Vertical Mass, Rocket Mass Heaters?

Hello All,
I am wondering how vertical mass, Rocket Mass Heaters would work? Could you duct all the way to the ceiling, then down, than up and out? Or would it have to be side to side over and over again, going up. Or maybe a corkscrew (this sounds like the easiest solution to me)?
How would the heat riser work? Do it need the entire barrel structure, or assuming we are going all the way to the ceiling, after a normal sized heat riser you simply leave off the insulation but continue going up? Instead of the 50 gallon barrel can you design a system that is 100% mass and 0 radiation heating?


What I was really thinking of was a Rocket Mass Boiler.  The design would built around reducing the floor space it takes up.

What I am thinking is like a hot water heater sized tank positioned directly above the heat riser. With some 1"-2" metal tubing inside for the fire, presumably about 3-6 of them to be equivalent to a 6'-8' duct. I want to minimize radiant heating. The tank would be hooked up to transfer the heated water around the house so it would be connected to quite a lot of mass.

It seems to me ideally, you would want the cap of the heat riser to be the tank itself (that is where it is hottest).  Than shunt the exhaust to the sides a few inches where it can go up  the corkscrews through the tank. When it gets to the top we put it out the chimney. But I am not sure if this is leaving out an integral part of the RMH system.

Does anyone have any experience with splitting the duct up into multiple smaller ducts (to increase the surface area)? Can you just replace a 6" duct work with 9 2" pipes? Other than greatly increasing the surface area, and moderately increasing the internal air friction they should be fairly equivalent.

Thanks for all your help.

You are asking a lot of questions, some of them are excluding others. To start with a sort of answer, a heated mass will also be radiating out, at a much lower temperature as compared to the well-known barrel construction. At that sort of low level the air molecules wouldn't heat up directly so it won't produce as much convection. A vertical mass is best implemented in brick, without ducts at all. I am a firm advocate of the technology which is known as a bell heat extractor. For a reasonably comprehensive explanation about the phenomenon see http://batchrocket.eu/en/building#belltheory. The workings of such a bell are entirely depending on the law of physics that hot air is lighter than cold air. What the above mentioned article don't explains is the fact that the excellent heat distribution is driven by another law: heat is always streaming from high to low, which means in this case the hot gases tend to give most of their thermal energy to the coldest spot around in the same horizontal plane. Until that is heated up sufficiently so another spot is coldest, and so forth. This goes on and on until there's an equilibrium reached or the laws of physics aren't valid anymore.

In short, for a vertical mass a bell construction is absolutely your best bet.

About your other questions: I fully intend to stay away from heating water with a rocket heater: too dangerous when done by an amateur. It has been done by other people with a varied degree of succes and sometimes the plumbing is ruptured or even the boiler itself explodes. The phase change from water to steam means the expansion factor is about 1500 times. Imagine every liter of water will be converted to 1.5 m³ of dangerously hot steam. In imperial measurement: every 2.11 US pint   turns into 396 US gallons of steam in a split second.

And oh yes, don't split up a duct into multiple smaller ducts, the gases always will follow the easiest route so part of that smaller ducts won't heat up at all.

When I read what you posted I thought of the rocket stoves used for cooking that are specialized for a cook pot of a specific size. The stove is insulated up to nearly the lip of the pot and the gap around the sides is optimized to allow enough air flow to get the fire running hot but close enough to maximize heat transfer.

Hello, and thank you

OK, so it works because it is vertical? You need duct work for the long benches they normal make, but they are simply unnecessary when you go vertical?

My father is a retired boiler system installer/maker. It has to heat water since it would be replacing/augmenting an existing conventional boiler.

If I can ever make this project work the multiple smaller tubes would be the easiest and most similar to how the professional boilers do it. I guess I will have to make sure the different routes are identical in drag/distance. But that Bell design might imply that I could just duct the exhaust into a large hollow in the center of the water tank. Then exhaust it out the bottom, presumably the intake and out-exhaust could be from the bottom (maybe a big "U", or just a big single hollow).

OK, so it works because it is vertical? You need duct work for the long benches they normal make, but they are simply unnecessary when you go vertical?

 It doesn't work just because it's vertical.  

You could make a 'Bell'  that is a long bench, if you design it with the right dimensions and it has the strength to handle the abuse of a bench, so you do not need ducting in a bench; you can use a bell there too.  

It works because hot air rises.  Hot air will rise to the top of your bell and that will be your hottest zone, and in order to flow out, the hot air will have to descend, and cool, thus being replaced by more hot air.  There are certain dimensions that you need to consider when building a bell, which involve the internal surface area that the heat will come into contact with.

Whatever you do, do not put metal pipes containing water anywhere near your burn chamber or heat riser.  These areas are far too hot for steel and will eventually (soon enough) cause the metal to fail.  If you are going to heat water, you will want it in your Bell, somewhere where it will not overheat.  Since your Father was a boilermaker, you should be able to figure out how and at what temperature to install the heat exchange pipes.   You should go to the link that Peter gave and read it thoroughly, if you have not done so already to figure out the bell.      

Thanks for the idea Duane Hylton, I will have to take another look at how those cookstoves are designed.

Roberto pokachinni wrote:Whatever you do, do not put metal pipes containing water anywhere near your burn chamber or heat riser.

Ya, I had just read that. Presumably, it could be a few inches above the heat riser, replacing the typical barrel, as unprotected metal is their in most designs?

http://donkey32.proboards.com/thread/1884/rocket-water-safety-system-version

http://donkey32.proboards.com/thread/1096/hot-water

 Presumably, it could be a few inches above the heat riser, replacing the typical barrel, as unprotected metal is their in most designs?

 While this is true, I would still consider that of all the metal in your barrel, that spot directly above your heat riser is likely to be the first metal to get overstressed and weaken and ultimately fail.  Many people have a snap on or clamp on lid in this position, which is easily replaceable.  Just saying.  I might be wrong, but I wouldn't put water there.

Great links Satamax Antone.

@Roberto pokachinni I was just thinking that. Perhaps, if I went with a design like that I could place some of the high-density (heat absorbing) Fire Bricks directly above the heat riser exhaust. That would allow decent convection to the tank, but take the brunt of the super high heat exhaust as well.

Is there a reason why you don't want to put the water pipes in the bell?  I would think that the inside of the top of the bell would be adequate to any needs you might have. It will likely be nearly as hot as it would be after being transferred through bricks above the riser.

Roberto pokachinni wrote:Is there a reason why you don't want to put the water pipes in the bell?  I would think that the inside of the top of the bell would be adequate to any needs you might have. It will likely be nearly as hot as it would be after being transferred through bricks above the riser.

I am mostly just stuck on the classic boiler setup. piping the exhaust through a water tank.


That, and a tank of water, with all the mass is better than just a few pipes. If we are talking 1000 degree exhaust, you really would not want the pump to stop for even a few seconds if you do not have a large mass of water to heat.
And I want to reduce the radiant heating, so that would basically mean insulating the bell vs just pumping the fire directly into the middle of a tank of water.

The one thing that comes to mind with incorporating that idea from your steam engine image with a Rocket Stove, is the Fly Ash, which might accumulate in those pipes that are directly off your heat source.  I don't know how this is addressed in the steam engine... perhaps with coal it doesn't happen.  

The multiple small tubes going through the water tank would take a lot of expert work to build securely. You would need many more of them than a direct cross section calculation implies, as the friction in a small tube is much greater than in a large tube.

By far the easiest to build would be a bell with a water tank inside it.

Do you have a chimney with good draft now? Also, where do you want to put this heater? They work best by far when near the center of the living space. Circulating water can be useful for heating remote areas when the building is not designed for single-source heat. Any other location will likely be significantly less efficient and possibly less effective.