TIny rocket mass heater build

Do you think its possible and how small could one go making a rocket mass heater. I'm talking one in a tiny house. I have a space that is about 18 " high and 48 long I could use. And it doesn't take much to heat a tiny house. What do you think and design ideas. I was thinking a 20 lb propane bottle and a 4 "burner. (well pipe or fire brick) into that and a firebrick run to about 48" away and out . I can corner the toothpick market maybe

Dave ; Think small but think 6" , no metal in burn areas , go with fire brick or a cast core. Get a copy of ianto evans book and read up. Small masses seem to work fine, but apparently a 4" system is problematical for a first timer. Give us some info about your location and the layout of where you want to place your rmh.

Dave Woods wrote:Do you think its possible and how small could one go making a rocket mass heater. I'm talking one in a tiny house. I have a space that is about 18 " high and 48 long I could use. And it doesn't take much to heat a tiny house. What do you think and design ideas. I was thinking a 20 lb propane bottle and a 4 "burner. (well pipe or fire brick) into that and a firebrick run to about 48" away and out . I can corner the toothpick market maybe

I think your area is too small for the standard RMH. In particular, the 18in. high. Can you say more about that? Is the space 18in. or is it 18in with space over top? The top of a rocket anything (in fact any wood burner) is the hottest place on it and you probably need close to 18in. between your heater and whatever is above it.

If you are using a 4in. chimney/riser, there may be difficulty having the flue go back down. I would instead set it up as a rocket stove and add any and all mass directly above so that the flue gas is always rising till it leaves. The riser for that setup needs to be about 10 inches high... it will not burn quite as clean as the standard RMH (that takes space) but it could do better than a metal box.

If you have the full 18in. height (with space over head) you may be able to even do a mini bell from cob covered brick. Let me do a drawing... you would need to have min, 12 inch depth I think... maybe more. 4 inch air space with heat shield next to the wall.

Ok, in the picture I have a pretty standard cooking rocket drawn in with two layers (at least) of brick over top of the riser. there is a gap, but that can be quite small so long as the exit into the "empty space" has the same (or a little greater) CSA as the riser. I would have to put real measurements in to know exactly. If I had my way the layer right above the flame would be a steel can full of tin as this should act as a better mass for the size than straight brick. In fact for this size, soapstone would be better than brick and could eliminate the amount of joins (potential leaks). The feed is horizontal... look up rocket stoves to understand the feed and separator... in fact start here. See how to build a rocket stove and then build the rest around it.

Let me know what doesn't make sense.... Len has been reading about these things too much and tends to assume you know what he does

Oops, couple of things to add... where the flue exits the box you want a good tight damper... think 100%, but be responsible, only close it when the fire is all the way out. Maybe empty the ashes before closing. Have a plug for closing the feed/intake air after a burn as well... close that first.

Second, this is a small heater and will get hotter than a normal high mass heater. I know this will sound counter intuitive but put a layer of insulation on the outside. Yes this will block the heat from the room there is a reason for that. Because you have less mass, the insulation will help it last a longer time after the fire is gone. The outside of the insulation should get to about 180 to 250F max. That way less of your heat will go straight through your walls to outside. None of your heater should be directly facing a window if you can avoid that because the heat will radiate directly outside. Rather something shinny (even foil on a board ... cardboard even) angled to put the heat where you will be most times.

Can you post a picture of the area you wish to fill with dimensions? We will do the best we can to make it safe for you. The small area worries me, to be honest, there needs to be space and insulation between hot things and walls/ceilings. Start with a diagram that shows safe distances for an iron wood stove and flue and see what is left... for something so small you need to stick to code I think.

I guess this is rattling around my head. Think of a space 18 high 24 deep and wide. the heat riser (4" sq tube) would be up in a 20lb propane tank. (its only about 12" high). This part would be encased in a metal box with and opening that would let the exhaust run horizontal for about 4ft. The horizontal could be a 6 inch well casing with a 4" pipe running inside. and out the wall. I'm thinking you would be burning like 3/4 inch sticks in it. I am thinking about making a mock up outside and playing with it to see how it works and how much heat is generated at different places. I guess in a sense it would almost be portible in two parts. I don't see it getting that hot with a small fire and limited air
Like I said its just rattling around in my head. but a little more since propane went up 50 cents a gal due to no inflation in the economy. Figure that??

Dave Woods wrote:I guess this is rattling around my head. Think of a space 18 high 24 deep and wide. the heat riser (4" sq tube) would be up in a 20lb propane tank. (its only about 12" high). This part would be encased in a metal box with and opening that would let the exhaust run horizontal for about 4ft. The horizontal could be a 6 inch well casing with a 4" pipe running inside. and out the wall. I'm thinking you would be burning like 3/4 inch sticks in it. I am thinking about making a mock up outside and playing with it to see how it works and how much heat is generated at different places. I guess in a sense it would almost be portible in two parts. I don't see it getting that hot with a small fire and limited air
Like I said its just rattling around in my head. but a little more since propane went up 50 cents a gal due to no inflation in the economy. Figure that??

Certainly try it, It would just be a rocket heater, no mass at all. The normal thing with a rocket is for it to run full with as much air as needed to get the hottest flame. For less heat use less fuel not less air. Less air just makes smoke. Also test the temperature of the exhaust... lower is mostly better, so long as it still drafts right. If you have a RV propane furnace, they are forced air and use that air flow to spread the heat around. A stove like you want to make only moves heat by radiation (there is a little convection too). You may wish to add a fan to move heat away from your burner as fast as you can, just like the furnace. It is easy to get a wood flame several hundred degrees hotter than the relatively cool propane flame your furnace uses. That is why wood burning appliances need greater clearances than gas.

But what if you used propane in a mass heater? As people get older wood is more difficult. Perhaps this has been addressed and I missed it? I imagine adding mass to any heat source is helpful, but is there a way to amp it up?

Welcome to permies, Christine. One of the greatest advantages of rocket mass heaters is the type and size of fuel they are suited to burning. Twigs, sticks, and offcuts are ideal. I don't think it's out of the question for an older person to collect dead branches and kindling as they walk around. If cutting is an issue, find a teenager or two and bribe them with baked goods.

A lot of what goes into my 4" RMH in the greenhouse is deadfall that I just pick up when I see it lying around. Unless something bad happens, I expect I'll still be able to do this at 70 or 80.

Hi Christine;
Unfortunately, propane needs a proper exhaust.
Around here it is a sure sign of aging when you switch from wood heat to propane heat.
With a small RMH an aging person could continue to burn wood, but probably would want a propane backup for the times you can not.

I have been thinking about small mass heaters.  Maybe not rocket styles, but a small mass heater.  I have seen the candle and terra cotta pot "heaters."  What if they had more mass and used a rocket-ish ventilation system?  Hear me out, a 10 lb can with a smaller can inside and filled the space between with mass then put that construction over another set of cans with vent holes with the candle in that.  It would kinda be like a j-tube and bell, but the mass would be the bell.  Then it could just sit on a coffee table and not produce enough exhaust to harm anyone, but be a personal heater.  Maybe even add some radiator fins to the exterior?  Just brainstorming I suppose.

A candle heater like you describe would store heat, but you would never get more heat than a single candle flame gives, i.e. it could be a handwarmer but would not actually warm a whole room unless that room was already so well insulated and sealed that body heat could warm it.