RMH / Rocket Stove - what happens at the end of the burn cycle?

Hello to All,

I have not yet built an RMH, though I have built a couple small RS's for practice. I have followed this Permies forum and watched many YT videos ... some very helpful (thanks Paul, Ernie, and Erica), some rather comical, and some downright scary. I've created several RMH designs in my head and on paper ... J-Tube (thanks Ianto), Batch type (thanks Peter), and some utilizing bells for heat storage. Hopefully soon, when my circumstances permit, I will actually construct an RMH (or several).

My question today has to do with the "end-of-burn" in an RMH. I have never seen a video, or even found a discussion thread that addresses this. If it's out there, maybe someone can direct me to it. From what I understand, the power and draft of the RMH is driven by the hot, rapid burn of the wood, which makes for clean exhaust and little or no "smoke-back". What happens at the end of a burn cycle when the rocket is winding down? When there are only hot coals left in the burn chamber, what drives the heat riser? Is there a danger of exhaust coming back out the feed tube?

Thanks in advance for any help on this topic.

Peace,

-Thomas

Hi Thomas! I'm just a little further along on my RMH discovery path than you are; I'm in the process of my first build. I had the same question when I was in my research stage and couldn't find any answers either, so I ran some test fires to observe what happened. As far as I understand, the actual power (or engine, if you will) of the RMH comes from the draft created by the heat riser. The smokeless exhaust and high efficiency combustion is driven by that draft, not the other way around. As long as there is hot air being pulled through the riser, it should stay at a fairly high burn efficiency. Ive let my core burn down to coals about 5 times now, and the only time I had significant smokeback at the end of a burn is when I was using softwood for fuel and I overloaded it. Everytime I used hardwood the coals were hot enough that the gasses were still combusting in the burn tunnel until the very end, and even after combustion it was still pulling air through because of the preheated riser. No smokeback! I'm running a 6" J-tube system, and now I only burn apple and cherry wood cause it's the most abundant near me. I'm a lucky bastard, I know Sorry so longwinded, I guess the point is that in a well designed system the End of Burn shouldn't cause any issues at all.

I also forgot to mention that near the end of the burn cycle I partially close off the feed tube with a brick to maintain the heat and to help the draft as it burns down. I probably close off around 50% once all my fuel has dropped to the bottom of the feed tube.

Thanks Jeff, that's helpful information. I was suspecting that some type of shutoff for the feed tube would be needed. In Ianto's book, he shows a lid for his feed tube, but it's not completely sealed ... it has a small hole. This will be something I'll experiment with.

Peace,

-Thomas

Tom; As long as it is properly built, a fully dry warmed up rmh will burn every coal until its gone . Once your mass is warm it will maintain a constant up draft in your chimney all night. The purpose of covering your feed tube is to slow down that drafting after burning , to keep as much heat as possible in the mass overnight.  Some people use a brick to partially cover the feed tube to aid drafting while burning.

Thanks Thomas Rubino,

That makes sense. All this great information goes into my design process.

Peace,

-Thomas