RMH near the ceiling?

I want a rocket mass heater, but space is a big issue.

I have a 20'x20' garage in western Pennsylvania with what was once a carriage apartment above it. The upper floor is now an office and workshop, and the lower floor is almost entirely a workshop. It has no natural gas service, only electric, water and sewer.

Floor space is at a premium, but the ceiling is 9'6" high in back and 10' high near the garage door. The least valuable spaces are in the back corners and anywhere near the ceiling. The garage wall is heavy block up to 4' high, double-walled brick above that, and wood on the second floor. I added internal studs and R-13 fiberglass insulation to all of the walls. The roof and attic crawl space are insulated. The floor is heavy concrete.

I want the place to be heated pretty much all winter, although it is rare that both floors are occupied at the same time, and I mostly need to heat the upper floor. There are already vent holes in the upstairs floor and an internal stairway with a trap-door top between floors.

The rocket heater would not have to be the sole heater, although the less I have to use electric heat the better. I have access to free hardwood flooring scraps, and I can vent the steam through the brick. (There are already vent holes about 4' off the ground on the garage level.)

I guess my biggest problem would be with the "mass" aspect, as I have no spare room on the floor, and building a masssive heat sink along the ceiling could create weight problems. I can, however, support such a frame with poles running up from the floor and tied into the walls.

Dan Sulivan : Often when trying to adapt the Rocket Mass Heater into an existing place we wish the ceiling was a little higher! The distance to the nearest flammable object

for any wood stove should be 3' and thats with a reflective barrier mounted to the wall or ceiling to create a 4'' air channel ! So the closest your Barrel can get to the ceiling

is There feet, four inches (3',4''). You are not really saving any great amount of floor space just raising your Rocket up to the height where it will be very difficult to feed

it wood, and stratifying your Heat close to the Ceiling!

I am not telling you you can not do this, only that you are creating additional problems for you to discover fixes for !! As always we can only propose ways to help you with

your Rocket, you must make the decisions and do the work.

The Rocket Mass Heater RMH, Works Best when it is located close to hand, in the very center on your home, from there it is easy to tend to your Heater as you go about your

daily living tasks, your ear quickly tells you exactly how happy and well fed your RMH is.


Starting with Small, very dry, fine split wood, within 15 minutes of starting Your RMHs Firebrick in your Combustion Chamber should be glowing red hot, after that, as little as 1

Hour a day can provide 12 -24 hours of Heat ! Extreme cold may extend your burn time or you may do it twice a day !


Think of your Rocket as a fine piece of built in furniture, something to be used and passed on the future generations - - - Or think of it like a well endowed Bank account, one

that takes small deposits of wood (on your schedule) and Gives dividends in the form of Heat, Bone-warming heat you can get close to or sleep on !


Conversely, Any attempt to place aRMH in an out-of-the-way corner is kinda dangerous and will require you to stop,break your routine, and go check on Your rocket, never

arriving at the right moment, turning into a Drudges task that you will find excuses to avoid !


My best suggestion is to take a blank piece of paper, a plot out a keyhole shape about 5' by 3' and consider if perhaps you can place part or most of it underneath workbench

or other little used space .


Again the final Decisions are yours ! Come back often, we all started where you are now ! For the Crafts ! Big AL

As Allen says, you can't locate your RMH close enough to the ceiling to avoid obstructing the floor space, but you can make a very small footprint by constructing a tall narrow masonry bell to enclose your rocket core. It would be possible to contain an 8" core in a 30" x 48" space, maybe less if you squeeze very skillfully, with a bell rising to 8' and having its hottest surfaces near the top. A steel drum bell could be even more compact, at the cost of loss of thermal storage mass to spread out the heat generated for several hours. If your upper floor is capable of having a good-sized hole cut in the floor framing, and your slab is sturdy enough, you could even make a two-story bell which would heat the upper space directly.

I absolutely cannot place it permanently in the center of the room. That is needed for large projects like building recumbent tricycles or occasionally working on my car. What you seem to be telling me is that I cannot do it at all. Is there another low-pollution, high-efficiency heat system that can go in a corner? Is there a way to build it on wheels with a detachable exhaust?

Also, the part that would be in either corner does not have to be off the ground, and the bottom 3.5' of the back wall is concrete block with no wood or insulation, and is actually below ground. I hate heating the ground, but I can add some kind of non-flammable insulation outside the stove as well. Still, I am limited in how much of the corner I can give up.

Does this system scale down and still work properly, or is there a minimum size? As I have no heat at all, other than pricy electric, any heat is better than none.

An RMH works best in the center of a space, but can be built in many less-optimal configurations and still be worth doing. If you can put it along the center of one of the walls, that should still work well enough, at the cost of the far corners perhaps being cold sometimes. Arrange it so that it works for your equipment and working layout considering the heat distribution. If you do not rise through the upper floor, the exact location of the RMH becomes less important to the second floor space.

I would put very good insulation along the exterior wall in this case, allowing an airspace between insulation and wall to let it cool the far face of the insulation.


A 4" system has been found to be about the minimum workable size, but this needs to be built just right to work at all, and has a very small heating capacity even then. It is considered an advanced project. You are better with an 8" system for the kind of space you describe. Alternatively, a 6" batch box style is said to give similar heat output to an 8" J-tube, so as long as you build it right (batch boxes have certain specs that must be followed for good operation, and are more touchy to build than J-tubes), it could be a good option.

Dan Sullivan : I really like the idea of building ether a conventional Rocket Mass Heater in 6" or 8'' and your space certainly does not need a n 8'' System.

I have not yet built any RMH with a bell /bells which Can replace most of the Thermal mass so Glenn is a better man to talk about that !

Some people who try to build any rocket system smaller than 6'' have major problems and a few have given up on the sub 6'' units - mostly people who

tried to go small with there 1st D.I.Y. Rocket. Again this is Your project ! Big AL

You need to consider your climate and the space's insulation, infiltration, and usage patterns to arrive at the best type of system for you. If you will often be using the upstairs, a J-tube would not be suitable as you cannot regularly pay attention to it and feed it. A batch box, however, is something you load and start, then let burn for an hour without tending. Often a batch box only needs to be run once a day, or maybe twice.

The western PA climate and "shop" use suggest a need for more heat than the square footage might otherwise call for. A 6" batch box may be the best system for you. (An 8" batch box has been found to be a monster, sufficient to heat 2000 square feet of auditorium/shop in Montana.)

Yeah, it gets pretty cold here. I just found out about these heaters because I was complaining to a friend that I couldn't afford the heat I needed. Today has been a crash course for me.

The good thing about being near Pittsburgh is that it is a big industrial center. Lots of sources for firebrick, ceramic flues, sheet metal, machine shops and so on. Once I settle on a bell design and specs, I can get a sheet metal place to custom fold and crimp it into the shape I want at a reasonable price.

I think I got a good enough idea from y'all, and I will go back to studying, with an emphasis on using an extra-tall bell. I am also thinking of running a small fan into ductwork that will blow across the top of the bell and prevent heat from building up near the wood above the bell. I would be worried about the fan shutting down and the heater still blasting, but I think there are ways to automatically shut down the air intake if the power fails. I expect the greater danger is from the effects of prolonged heating anyhow, as this makes wood more brittle and flammable.

Anyhow, back to studying the basics. When I have a tentative design, I will post it. Thanks for the tips.

You can perhaps have a metal bell, but it will not hold heat except when the fire is burning. If you want to keep the space warmish overnight you need some mass in the bell. If you do want a metal bell, two steel drums stacked will give an effective, cheap and rugged system. It is also possible to cob or put other masonry mass around part of a bell like this to absorb and store some of the heat.

Have a look at this workshop heater.


https://permies.com/t/51057/rocket-stoves/build