Miniature RMH style Rocket Heater

Hey all,

At risk of sounding like a broken record, I've not yet built a RMH and have an idea. I'm trying to build a prototype just to aid in my own education. I have an emptied 20 lb propane tank that I will use for my bell. I will cut a 3" or larger hole in the bottom for the heat riser. I plan to cut an identical size hole near the bottom of the bell for the exhaust. I plan to use thinwall conduit or 3" exhaust pipe for the burn tunnel and heat riser, keeping the top of the heat riser about 1 1/2" from the top of the bell. I know using regular steel won't hold up, but this is strictly an experiment. I don't expect much in the way of performance, but I do want to be able to burn sticks similar to the way I can with a "soup can" Rocket Stove. I figure it's worthwhile to experiment a little, maybe even use the heater in my fishing shanty until I burn it out.

I don't expect this to be a long-term appliance, nor do I expect to do much more than build a really cool miniature wood stove. I've seen a lot of discussion on RMH design not working very well below a 6" burn chamber/ heat riser. Understanding my low expectations, does anybody have any advice? I will try the build anyway, but I hate to overlook something obvious.

Thanks, this forum has been made great by great people.

Dan

A friendly exhaust shop might let you scavenge their junk pile for a piece of 3" stainless, might find a good shape and it would last longer.

Dan Henn wrote:Hey all,

I figure it's worthwhile to experiment a little, maybe even use the heater in my fishing shanty until I burn it out.

Which, the heater, or the shanty?

: )

You mentioned it being a cool woodstove. Part of what keeps a woodstove "safe" is that the so much hot air is exiting the system that it carries with it any toxic/explosive gasses.

I have not built a stove yet, but I have been doing quite a lot of research, and the thing that concerns me most about trying alternative designs is that they have not been tested by anyone, so basically, I would be the guinnea pig.

Building an experiment that doesn't work will aid in your education. Building an experiment that injures you or someone else I suppose is a type of education too.

I saw a guy on youtube who built what appeared to be a functioning 3 inch heater in a tank of some kind, but he used a lot of the tested principles of RMH. He insulated the feed tube and burn chamber and riser really well. He mentioned that he initially planned on using just use steel pipe, but decided to use firebrick instead after so many people warned him away from the steel pipe.

His design also had a REALLY tall riser. Much taller than your propane tank would allow.

He also was following formulas from Ian Ianto's book pertaining to the areas of the different sections of the stove and how they should relate to each other in order to keep the stove functioning safely.

His stove APPEARED to meet the minimum requirements of functioning safely, but probably would not fare so well if tested for efficiency.

Since I am new to the math, and have no experience, I dont want to advise you on any part of your build. But definitely look in to exhaust ratios, you don't want a constricted airpath. Hopefully the worse case scenario is that your stove just wont work, or will function as a smoke generating device.

In any case, it doesn't hurt to do lots of research before you build anything.

You can learn a whole lot by researching before you even touch your tools.

Bob Jackson wrote:A friendly exhaust shop might let you scavenge their junk pile for a piece of 3" stainless, might find a good shape and it would last longer.

That's a pretty good idea!

Regarding Jacob's comments - all taken in good faith. My main goal is to get something that works for the specific goal I set, this one being a vessel with a closed combustion chamber. I'm simply expecting to burn small sticks and other fuel similar to a rocket stove used for cooking, but if done right it will contain the smoke gasses and vent them how and where I want.

The goes-out must handle the goes-in so the obvious starting point is to make intake and exhaust the same size. This should work in regard to air in = air out.

Picture a 20 lb propane tank safely decommissioned, valve removed. Where the valve once was, cut a 3" hole for the steel riser, weld riser to tank about 1 1/2" from bottom of tank. Flip over tank, add legs as required, add burn tube, J-tube and elbow from burn tube to heat riser. Near the bottom of the tank (formerly the top) cut another 3" hole for the exhaust and weld in a bulkhead/ pipe stub. Then add an elbow and enough vertical pipe to get comfortably above the level of the top of the heat riser and it should be ready for testing.

The design is crude, uninsulated and likely inefficient. However, before I attempt to build a full-on RMH with insulated burn tube and heat riser and thermal battery bench, I'd like to know that I can make a gas-tight vessel that applies at least one principle, that being the rocket action. The added benefit of using the propane tank as a bell (hopefully) is to allow for a greater area for the heat from exhaust gases to radiate out before being vented to the atmosphere. Crude, but a baby-step if you will.

Thus far, the scariest part of the process has been emptying the propane tank safely and burning off the residual gas that remains in the tank. Fortunately I didn't create a bomb and the remainder of the project shouldn't have anything near the risk level as decommissioning the tank.

Thanks for the input,
Dan

Maybe someone with experience can verify this for you, but I think you will want less pressure on the back end of the system to help gasses move through the system.

Especially if you will be having such a constricted system.

maybe 4 inch exhaust would be good, and that also happens to be a real stove pipe size, so finding suitable material would be easy.

The only thing I wonder about by your description, and maybe my lack of knowledge is making me overly cautious, but if your stove doesnt move air through the system, and you try and start it up cold, so you got a cold stove, starting with a really cold burn and not much is happening, I just hope woodgas type of vapor does not accumulate in your tank while fire is smoldering/ going out. Then when you try and light the stove it causes a problem.

The lack of insulation and tall riser combined with your tiny system diameter it seems like there wont really be much physics to move things along in your system.

There does not seem to be much of a "heat pump" effect. Without the space in your tank being cooler than the air going up your riser, what will suck the air along?

The stove works on pressure differentials. I am not sure your system will have any.

maybe you can build a mock up without welding it first and see what happens.

Dont forget to add the exhaust in the mockup and see if gasses will move all the way through and out the chimney.

Dan Henn : Any sub six inch system is problematical, a 4'' system that goes exhausts vertically immediately after the barrel, will nearly always work, but will
not store any heat, we usually figure that 40 % of the heat energy is radiated off of the barrel and 60% should go to the Thermal Mass.

With a well primed system a 4'' Rocket W/Thermal mass is difficult, but not impossible, the best plan, it should be put off s a 2nd, or 3rd RMH Build, the last
thing anyone here wants is to point you down a path so likely to end in failure !

The only well working system smaller than 4'' was a one off unit built like a 3'' Rocket cook stove with a open funnel shaped hood over it Like a Lab's fume
hood or a hood in a shop or the range hood over your stove, with a gap of about 4.5''- 5'' just big enough to allow a small fry pan or grill to be inserted into
the gap and still have the hot cooking gases rise through the 4'' naturally vented exhaust stove pipe. In an average Ice fishing shack there should be enough
air exchange to allow the safe operation of such a unit, the one i saw is used out doors, on the back kitchen/pantry porch about 8 months out of the year

Good luck, i hope that this was timely and helps ! Big AL

Allen - Thanks for the tips. I have been tinkering around with various rocket stoves for the past year and have been learning how things behave. One of my favorite little projects was a wood gas stove, I think a TLUD stove. All these different gizmos are very fickle and my clumbsy nature doesn't work well on it's own so I've had to pay much greater attention to detail than I originally thought. With this little project, I'll likely tack-weld things in place and test it out in the wide open outdoors. I was considering a 3" inlet and a 4" outlet and Jacob suggested that as well. Having the stack run vertically was another thing I was somewhat certain on to avoid failure. So all the suggestions are welcomed.

I think I'll need to draw up a diagram to illustrate my plan. I'll also need to take some photos to post later. I'm hoping this experiment ends up being fun if nothing else and if I'm lucky it'll burn wood as well. If things go well, I'll move on to a larger scale project with proper materials and insulated components, but what you said about most of the heat going out the stack sounds reasonable.

Thanks again guys,
Dan