be at least as long as the feed and burn tunnel combined.

Thank you all for the replies - I have to work the numbers out a little bit with the ratios that you've given me. The heat riser is sealed with clay although the feed is not so maybe that is part of the problem. Hopefully I will triumph!

Good luck!

Ok - so I rebuilt my system changing some of the measurements. I know have a system that has a 15" high feed, 22.5" long burn tunnel and 45" high heat riser. This goes along with the 1:1.5:3 ratio. My openings to my feed and riser are 7.5". I attached the barrel with my exhaust and it worked! I was so thrilled. I checked the flue and steam was coming out. Hurray! So I waited for the barrel to get hot and it did a little, but after hours of running it I could still touch the sides for about ten seconds. It just wasn't getting hot enough to radiate any decent heat. I'm wondering if it has to do with the draft. Sometimes the fire starts to climb the logs and burns upwards rather than sideways - is this a thing that happens? Also I'm guessing if all goes well there should be no heat coming from the top of the feed? I thought my draft was good but maybe I am wrong?

My heat riser is made of bricks insulated with clay/Cob

I am using flexible aluminum ducking for the exhaust and flue. I can put it directly in the bottom of the barrel and seal it in with more clay. I have it running along the floor and exiting through the bottom of the wall where where it extends upwards vertically about two feet. Mind this is not in a home in the traditional sense but in a temporary winter shelter in the woods.

When you say flexible aluminum ducting, do you mean the crinkly kind that you can move every which way? From what I've read, all those ridges seriously affect the draft, and may be causing your issues.

As far as the wood creeping back up the feed, that may just be from overloading. During all my test fires I learned I need waaay less wood in there than I thought I would, mostly cause my experience with wood heat comes from traditional woodstoves, and RMHs are a whole different beast. Let me see if I can find a photo of my feed going full blast...here we go. I'm running a 6" system, so a little smaller than yours. But you can tell I only have maybe 6 pieces of wood in there, all around 1"-2" in diameter.  I specifically took a photo of this burn because it was the hottest, cleanest burn with the best draft I had done. Hope this helps!