Rocket mass heater for workshop

Hi, I've built rmh for my workshop but I have a problem so I hope that you can help me.
when it burns without the barrel, it burns great, no smoke is coming out of the riser, just hot air. but, when I put the barrel on it starts to smoke and fire dies in a few minutes.
feed tube is 11" high, burn tunnel is around 12" long and the heat riser is 35" high. square tubes are 4x4" and heat riser is 4,3" in diameter + 2"of isolation (stone wool). between heat riser and the barrel there is 2" of space. barrel is 23" wide. exit pipe, for the gasses is 5" in diameter.
I was reading on this forum and I saw that feed tube is too high and it should be 8". Is the barrel too big and the exit pipe too small? what do I have to change so I can get enough draft with the barrel on? will it help if I put exhaust 3 meters high?, because now I have only 1m height.

This is my rmh

Hello.

So, i tell you straight away, you won't like what i have to say.

First of all, metal is doomed!  https://permies.com/t/52544/metal-burn-tunnel-heat-riser


Secondly, 4 inches systems are known to be temperamental. And not work well.

35 inch riser? I can't see how it could work, since the barrels are normally 33 to 35

https://en.wikipedia.org/wiki/Drum_(container)#International_standard_size

Check your top gap, to make sure. And even, enlarge it to 3" just in case.

Then, you don't tell us anything about your piping afterwards.  Do you have a chimney? Proper one?  What is the length of your bench ducts? How many elbows?  Already cobbed?

Any more questions? ;D

yeah, I saw that today about steel rocket stoves. I will use it around 1-2 weeks during winter so I think it will last at least a few years.
pipes would have to be bigger than 4 inches?
between top of the riser and the top of the barrel, there is a gap of 2". ok, I will increase it to 3". can the top gap be too big? what would happen if I make it 4"?
I don't have a proper chimney. my workshop is about 13ft x 26ft. the idea is to place the rmh in the middle of the workshop. So, that means that I have to have 13ft of pipes to get outside of the workshop. is it better to lay the pipes first horizontaly, and then verticaly or other way around? I will have 4 elbows. Sorry but I don't know what does cobbed mean? I don't have bench ducts.
thank you for your help.

Well, steel might not last 2 weeks!

You need a chimney.

Cobbed, means that you have applied a mixture of mud, to pipes, walls or else. In this case, pipes.

Here is a workshop heater i made.

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

Here is Peter's http://batchrocket.eu/en/applications#threebarrel


Here is another workshop heater that i like.  

https://www.youtube.com/watch?v=-jzKKIHhTU0


Theory of bells. http://www.stove.ru/index.php?lng=1&rs=16