RMH in a partly underground round house made of earth bags

Maybe a little complicated.  If I have as earthbag house that is mostly underground - roof is covered with soil also....inside, before covering the earth bags...would it be efficient to make a rmh on the floor, right up against the earth bags....with a long flu (if that is what the exhaust tube is called) that is placed at a slight incline?  The earth bags would probably be covered with some kind of mass to protect the bags...the rmh would be 6" wide...and maybe put papercrete on the walls above the rmh against the earth bags.

Hello Tom,

Of course you can put in a rocket mass heater in a earthbag home.

Here's somethings you might care about:
-insulate the bottom of the rocket stove from the floor (some do this by using two layers of three hole bricks underneath the firebrick...not the best insulation but eco-friendly and passes air from the room through the bottom);
-insulate the mass around the flue from any outside wall (doing this in an eco friendly way takes some creativity like papercrete);
-papercrete on the wall around the barrel isn't such a great idea unless you've tested your paper create up to 1000F;
-no to the incline unless you decide to go with a stratification chamber and an exit flue a half inch from the barrel.

those are my thoughts!

good luck!

Hi Tom,    As Orin stated a RMH would be a great idea. Insulating the floor would also be a great thing to do as the Earth will constantly be sucking up heat that you produce and not give much of it back. A few simple ways (under the core is the most critical) to have either some air flow using bricks turned on their sides and covered with a cement backer board or making a perlite/clay foundation.  

The horizontal exhaust pipe can be put on a slight incline towards the vertical stack but is not absolutely necessary. You could also create a bell or stratification chamber which would eliminate all the horizontal piping. It can be pretty much be any shape you wish and tends to put much less drag on the exhaust. Let us know which one you go with as each one has their fine print to work with.

Not sure the temperature rating of papercrete and how close it can be to the combustion chamber but would probably think it would be fine in the heat extraction area.

I had a similar question.. didn't want to convolute the forums with a new thread, but I'm also curious about "rise" on the flue design.
I am building a rocket heater in an external rock/cob room and hoping to integrate the flue into a trombe wall of sorts and vent warm air into my primary living space.
It's a small 4" set-up with a 30 gallon bell.
My plan is to pipe the exhaust with maybe 2 elbows from the bottom to top (imagine a cob bench turned on it's side.)
Does the flue/exhaust pipe necessarily need to run on a horizontal plane or is a more vertical design just as efficient? I suppose I won't know what kind of draw it will have until built.
Thanks for any advice!

What your describing Nicolas sounds more like a bell or stratification chamber than a piped system for your heat exchanger.
I'm assuming your referring the 30 gallon "bell" is your barrel.
Efficiency can be better in a bell than a piped system as the gasses (hot and cold) don't all flow together but rather have a chance to separate out and release more of the heat before exiting the chimney stack. Bells also don't put any drag on the exhaust so they can maintain better overall draft.

Gerry Parent wrote:What your describing Nicolas sounds more like a bell or stratification chamber than a piped system for your heat exchanger.
I'm assuming your referring the 30 gallon "bell" is your barrel.
Efficiency can be better in a bell than a piped system as the gasses (hot and cold) don't all flow together but rather have a chance to separate out and release more of the heat before exiting the chimney stack. Bells also don't put any drag on the exhaust so they can maintain better overall draft.

Okay, I think I get your drift (or is it draft ;p). But I'm assuming there's still some amount of heat to be grabbed from the chimney stack as well, no?? Thanks for the quick reply.. It's definitely hard to conceptualize the working results without the thing built, but that helps a lot! Hope you keep warm up there.

Around 200F and up to 300F is considered an ideal stack temp (after the system is all warmed up) in order to maintain good draft in all weather conditions.
Sure, you can go lower for more efficiency but at the risk of smoke-back and possibly not having enough 'rocket' to maintain a hot and clean burning fire.
So you don't want to steal too much from the chimney stack but certainly extract enough so that your not wasting more than necessary.

Its been hanging around 70-80F during the day and cooling off around 40F at night so still enjoying summer here. Looking forward to waking up the shop dragon though!

You mention a 4" system. Is this a J-tube or a batch box? J-tubes smaller than 6" tend to be unreliable and hard to use, and one would give a very small amount of heat - not 2/3 of a 6", but probably closer to one third. Batch boxes do scale down, though again the heat output would be less than half of a 6" bb.

Glenn Herbert wrote:You mention a 4" system. Is this a J-tube or a batch box? J-tubes smaller than 6" tend to be unreliable and hard to use, and one would give a very small amount of heat - not 2/3 of a 6", but probably closer to one third. Batch boxes do scale down, though again the heat output would be less than half of a 6" bb.

Yes, a J-Tube (Dragon Heater Core). I have to say, it looks better than anything I could've built myself. Also, only looking to supplement propane heat in a very small living space. The primary function will be to keep a water reservoir from freezing. If I can warm up a mass of cob and stone, that would be ideal, but I don't expect to use it as a primary heat source.

A 4" Dragon Heater core should function as well as one that size can, but the heat output will be small enough that I would want to ensure good draft, especially at startup. If the mass gets cold and the outside air is warmer, it might be impossible to start without major smokeback. I would incorporate a bypass that allows a short, direct exhaust path going up, so that the system can be reliably lit. Once going, it may be able to handle some zigzag ducting through a mass. I would definitely go for a hollow brick box rather than a duct running through masonry, though, to reduce drag as much as possible.

A 6" J-tube system is supposed to be good for 35' of horizontal, minus 5' for every 90 degree elbow, so I think a 4" system would be good for no more than 20' of straight run 4" duct, maybe only 15', minus allowances for elbows. In other words, not much length. A bell cavity will have much less danger of friction restricting your draft.

Glenn Herbert wrote:A 4" Dragon Heater core should function as well as one that size can, but the heat output will be small enough that I would want to ensure good draft, especially at startup. If the mass gets cold and the outside air is warmer, it might be impossible to start without major smokeback. I would incorporate a bypass that allows a short, direct exhaust path going up, so that the system can be reliably lit. Once going, it may be able to handle some zigzag ducting through a mass. I would definitely go for a hollow brick box rather than a duct running through masonry, though, to reduce drag as much as possible.

A 6" J-tube system is supposed to be good for 35' of horizontal, minus 5' for every 90 degree elbow, so I think a 4" system would be good for no more than 20' of straight run 4" duct, maybe only 15', minus allowances for elbows. In other words, not much length. A bell cavity will have much less danger of friction restricting your draft.

Great advice, thanks! I will probably go with something along the lines of a brick box. I have about 4' of this wall built and was at the point of considering ducting, but that's an engineering catastrophe waiting to happen. I will consider draft / combustion air for sure. Probly a few weeks out, but I'll post pics if and when it is successful lol : ) Appreciate all the info on these boards!