Rocket mass heater for my shed

I have a 12x20 shed I wish to heat. The coldest it gets here is 0 but mostly stays around 20-30 during the winter. I've been reading a lot and I have the concept down I think but I don't want to to over do it. I'd like to build something that could keep the space warm efficiently while I work. My main concern is cost. I want to keep it down. Is a 55 gallon drum too big for this shed? What is the cheapest option for riser and firebox material. I have a ton of landscaping bricks laying around. Thanks guys.

If the landscaping bricks are the kind made by putting sand and a little binder under a lot of pressure, no. If they are regular old solid bricks that have been fired, yes. When I built my RMH, I got used bricks from a 100-year old schoolhouse that was being torn down. They work fine.

A 55-gallon drum may be too big for such a small space -- well, unless you plan to raise tropical plants in it during the winter. Another possibility is to use an old meat smoker, they are about 30 gallons in size.

I have watched a lot more videos and have been reading for hours. I understand construction a lot more now. I have a 55 gallon, 30 gallon and 16 gallon drum. Though it is my understanding that the thermal mass holds most of the heat. My biggest concern now is whether or not my shed can hold the weight. It's a prefab shed so it's built well but it is on a gravel and cinder block foundation.

Counting all the bricks and cob I used in my RMH (built with an old meat smoker), I figure it weighs maybe 500 pounds. That's with a footprint of maybe 4 square feet. Not too bad on floor loading, but if you are worried that the shed floor is going to sag, go put some concrete blocks under the area where you are going to build it and they can act as piers.

I was thinking of running the bench down the entire back wall. That's 20 feet.

The bench is just a lot more mass to hold a lot more heat. Unless you plan to live in the shed, you don't really need it releasing the heat it has held at 3 in the morning. If you are just working in it, an hour after you have fired the RMH up, it will be releasing enough heat into the room so that you can take your parka off and be comfortable.

I am finishing the shed as a permanent work place and man cave. It's for leather working mainly but I have 2 sheds and at some point I plan to move the shop to the other. I want this to be a really efficient heater and a room I could live in if I wanted. I am however leaning towards making the bench and stove span the 12ft wall instead. This would allow me to build in a small bed under the bench and shelves above it. I also had the idea that I could place 2 "clean outs" at the end of the bench. This way, if I later decided that I needed a bigger thermal mass, I could remove the clean outs, plug the space between them and extend the bench in the other direction.

Also, I am thinking that I should use the 16 gallon drum as the base for the riser(using the heat resistant concrete/perlite casting method) and the 30 gallon barrel instead of the 55. I'm open to suggestions and other ideas. I am not 100% sure how I'm going to build the firebox but I'm leaning towards fire bricks.

One thing you might want to consider in your studying is the traditional New Mexico kiva style fireplace. These adobe wood fireplaces scrunched into a corner can keep a room quite warm with all their thermal mass. There is no reason that you can't make it a hybrid of a kiva and a RMH, if you set your J-tube coming out of the corner and arrange your barrel and mass and exhaust ducting appropriately. If the exhaust piping ran along the short wall before exiting at the other corner, then you would have a sleeping bench that would retain a good amount of the heat.

I built my firebox out of the 100-year-old bricks and used refractory mortar (recipe can be found at many websites) to assemble them together. I left out the small drum and perlite insulation and just went with a brick riser. I suppose it is less efficient for doing so, but it still works fine. What I lost in riser efficiency I think I have made up by covering the whole RMH in cob. A steel drum lid with a fire burning right under it can get dangerously hot. Two inches of cob over the lid and even with the thing going full out, it doesn't get much above 130F.

Thanks. I will certainly look that up. I love the idea of covering the drum in cob as well. I assume this would work as even more thermal mass. I did kinda like the idea of having a small flat surface to heat a pot of tea on. What do think of embedding the bottom of a cast iron pot in the top?

Also, is there anything wrong with looping the exhaust pipe down the wall, back then down again before coming up for the chimney?

Thomas Burgess wrote:Also, is there anything wrong with looping the exhaust pipe down the wall, back then down again before coming up for the chimney?

As long as the riser works correctly, drawing air in and combusting efficiently, then the exhaust gases have enough momentum (otherwise known as "oomph") to carry 20 or 30 feet before being let loose to the outside.

What do think of embedding the bottom of a cast iron pot in the top?

Yes, I had thoughts along those lines. That would be a reason for leaving some steel of the lid exposed.

If you want one end of the shed warm and the other end cool, you could run the bench along an end wall. If you want the whole thing warm, I would put it along the long back wall, possibly with the feed and barrel near the center and the bench running to one corner. To get less drag, less weight and better heat transfer, you could take your 55 gallon barrel and split it in half lengthwise, and build a "half-barrel bell" in a bench 3' wide x 8' long. The outlet from the 35 gallon barrel goes into one end of the pair of half-barrels, the heat rises to the top, and as it cools it sinks to the bottom where the exit to the chimney is. The chimney can be at the same end so you can put in an adjustable bypass straight to it if you want to make starting easier or not heat the bench so much. I would check out Ernie & Erica's "Daybed" plans which give all the details you need.
http://www.ernieanderica.info/shop

Cobbing around the barrel will reduce the immediate heating effect while increasing storage, if you want to do that. It has been reported by some who did that that their systems lost some strength, due to the lessening of the cooling effect of the bare barrel after the heat riser. Also, if you cob the barrel completely over, it becomes impossible to remove it for inspection and cleaning. A good compromise might be to build a cob heat shield behind the barrel to protect the wall, leaving the barrel unencumbered. In any case, leaving the top bare will give you a nice cooking surface.

OK. That was my original plan (running along the back wall). I am still very far from being able to build it and I'm really still in the planning stages. I haven't even finished the inside yet. I'm just collecting materials and trying to savage as much as possible. Thanks again for the suggestions. I was actually just about to ask about removing cob once it had dried (if I needed to inspect it).

So I have a question: I've been told by a few people that a rocket stove without the thermal mass would be more than enough to heat my shed, it would just burn a little more wood and wouldn't keep the shed hot for hours. Why would the bench on the short side not keep the entire shed at a decent comfortable temperature. I'm going to be insulating it really well. I'm sorry, I don't want to be annoying. I know I don't understand everything yet. I've seen several videos of people heating greenhouses with rocket mass heater. They are very long greenhouses but the stove is on the short side.

If it's very well insulated for the climate, a mass radiator at one end could possibly make the whole thing comfortable, but one end will definitely be warmer. The RMH is principally a radiative and conductive heater, not an air (convective) heater. The farther you are from the radiator, the cooler you will be.

And a rocket stove without mass certainly could warm your shed, just as a woodstove or RMH would. You would still be warmer near the stove and cooler in the far corners.

I understand. I just figured that the barrel worked for "quick" heat and it's such a small space. I mean it's 240 square feet. 12x20. i mean my idea was that the barrel would keep the shop warm while I was there feeding the fire and the thermal mass would have a nice bed on it that would keep my butt warm when I slept. But again I haven't ever built one. I plan to do a lot of research and testing.

Yes, your figuring is correct. Research and testing are good things.

I Thanks. I appreciate your patience and advice. I just learned of these heater last night when I was looking into the option of installing a cast iron wood stove. I was looking at the cost of the stove and didn't consider the hidden costs of proper stove pipe and chimney. The efficiency and low temp exhaust are extremely attractive. I've been scrounging all kinds of materials to do this shed. I intend to make it a workshop for my leather working hobby and a living space as well.

Add a little mass to something like this? http://www.mha-net.org/visit-with-peter-van-den-berg/

I'm wondering what the most durable material is for constructing my riser. (At least something I can reasonably construct). I see a lot of different designs. I gather that anything metal will eventually burn out due to the high temperatures involved which seems to leave fire bricks or some concoction of refractory cement and perlite. Is there other options?

Peter has used vermiculite board for prototyping. IIRC he said it warped after a while. There's some people using rigidized superwool or other high temp fibrous materials. I use refractory clay tubing. Makes my life easier. Tho, they're not the best. And are hard to find in some pats of the world. You cn find tubes at foundry suppliers too.

The most durable (commonly available) I think would be firebrick splits (9" x 4 1/2" x 1 1/4"), with insulation surrounding it. You get the hard firebrick strength with no more mass than necessary. The most efficient would be a cast insulating refractory or perhaps some rigidized blanket insulation.

I just saw a craigslist posting for firebrick the other day. Only problem was they are all odd shapes and sizes according to the user. I do have 3 drums. How well do the casting methods hold up? I was thinking of using the 16 gallon drum along with a 6 inch pipe to cast the riser. I see a lot of folks using perlite and refractory concrete. Does this hold up well?

Unless the firebricks are mostly jagged fragments, you could probably put them together to make a good J-tube. It depends on how cheap they are, whether it's worth your time to play with them.

The 16 gallon drum and 6" duct would work fine. Perlite and fireclay with an outer metal shell make a good insulative yet durable riser with the right mix. Perlite and refractory cement would be stronger, but also a lot more expensive. For the feed tube and burn tunnel, the strength would be important, for the riser, not so much.

Would it be acceptable to make a base of normal landscaping bricks then build the j tube from fire bricks and make the riser from fireclay/perlite mixed topped with a 30 gallon drum? I want the system to last for years if possible.

Thomas Burgess wrote:Would it be acceptable to make a base of normal landscaping bricks then build the j tube from fire bricks and make the riser from fireclay/perlite mixed topped with a 30 gallon drum? I want the system to last for years if possible.

That sounds like a plan. Only thing I might suggest to add is a piece of black iron pipe in some unobtrusive place to act as a clean-out. Nothing like being able to take off a pipe cap to access parts of the system and get in with a shop-vac hose.

Thanks. That sounds great. I have a lot of time before next winter so I can build a few j tubes to test Outdoors.

I have two thoughts for an Rocket Mass Heater for a workshop. This is where I find the bum heater model does not suit my needs. Space is precious in a shop, almost no mater the size, a large couch is out of the question for me:

One idea was to make the bench a workbench. I already have a bench that works just fine in tactical mode. Vises, smooth maintainable surfaces for planing, etc... But I have several other benches that do things like support general fabrication or assembly, or support tooling like metal lathes. These benches could be more generic, and some of the surfaces could be metal and radiate heat, while other parts could be mass. Three options strike me:

A convertible bench. So for instance, it could use a drum in conventional form, but it would come off in the summer so that a plate could take it's place and return the bench to full function. If one's bench were 6-8 feet long, and 30 inches tall by 20-24 inches wide, it is not too hard to imagine a conventional construction of an Rocket Mass Heater, and therefor one is playing with proven concepts and materials, often site available. The bum warming equivalent surfaces on such a bench could be used when the bench was fired up as an Rocket Mass Heater, they could even accelerate processes that require heat in the shop. Some interesting fixtures and cavities recommend themselves there. But the drum section would be too hot for most, though not all functions.

Similar idea but in this case the bench is a functional bench and a functional Rocket Mass Heater simultaneously. I imagine such a bench with a steel top surface a bit like an industrial kitchen counter and storage unit, except that rather than storage, it contains the Rocket Mass Heater. Ebay has some interesting hardware that might be adapted to this cause, at low cost. Integrated into this unit is a fabricated alternative to a drum that would perform the drum functions while a part of the bench itself. This would require some modification to normal procedures, and it is assumed the top would at times be too hot to place work on, so while it is a bench and Rocket Mass Heater at all times it is not usable as the bench at all times. The large metal surface would create significant quick heat radiation that appeals in certain applications. However there are alternatives to steel that are excellent as radiating masses and work surfaces.

The last alternative that I have been toying with is basically 2, but with machinery supported on the bench functioning essentially like radiator fins. Certain tools could take the milder heat, such as drill presses, maybe lathes though the oil in them might not cooperate. The hot points would be too hot for anything other than tools that use heat, such as benders and steam boxes (steam is not contained).

My second idea is based on my shop that is 42'x 15'. I am not all that anxious to implement this now that my shop is built but were I building it again I might. Mid way down the shop put against a wall, a drum unit and feed tube, all that stuff, but built into the floor as required. Then put a tube through the floor 15 feetish as the mass (floor is poured concrete in my case). Then at the end of that there is the stack either through the wall or conventionally up through the roof. The heater would be like a You mostly in the floor from wall to wall.

The idea here, and it could be done many ways, is to just minimize the footprint. I gather this has been done in wood floors, don't know if it has been done in concrete. The channel would presumably have to be insulated so that ground and slab would not simply draw the heat to areas where it would not do the owner any good. But enough concrete (or other) mass would surround the tube to radiate and store significant heat. Basically half way down the slab, the slab would take a dip to form a trench and the Rocket Mass Heater would be built into the trench.

What say you all?

(Apparently you can't use a single letter but must use "you" even when the point is a shape. I also had to pull some formatting cues that were designed to make it easier to read)

For a workshop, a mass bench would often not be the best configuration, and there is no need to make an RMH in bench form anyway. Many shop heaters have been built as bells, metal or masonry depending on the fast vs. stored heat desire.

You can make a bell 3 feet square or less in masonry; a metal bell can be as small as a stack of barrels.


BTW, the forum doesn't enforce spelling; wherever you have spellcheck enabled, you ought to be able to turn it off.

I haven't decided what I want to do yet. I have 2 sheds. One is 16x16 and this one is 12x20. I am leaning towards finishing this one like a small cabin (full insulation, walls,ceiling, etc) and turning it into a leather working shop/bedroom/man cave. If I go thus route, a mass heater would be welcome because I'll have a bed in there anyways. Might as well make the bed sit on top of the mass.

Thomas, Just pasting the link below as a re-bump. I'm still using this design and it's working well for temporary shop heating, but uses a metal core which is frowned upon for long-life in a RMH.

https://permies.com/t/42529/rocket-stoves/FWIW-Rocket-stove-heater-concept