I built my RMH to the spec.s I got from you free download. I tried it for the first time yesterday and it wouldn't draw at all. The space between the top of the bell and the heat riser is about 2.5 inches and it got a bit off center. Could this be the problem. Also it was windy outside and I haven't installed a cap at the end of the flew which comes out horizontal. Should I try to center the bell better or get rid of the bricks and make a stabilized perlite riser? Or what. Please help. I want this to work.

Well, two things, if your "chimney" is coming out of the wall, and not above the roof peak. You are asking for trouble. Even more if there is wind.


And i can't see how you made the transition from barrel to flue.

Ohhh, and i forgot. Insulate that vertical pipe, with glasswool batt. That would may be increase the temp differential, and get the thing to draw.

Other thing is ... HOW DRY is your mass ? wet mass will require apx. 6 weeks burning to totally dry out . During that time, especially the first few days you may need to help the burn with either a hand held fan blowing down the feed tube or a fan at the exhaust to help pull. After your mass drys , then you can see if your chimney arrangement will work. Personally I vote that you go indoor vertical thru the roof and then you will not have draft issues. off center riser is not your problem. Always cap your exhaust. Fireclay / perlite risers get my vote over brick , especially full size heavy brick. good luck keep us posted

Is there insulation around the feed tube, burn tunnel and riser? When affirmative, what is the space between the insulation and the barrel? Brick isn't insulative at all, so what's the nature of the insulation?

What is the ratio of feed, tunnel and riser? Is this at the very least 1 : 2 : 3.5 measured along the centerline?

Is the cross section area the same throughout with a lot of space (typical 200% of system size) in the transition area from barrel to horizontal flue?

Is there a vertical chimney (preferably insulated) coupled to the end of the horizontal flue which does reach all the way above the highest point of the roof?

All these are requirements, the J-tube rocket mass heater is an extreme kind of heater, it need to be done just right when you want it to work right out of the box.

Nearly forgot: your riser and feed seems to be dry stacked. This isn't any good, all the crevices, however small, need to be sealed.

I notice that your system is built on a wood floor. What kind of heat protection do you have? Just insulation will not keep the floorboards from getting hot over time and eventually burning. Insulation does not stop heat flow, it slows it down. You have to have some airspace under the mass and the insulated burn tunnel especially for heat to escape.

I see some styrofoam between the mass and the wall in one photo; styrofoam is combustible, and if it ever gets too hot you could have a real problem. Airflow is required to let heat dissipate.

Thanks everyone. Boy I thought I'd read everything, but you guys have increased my knowledge 10 fold. I won't say too late, but in order to institute all of your suggestions, I would have to start over, especially on the insulation parts. I don't have any insulation around the J tube, only cob. I insulted the redwood floor with 4 inches of perlite stabilized with cement and the wall with standard foam wall insulation. I guess I figured that if you can sit on the bench, the heat given off can't be great enough to start a fire. Taking the J tube apart and replacing the cob with stabilized perlite won't be hard. Replacing the riser with a stabilized perlite tube I can do also. I couldn't find 10" pipe and 8" was impossible to pack uniformly. I can replace the exposed flue pipe with double walled pipe. I don't want to wrap it in glass wool insulation being too ugly. If going straight up is the answer I'll do it, but it's a hole in the roof I didn't want.

Given that it looks like the ducts next to walls are the last in line from the combustion zone, the insulation there, both against walls and on floor, is probably going to be okay. How thick is the material below the ducts to the floorboards? The closer, the more hazard.

The insulation under the J-tube, no matter how good it is, is likely to transmit enough heat with long burns to be dangerous over time. (The ignition temperature of wood exposed to heat declines over time.) You might be fine for six months or a few years, then one winter your floor starts smoldering under the heater.

I think it is critical for you to take up the J-tube area and make air channels under it (space the mass up on something like widely-spaced bricks or metal channels), then put in your bottom insulation and rebuild the core with good insulation.


You wouldn't just wrap the flue in fiberglass, you would surround that with a larger sized duct. Something like a 10" wrapper for an 8" duct would give you 1" of fiberglass, which will make a significant difference and be quite inexpensive.

I tore out the J tube area and put in metal pipes to help disipate heat better. I packet this area in an average of 6" of stabilized
Perlite and increased the length of the heat riser to get a better length ratio as suggested. I haven't vented straight up through the roof or insulated the flue yet, but I plan to. I didn't try a burn yet, but will once I have a hair drier in hand to help start the flow. The cob is still wet being only a few weeks old. I was hoping to get the beast going to help in this process.