losing draft when I put the lid on

using 6 inch system. burn tunnel is 8 inches, feed box is about 8 inches. Riser is 32 inches, 2 inch gap down the sides as well as 2 inch gap on the top. I had to bring down a 55 gal drum to a 16" diameter, so I have the lid loose. Ive been playing with increasing or decreasing the top gap all afternoon and night now. When the lid is off, the draft is extremely robust. As soon as I put the lid on, really with any sized gap, it reduces significantly. Some positions are worse than others, but even the best isnt that good. Having gone back through many many threads this afternoon, Im rather stuck.... Is there something Im missing?

on edit: would it matter if the bell is not enclosed yet and simply venting out of the bottom? would hooking up the rest of the exhaust possibly make a difference. Ive been messing with Mk II now for several days. I may try an inline duct fan at the end of the exhaust to give it a boost.


Mr wisner, I assure you that Ive spent many hours pouring over the RMH posts here!, taking notes. Actually find that combined information much more useful that the Evans book. Reading trials and error, experiments is extremely useful.

match your burn tunnel csa to your riser csa. CSA= cross sectional area. you are going to get a slow down any way but you wont have near as profound one as having a big easy path for the fire right above it.

its no joke that the cross sectional area needs to be close to the same. for a six in system it works out to about 5 and 1/2 to 5 and 3/4.
also give the bell a bit of time to heat once it goes on you are now depending on how much temp differential you have between the outer skin of the bell and the core of the heat riser.

my burn tunnel dimensions worked out to be 5.75 x 5.25 and 8 inches long. Thats 30.18 square inches. 1.92 inches difference. from the 6" round duct. Which may explain the intermittent pulsing.

that would be it the pulsing is the stove trying to get more air. the system has two pumps in it one is the hot stack effect and the other is the rapid heat loss from the radiant surface of the barrel. the stock stove is a balance between the two. you can play a bit and get other effects for your situation but those two pumps are vital to good system performance. hmm thinking about it you might decrease your gap by about 1/4 inch that may fix much of the problem. most slowdown problems are in this area or in the manifold setup. since you dont have a manifold i would suspect the gap.

Ive had it burning for about 90 minutes now. Maybe my expectations are too high. While still "rocketing", it's no where near the super rockety-ness with the lid off. I mean with the lid off it sounds like a genuine rocket. Ive been able to keep it from smoking back today by changing wood to kiln dried 2x4 scraps, and choking the air intake greatly. Maybe 75% choked.

I want to slice some firebrick to install in the burn tunnel and get a closer match to the CSA of the heat riser. I will also see if I can get the gap down a little. btw, when I use the "divide CSA by circumference" cited many times in these forums, I get 1.5 inches for the top gap.

yep and in a perfect world where there is no variation in materials 1.5 would be exactly what would be needed. but w live in an imperfect world and that means we have room to play a bit with our dimensions to make different effects. the Rockety sound does change with the barrel on but thats expected and accounted for. you need things to be a little slower or you wont get much heat absorption in the mass. I still dont understand why folks think the sound is important. *sigh*

going to spend some time hipping up those numbers. Thanks for your input today.

I got the top gap to an inch and five eighths, and put some tile shims in the burn tunnel so my CSA there is 5.25 x 5.12 (an eighth). It is has a much stronger draft. very nice. Thanks again for your feedback.

yikes. Apparently fixing the top gap really helped. Yesterday I discovered one brick at the very back of the burn tunnel that was 3/4" lower than the rest. I'm not sure how I did that, but it's also not that surprising. It was reducing my csa almost 5 square inches. Crazy. getting a grinder back there to fix it wasnt easy but seems to have done the trick!

"the system has two pumps in it one is the hot stack effect and the other is the rapid heat loss from the radiant surface of the barrel. "

just read a reply from you on another thread where you mentioned putting fins on the exhaust duct to move the heat into the mass. Ive been wondering if adding cooling fins to the bell would help drive that second pump you've mentioned.

3 days of firing so far with no problems. Time to start building the mass

yes fins on the outside would help but are not needed. nice job on the stove. its pretty nice when you get it right and see how the system can work.