barrel size?

does the barrel have to be large? or i should say tall. what im asking is weather can use a metal 5 gallon bucket as a barrel for a small 4" core. is there enough area to start gases cooling and flowing into the bench pipe?

Ronald Bush : You are certainly thinking outside the box, and that is generally a good Thing ! Remember, even with a 4'' system you are going to have to have
Heat Riser walls that are are both durable and thickly insulated.

Again there must be a strong difference in the Temperature of the Ascending and Descending Exhaust gas Flows to create the push-me pull you effect that Drives
the horizontal flow of hot exhaust gases ~30' ~ (30' or a little less for a 4'' system )

So, A 4'' opening, and Heat Riser walls 2.5 -3'' thick, you are talking 10ish '' and then you will need a minimum 2'' air space between the outside wall of the Heat
Riser and the inside wall of your barrel. So you are looking for a durable metal barrel with an inside diameter at least 13'' !

Generally speaking, a 4'' system would be considered an advanced build, most of your Fellow Members would recommend building a 6'' or 8'' system out of doors,
learn the basic skills and the relationships between the various parts and then go on to build a separate 4'' system !

This is where I Recommend that you go to rocket stoves.com to download Your Pdf Copy(s) of the Brand new Third Edition of Ianto Evans' Great Book '' Rocket
Mass Heaters" This book will answer most of your questions, save you Time money and frustration, and teach you a new vocabulary so that you can come
back here to your fellow members and use the same terminology to Describe the RMHs parts, their size and shape and orientation to each other ! The little bit of
Math required can be quickly boned up on as it is all 3rd or 4th grade stuff !

And stay away from the Crap on you tube, most of the Units found on there have profound flaws that have already doomed them! For the Good of the Craft! Big AL

The heat riser for a 4" Dragon Heater is 24" tall. We recommend a 30 gallon steel drum, if one is using a steel drum. A small bell is also an option. In the attached photo, the heat riser is in the short stack behind the feed tube. Exhaust leaves that stack and rises into the 6' tall stack next to it. This unit has excellent efficiency and draft.

ok ,i see with the risers outside dimensions it wont be enough room for the bucket. what about just the height aspect of the barrel. can you get away with a short barrel with plenty of space around the riser? im trying to figure how to send more heat to the bench, rather than off the barrel.

thanks for the replys

ronald bush : As soon as you have a working Rocket Mass Heater RMH You can simply add-on a thin layer of Cob starting at the Bottom of the Barrel.

In the Case Of Dragon heaters (Dragonheaters.com ), The Heat Riser and the Final bell can be finished with an appliqué of dressed stone which will 'store'.
and re-radiate heat energy off of its mass much like the much more massive and heavier Rocket Mass Heater !

Hope you have found this helpful ! For the Good of the Craft ! Big AL !

Ronald, you also could do a rocket with no barrel, just a bell. Or, cover your barrel with cob, or firebricks. There's plenty of solutions. Tho, if you cover the berrel, or use a bell, in my opinion, you need bigger gaps. A taller heat riser is a good idea, and proper chimney draft is a good thing too.

ok, thank you for the input.

Regarding the gap between heat riser and first bell...Satamax is correct, you do need a large gap. If you look at the image above, there is a line of fireclay mortar between the heat riser stack and the first bell stack. That is the height of the opening between the 2 stacks.

ok does the bigger gap mean more flow too?

ok does the bigger gap mean more flow too?

Yes. The large openings are important.

The exhaust from the combustion has plenty of room to stratify by temperature once it moves to the bell. The advantages of bells over flues are explained in a post on our blog. One fact about the exhaust is that it still has 79% nitrogen which is not involved in the combustion transaction and the molecules never get as hot as the water and carbon dioxide molecules. Consequently, if you give them a chance, the cooler gases will fall out of the exhaust and exit faster than the warmer ones. This results in the warm gases staying in contact with your mass longer and better heat transfer.