Minimum riser height for 6" RMH

Hi All,

I'm playing around with making a RMH and I had a couple of questions if you could assist me...

1. What's the minimum effective riser height for a 6" riser? I need this RMH to achieve quite high temps.

2. Can I have 6" feed and 6" riser and 6" exhaust? Or do I need to increase one of those components?

Many thanks for your help. Jim.

Maintaining a 6" CSA throughout will be no problem. Riser height depends on the construction materials used, how the system is built, and whether including gas mixing build modifications, chimney system design and etc. Tallest of 6" risers range from 44 ~ 48 inches. But there's lots more to consider. Here's "the book" every RS/RMH builder/tinkerer should have in his/her library. It addresses all the details of construction, materials choices, and also covers the RMH foundation right up to the top of the chimney:

http://www.newsociety.com/Books/R/The-Rocket-Mass-Heater-Builder-s-Guide

Thanks Byron. I'm actually looking for the shortest height, not the tallest height.

Any ideas??

Have you checked out Matt Walker's riser'less core design? -->

http://donkey32.proboards.com/thread/1624/walker-riser-less-core?page=1

I 'd like to bring this thread back to life because I have the same question as the original...   What is the SHORTEST riser height possible for a 6" J tube RMH?

I was thinking about trying to put the barrel kinda centered in the room to be a coffee table during the off season. How low can it be?

The standard formula is 1-2-4 so if you are not bothered about loading the stove every 10 minutes … a 10” feed tube with a 20” burn tunnel would match a 40” riser.
Then you have the thickness on the stove base and the clearance under the barrel top so around 4’ high…. to high for cooking or a coffee table!
It is possible to get that a bit lower, if you have a tall insulated and straight up chimney with no mass, then maybe you could get it down to 3’
But there are many other designs that can be built much lower.

Hi Jon,    You could also consider Matt Walkers Riserless Core

Thanks for the tips.  What if I sunk the core down into the earthen floor a foot or so? Would that be a problem?

That would be the best way to get the top of the riser not too high above the main floor. If you can dig down to set the core into the floor, why not just go farther so you can have a decent height to the feed tube to be able to use standard firewood?

A 1:1.5:3 ratio would give you a 16" deep feed, 24" burn tunnel floor, and 48" riser. This would yield a 34-36" high bell top if metal... not coffee table height, but you couldn't use that surface as a coffee table anyway, it will be too hot for anything but cooking. For a 6" system, you wouldn't need more than about a 14" diameter section to be tall; the rest of a coffee table/bench could be whatever height you want. To make the tall section comfortable to be near, you would need to add masonry walls around it giving a total diameter around 22" or so. A masonry screen with gaps for airflow could be thinner and still be comfortable.


If your chimney has a reliable strong draft, you could shorten the riser a few inches and still have it work well. My 8" J-tube has 16"/24"/45" dimensions, and with my constant positive draft chimney it runs beautifully. My experience with peepholes to watch the tops of rocket risers is that the flames in a finished system rarely reach the top of the riser at 45", so combustion is complete and you don't need more height unless the draft requires the boost.

All in all, I don't think it is a good idea to try to make the top over the riser configured like a coffee table, even in the off season. If it is convenient to put things on, someone will do it while the heater is running and cause a fire. Also, the core would be dangerously close to people sitting if it is close enough to ever use as a coffee table. A sunken feed tube in the middle of a living room would also be a hazard. Somebody will inadvertently step in it, maybe while the fire is out, but that could still be dangerous.

Thank you for the suggestions as well as the cautions. I am completely new to this so I don't know what I'm doing

My situation is that I'm looking to heat a very small space. Only about 8'x13'.   I'm finding that RMHs are not very easy to scale down, and a normal one will take up a lot of my very limited space.

Considering the small quarters, and clearances to the combustible walls, there's really no way to avoid having the hot object out in the middle of the room. I figured that if I can get the height of it down below 3ft or so, it might not be too intrusive.

Your warnings about the dangers of a RMH coffee table are well taken.  I might be able to get past them, considering that the space is a fairly private space, so there won't be a lot of unfamiliar visitors. I could fabricate a metal/masonry tabletop or barrel surround to further protect from accidental contact.   Maybe also a metal grate to cover the feed hole.

I'm not sure though.. certainly not ideal, but I haven't been able to figure out a better alternative.

How long of a bench are you thinking of for this 8' x 13' room? And what placement in the room?

I think you would have better more flexible space by integrating the riser surround into the bench, or partly into it. If the riser center is at the front edge of the bench, you would have a small bumpout, away from the back wall, and the feed at floor level could be tucked into the angle next to the bumpout so that it is not easy to walk into/over. Brick or cob wings could partly surround the hot metal surfaces to keep combustible things from accidentally touching metal.