Portable SS Rocket mass Hot tub water heater

I'm picking up an old project and needed some guidance!

I wanted to design a portable rocket mass heater (26"x18) to heat up our horse trough tub.

There is a riser bypass for easy ignition, and the slider pushes in to activate the ceramic riser.

Wrapped around the riser is 1/2" stainless tube heat exchange.

I am mostly curious in ball park heat riser and chimney dimensions so I can slap together a prototype (considering that it's short)

also, would 1/4" 304 stainless survive the the horizontal chamber or should that be plated in ceramic as well?

Any help would be much appreciated!

Hi David;
What I think I'm seeing, is you're using  4" square tube.
You have a very tall (too tall) feed tube.
It appears you have a tall, possibly a  2" metal "chimney"?
Then you mention using the "slider" to activate the ceramic riser with the heat coils.


Is that all correct?

As to your questions,  as you will not be burning this like a home heater, I think your stainless tube will hold up for a good long while.
The riser dimensions must be the same or a bit larger than your burn tunnel.
The "chimney" seems like it is the third wheel... why is it there?
The burn tunnel would benefit from being wrapped with a ceramic blanket.

What is your plan to move that water thru the riser?
A pump?    Convection?

Be aware if the water stops moving, it will flash to steam, expand and blow out your copper coils...  spraying steam hot water, and shrapnel...
We call this phenomenon (Boom Squish )   something you do not want to see firsthand!

thomas rubino wrote:Hi David;
1. You have a very tall (too tall) feed tube.

2. The "chimney" seems like it is the third wheel... why is it there?

3. What is your plan to move that water thru the riser?
A pump?    Convection?

Thank you for your reply.

The current drawing is 4" .25 wall.
1. The long feed tube idea came from the Liberator rocket heater.

2. I believed the The Chimney height was important to create  draft through the system. 3" tube.

3. Water conveyance was thermosiphon.

The liberator stove attached has these heat fins on the burn chamber, attempting to dissipate heat build up. Is there any reason the burn chamber can't run cool and let the heat build up in the riser?

Hi David;
Let me start with why the Liberator has the cooling fins on the feed tube.
The liberator is a mass heater, by design, it is going to get hot. Without cooling fins, the metal would start warping quickly.
This is an unfortunate result of building a super hot stove out of metal.   Metal will always warp when overheated.
The liberator folks have done a super job of taking the RMH principle and adapting it to meet UL safety listing.
Unfortunately, it had to be made out of metal.  It cannot reach and maintain batch box temps without damage.

With a long feed tube made of metal, you risk heating to the point of combustion. Your tall feed tube could suddenly decide it wants to become a chimney.

The tall 3" pipe is highly unusual; personally, I can see no benefit from it at all.

So about cooling your burn tunnel... not recommended.  
With your J-Tube design, you only have the length of the burn tunnel and the riser to create the inferno that a rocket stove is famous for.
Trying to run cool in the burn tunnel is counterproductive to reaching the high temps you desire to heat your water.

I would put a pump on that water system, trusting thermosyphon is a gamble in my opinion.
You can make this work, just be careful around heating water it can hurt you very badly.

Keep us posted on your progress with lots of photos.

It's an interesting design, but I wonder if it's much more complicated than it needs to be. Before electricity, farmers here used a simple immersion heater in their stock tanks. The heater was a heavy cast iron tub that sat right in the water, with a downdraft design and a chimney of adequate height to maintain the draft once established. Both wood and large lump coal could be burned. I'm sure the idea could be adapted to modern materials.

Edit: a search for stock tank heater wood fired turns up some commercial options, lots of discussions, and an example of a DIY project: https://www.backwoodshome.com/build-a-wood-fired-stock-tank-heater/

Douglas Alpenstock wrote:Before electricity, farmers here used a simple immersion heater in their stock tanks. The heater was a heavy cast iron tub that sat right in the water, with a downdraft design and a chimney of adequate height to maintain the draft once established. Both wood and large lump coal could be burned. I'm sure the idea could be adapted to modern materials.

I believe the primary concern with emersion is sucking too much heat away from the combustion area and not getting a smokeless burn.

Also we would lose the ability to run a thermo syphon to cycle the water...