Clearance required between barrel and riser?

how much clearance is requird between a 30 gallon barrel and the insulated riser is required for proper flow to mass? I have come across some commercial insulating bricks (very light weight) and have very little clearance left and I feel its restricting flow. When I put barrel on its not near as rockety and I have fire creap in by feed tube. My riser is 6 inches and 48 inches tall. I have more than double the combined length of feedtube and burn tunnel. Any suggestions, Thanks.

did you put insulation around the kiln bricks? please dont they are insulation enough.
did you seal the kin bricks up as you built the heat riser? I hope so.

Where did you score a 30 gal drum?
what orientation are the kiln bricks laid?
Can they be reoriented on there sides?
is the gap at the top between the barrel bottom and the top of the heat riser 1 and 3/4 inches?

Ernie, What I did was start with 6 inch stove pipe and put 8 inch stove pipe around it and packed it with perilite and clay. So I only had 1 inch insulation which was a mental mistake for only having so little insulation. At work they were rebuilding a silver kiln and were replacing the light weight fire brick which I was able to take. They are 3 inches thick and varies from 18 x 12 inches and down. What I did was using refractory cement cemented it into a square and put this around my riser. What I figured out finally was while test burning I failed to mud in the gaps between riser and square firebrick which I believed started the cool air to flow up between riser and brick because it started to smoke badly. But in order to get the 30 gallon drum over the square brick I had to shave the corners off the brick, how much is needed is the question. What I thought was a bottle neck due to lack of room between barrel and riser causing back flow and poor burn. Which Im still not sure what is a acceptable gap especially at corners of brick and barrel. Which is why I asked. My gap at top is 1 1/2 inches. Evi
evedently I got real lucky on finding the 30 gallon drum. The guy I bought it from had lots of 55's but happen to have 1 lonely 30. I was under the impression that I had to use a 30 for a 6 inch system? Can you use a 55 on a 6 inch system? I also shortened my firebox by a brick and test fired it today and it seemed to work fine. I am open for any and all suggestions. I hear you will have a new book coming out soon on rmh's. Let me know when its available I would like to have the latest and greatest addition. Hope you and yours have a wonderful Thanksgiving!

yes a 55 is fine on a 6 inch system; it will give you way more volume. you want a total area at the min to be the same as your system, its better to have more at this point it sounds like the corners are still blocking the flow.
I would just set the kiln brick at the same measurements as your burn tunnel (6" should be around 5 1/2X5 3/4) i would get rid of the pipe since its not relevant if you are using kiln brick.
make sure the heat riser is sealed up (one of three most common problems.

Ernie, I built a new square riser out of kiln brick as you suggested and test fired it today without the barrel. It is the most rockety that I have seen. I notice that since this riser is shorter than my round one that when I set the barrel on tomorrow it will almost sit on the burn tunnel. I plan on putitng an extension of brick on top to get more clearance for barrel, and I like to know what is the minumum and best distance off ground do I want my barrel bottom (top) to be to get the best feed/flow into manifold and into mass bench? I also went from 5.5 square burn tunnel to 5.75 wide by 5.5 tunnel as you suggested. Aslo, I see in Ianto's book that he uses a ash trap at the bottom of his feed, what is your take on this? I haven't seen one on any of your builds and I'm sure there must be a good reason. Thanks so much for your input, Bob.

other random questions. How hot should top of barrel get? Can burn tunnel get to hot? What are causes of fire creep in feed? What should exit temp. coming out of bottom of barrel? How fast should exhaust at bottom of barrel be traveling? After putting barrel on and it heated up I had fire creep again. Temp. in burn tunnel was @ 425 F. Want to put finished stove in basement and exit out window, if this is possible how hot do I need to keep exaust temp. for a positive exit temp? I have a fireplace in basement but I don't think you can expect to exaust up chimminey that high. Also, I failed to mention that my burn tunnel sides are solid pieces of hard firebrick material that was used in kiln, could they hold to much heat? It seems like once I get stove running it works fine until I stoke feed full and its seems to get tunnel to hot and I lose some draw. Top gap was 1.75 to start but raised another .25 inch and it didnt seem to help with the fire creep. Going to let it cool down and try not feeding so much wood at the same time, or is this something I shouldn't have to worry about if stove is working correctly. As you can see I'm starting to guess and really could use your expertee's. I wish your new book was finished so I wouldn't have to bug you so much, LOL. Thanks, Bob

it sounds like you need to build the heat riser taller. make sure you have no air leaks in the barrel and the heat riser. you will get wood stove temps at the top of the barrel and the fed tube will be OK at 400 as long as things are flowing. it surly sounds like you have a long burn tunnel and a short heat riser. however make sure you have no leaks first before changing everything around.

Ernie, My riser when measured from outside corner to outside corner diagonally is 13.5 inches. 30 gallon drum is 18.5 through middle ID. I only have 1.5 inches from the corners of the riser to inside of barrel. Without barrell on it rockets real well, when drum is placed over running stove you can hear it calm down and not roar like it did. I'm thinking the drum is to small to let good flow around square corners. I've checked for leaks and put more mud on, also. What is your opinon? I've decided that this weekend to get a 55 gallon barrell and try it. I really want to use the kiln brick riser, I think it will last longer than metal. And if that works will probably end up using the 30 I looked so hard for to store wood.

Rob i would go to the 55 gal. it cant hurt you and most likely it will help. let us know the result.

Ernie, I added 8 inches to my riser and put the 55 on and that seemed to do the trick. I kept fire fed hot and often and it kept drawing just fine. I tried the 30 gallon drum again and it chocked down after a bit and the draw dropped as soon as the I put the barrell over the riser. I'm at a 2 inch gap on the 55, would a 1.75 gap on the 55 make it draw even better or does the gap change when using a 55 gallon barrell? I have my doubts about putting this system in my basement and venting out basement window due to 7 ft. rise and higher to possible venting issues, so I will have to settle for the garage unless you say it can be done. For the exit out the bottom of barrel leading to my mass I'm planning on a third of the barrel open to dump into mass. Is this adequate? Thanks, Bob