The End Of The Core
If you stop your build right here, and go no further, then you have a Rocket Stove. The temperatures at the top of the Heat Riser can be hotter than most ovens or stove tops, and you can bring even industrial soup pots to a boil in a hurry with this kind of energy. Rocket Stoves are remarkably important things, dramatically reducing fuel consumption and pollution created in 2nd and 3rd world countries.
These three components together — Feed Tube, Burn Tunnel, Heat Riser — are commonly referred to as the “Core” of the Rocket Mass Heater. However, it is the next series of components that converts this from a Rocket Stove, to a Rocket Mass Heater.
Barrel Cover / Radiator
In a Rocket Mass Heater, a “bell” is placed over the top of the Heat Riser, open side down. With 200mm / 8in diameter systems, a 210L / 50gal steel drum is commonly used. Whatever you choose for your system, it has to be able to contain temperatures that range from 600° C / 1112° F through 315° C / 600° F down to a “mere” 175° C / 350° F, over a span of 1m / 3ft.
Plastic is unlikely to be a good choice. Nor is aluminium.
A flow gap of approximately ⅓ the diameter of the system is left between the top of the Heat Riser and the top of the bell. The high-speed column of exhaust gases from the Heat Riser slam into the roof of the bell, commonly called the Barrel Cover, and then surge outwards in all directions like a mushroom head.
As this extremely hot column of gas moves along the underside of the roof of the Barrel Cover, it is bleeding heat into the Barrel Cover itself. As it reaches the joint between the roof and the walls of the barrel, it is already noticeably cooler compared to the muzzle of the Heat Riser. Having lost temperature, it begins to contract and thus become more dense, and then flow downwards between the outside wall of the Heat Riser and the inside wall of the Barrel Cover.
This creates another low-pressure zone in the system, dragging more gasses out of the Heat Riser, accelerating the gasses moving through the entire Core even further. By the time this sinking gas column has gotten to the bottom of the Barrel Cover, it is about a third of the temperature of the muzzle of the Heat Riser — “merely” hot enough to run a bread oven.
At the bottom of the Barrel Cover is the Manifold, the only route for the gasses being pressed upon from above to continue moving.
Insulation and air-sealing are commonly applied to the bottom third of the Barrel Cover. It is important that the exhaust gases have no way to escape the system here, save through the Manifold — particularly since they are essentially pure CO2, neutral atmospheric gases and super-hot steam at this point. None of which you want to be filling your home or greenhouse.
The more the Barrel Cover is insulated, the more heat energy will be pushed into the Thermal Storage Mass. Likewise, the wider the gap between the Heat Riser and the roof of the Barrel Cover, the more heat energy will be pushed into the Thermal Storage Mass.
The outside of the Barrel Cover is a radiant and convection heat source for the space around it.
Word Of Warning – It is not uncommon for the outside of the Barrel Cover to be hotter than a pan fresh out of a broiler oven … 260° C / 500° F.
Do not allow pets or children, or anything else you do not wish to cook, to come in contact with the Barrel Cover, ever.
A common safety tip is keep a full kettle sitting onto of the Barrel Cover. It visually clues people in that this is a hot surface.
The top of my own Rocket Mass Heater in my greenhouse has hit barrel-side temperatures of 450° F (230° C) and barrel-top temperatures of 600° F (315° C) for a period of an hour or more. That is a good indicator that my system is running as well as I can possibly hope for — hence the title of this blog series; “Searching For 600”.
The Manifold is the 90 degree union point between the Barrel Cover and the Thermal Storage Mass. Here, downward flowing exhaust gasses are turned horizontally again, to flow through the pipework embedded in the Thermal Storage Mass.
Cross section here is crucial. The way out here cannot be smaller than any point prior to it, or you will begin to create a back pressure of fire-extinguishing gasses. At some point, your system will actually “switch off” as the back pressure starves the after-burner process in the Heat Riser of needed oxygen.
A bit bigger is OK. It bit smaller is not.
Thermal Storage Mass
As the exhaust gasses enter this part of the Rocket Mass Heater, they are in the vicinity of 175° C / 350° F, plus or minus 28° C / 50° F. This is well below any conceivable combustion temperature, which means that you do not need steel stovepipe for this part of the equation.
The Thermal Storage Mass is a “heat energy battery” and what really drives the efficiency of Rocket Mass Heaters into the realm of magic.
A “U”-shaped loop of pipework extends away from the Core on the “towards room” side of what is usually a bench (in a home) or seedling table (in a greenhouse). At some distance between 3m to 4.5m, it does an about-face turn back towards the Manifold. This gives a total pipe run of between 6m / 20ft and 9m / 30ft.
Here, the pipe does a 90 degree vertical turn, and connects to the Exhaust Pipe, usually within 30cm / 1ft of the Barrel Cover.
The purpose of the pipework is to put as much of the flow of still plenty hot exhaust gasses in contact with something cooler and comparatively slow to heat. A very thick clay-sand mixture is a common material. Heat, transferring through the walls of the pipework, heats this material. This mixture itself, on all faces exposed to the air of the space to be heated, is usually covered in an adobe-like mixture of clay, sand and straw called “cob”.
Cob is a slight insulator, incidentally, and slows the release of heat out of the centre of the bench. For our purposes, that’s a good thing.
I have seen many examples where this thick cob outer layer has been inset with colourful tile, glass bottle bottoms, slabs of slate or other flat stone, or sculpted and painted. Whatever you do, make it yours. This is a permanent addition to your space, and you will likely be looking at it almost daily.
The “magic” here is the mass itself. A bench-like structure that is 45cm / 18” thick, 60cm / 2ft wide and 4.5m / 16ft long made primarily of clay will have a mass approaching 2000kg / 5000lb. This gives it a huge amount of inertia, in terms of the amount of energy required to raise it’s total temperature up or down 1 degree.
In the space of an hour, burning 1kg / 2.5lb of wood can be expected to yield a theoretical average of 2kW / 8600BTU. At a nominal weight of 11kg per 27L, or 28lb per cubic foot, of Canadian Pine, we can work out the energy in a bundle of firewood 20cm x 20cm x 45cm (8in x 8in x 18in) — common sizing for an 200mm / 8in Rocket Mass Heater. For perspective, that easily fits in a standard 19L / 5gal bucket.
That is around 7.5kg / 19lbs, which is 15kW or 163,400BTU. My 200mm / 8in (it) greenhouse Rocket Mass Heater will chew through a bundle that size in about 90 minutes. For those of you accustomed to space heating, that is a lot of available energy. The problem is that it is all compressed into a single hour of heating — that would make the room too hot to be pleasant.
Instead of being wasted by being vented outside for comfort, or forcing the operator to run several small burns throughout the day, instead, all of that energy goes into the Thermal Storage Mass. As the over-all temperature of the Thermal Storage Mass rises above room temperature, it begins radiating and convecting that heat into the room. However, it does so slowly, taking hours to push that heat into the room at a constant, comfortable rate.
The bigger the difference between the room temperature and the Thermal Storage Mass temperature, the faster the heat moves from one to the other. It is not uncommon to hear testimonials where a 2 to 3 hour burn with a 200mm / 8in Rocket Mass Heater has the Thermal Storage Mass still warm to the touch 18 to 24 hours later; still keeping the room comfortable.
I have seen one design where the Thermal Storage Mass was the “box” in a box-and-spring mattress in a master bedroom for a queen-sized bed. A wonderfully warm bed to climb into on a winter night, no matter what is going on outside.
Another design had the Thermal Storage Mass as the “U”-shaped seating around the dining room table.
Thermal Storage Mass works best when you heat the people, not the air around them. Use your Thermal Storage Mass as furniture, not obstruction.
The Exhaust Pipe is the last stretch for the now well-used exhaust gasses. At this point, in a well designed system, 90% of the heat energy from the Heat Riser “afterburner” has been extracted.
The gas temperatures should be above 101° C / 213° F so that the steam does not condense inside the system at any point.
However, anything higher than 110° C / 230° F, and you are “paying to heat the outdoors”, as my father’s saying goes.
You should run the Exhaust Pipe as straight up as you can, and ultimately be vented outdoors. Ensure you have a rain cover on it, or wind-guard if possible. Since the gas temperature is so low, you do not need to worry about it touching wood or the like — however, you do need to be conscious of what your personal risk / comfort zone is. A sheet-metal cuff or thimble where the pipe goes through a wall or ceiling can be inexpensive peace-of-mind for your home.