Rocket Stove Boiler v.1

I am presently in the design process for a rocket stove boiler for heating/hot water at my transition property. I will be posting sketches and notes in this thread for people to comment on and hopefully let me know what I am missing in my design before I commit resources to construction. The basics of the system are:

* 2x 55-gal plastic drums to function as radiators/heat batteries one of which will also function dual purpose as a hot water heater through an immersed heat exchanger that I can run my potable water through and back into the home water piping. I know. Plastic yuk, but the water inside will not be for drinking, just heat storage and radiation. Eventually I will transition to no plastic systems, but this is a transition property design so... Anyway, I have these barrels on hand from the previous owner and they will work great for now instead of me having to source and buy radiators to pump the hot water through.

* Water pump(s) capable of pushing the water from one of the tanks, through the boiler and back to the tank. It needs to happen at a flow rate high enough to keep steam from building up in the boiler heat exchange portion. Current thinking is 2.1gpm on 1/2" tubing "might" be enough.

* The rocket boiler itself. Many questions need answered here. I am using a modification of a simple rocket stove boiler design done by  Robert Murray-Smith on his YouTube channel. (Seen Here: https://youtu.be/GWKBWROF8P8 ) Either an 8" or 6" j-tube design, with an inverted propane tank cut out and wrapped around the burn tunnel at the elbow (his is not inverted) and a ways up the long arm of the J. Another modification for this unit from Mr. Murray-Smith's design is that the top including most of the heat exchanger, will then be cased in a bell (likely a portion of a 30gal or 55gal barrel) with exhaust ports near the bottom piped into flexible metal exhaust ducting and from there out the chimney of a wood burning fireplace.

* Current wip design has water flowing in and out through separate ports on the bottom of the heat exchanger (perhaps with internal baffles installed on the inside of the heat exchanger to ensure full mixing of water inside) below the bell. Still unsure how/if that is good or if I need to figure out how to come in/out from elsewhere. Bottom was chosen as I can leave a portion of the heat exchanger outside the exhaust bell and do my piping without having to figure out how to get the pipes through the bell and sealed. Also, steam buildup could potentially be an issue, so need to figure out how to mitigate that if so…

So that's the basics of what I have planned so far. Basic sketchs attached. If anything is unclear or if there's a glaring error I have committed already feel free to point out. Thanks much!!!

Oh I should mention in the boiler sketch, I left off the fuel feed Hopper from the middle and right versions. This was just to make it easier to sketch everything else. Pretend it's still there, though it's shape may yet evolve. Them batch boxes sure do look interesting... Lol

Hi Michael;
We do not normally encourage folks to try heating water with a rocket stove.
The potential for water to flash to steam and implode/explode the water lines is high.
Having said that.
Consider heating a tank of water with a batch box RMH inside a brick bell, it is a proven safe system.
Pumping that water into other tanks would be safe and easy.

I can think of a few quick problems you might have...

With a design that is completely dependent on flow of water to keep it from going to steam and exploding, I assume you have a back-up generator that comes on automatically a few seconds after the power goes out? This is especially so with a rocket stove that could potentially be charged with wood, start to roar, and then... the power goes out making the water in your pipes flash to steam.

You could also have the same problem with your pipes freezing if there is an extended outage and you have no automatic back-up electrical supply.

Another is having some sort of automatic make-up water. You will be surprised how many gallons of water are evaporated at temperatures above 212 degrees.

I can see why people would be inclined to think that a hydronic heating system, coupled with the design of a rocket mass heater would be the best of both worlds, but in reality, it seems it would be the opposite. It is also incredibly dangerous as steam explosions are extremely dangerous.

This is a great video as it is short, but shows exactly why boilers can be explosive so easily. Making one yourself is not only illegal, but for good reason...

Home Boiler Explosion

I have been watching this thread for a while.  Steve's notes of caution are well founded.

The volume of water past a certain point at a single point in time is what, I think, most people are looking for in a shower - ie, a good pressure, rather than the volume of water.
The best way to create the pressure is either to raise the head or use a pump which can break or lead to an explosion if the water boils.

We have a solar hot water system which works much the same way as a radiator in the car.  In the winter, the fire has a sleeve in the chimney which works using a thermosyphon.  The shepherd's crook acts as a relief so if the water boils, the steam is released, averting an increase in pressure.  Our header tank and  shepherd's crook are 10 M (30') in the air so we get really good pressure.  I think that a coil in a RMH can act as a water heater.  If the exhaust is 170 deg, it is plenty hot enough for hot water heating.  The picture below is our roof set up.  We have a 12 volt pump that tops up the header tank three times per week.

thomas rubino wrote:Hi Michael;
We do not normally encourage folks to try heating water with a rocket stove.
The potential for water to flash to steam and implode/explode the water lines is high.
Having said that.
Consider heating a tank of water with a batch box RMH inside a brick bell, it is a proven safe system.
Pumping that water into other tanks would be safe and easy.

Thank you very much for pointing all this out to me! I was aware of some of the risks inherent in a system like this and posted explicitly find out what I didn't know I didn't know. I have zero desire to create an accidental bomb. I appreciate your assistance in preventing me from doing so! Lol

Steve Zoma wrote:I can think of a few quick problems you might have...

With a design that is completely dependent on flow of water.

Thank you very much for sharing your wisdom and knowledge! I had considered that and thus the pumps in my v1 design were all 12v dc run off my battery bank. The plan was to never run this system unless well charged and have a flame cutoff and water flow/temp sensors, etc. I was trying to do it as safely as I could envision.

However, given all the excellent safety challenges and flaws you and others brought up I think I am going to shift gears on the "boiler" and work on an open air design for the water heat exchanger, a la the Dutch oven rapid boiler on the Lorena. Will upload v2 sketches once I make them.  Thank you once again! Me no wanna go boom... Lol

Paul Fookes wrote:I have been watching this thread for a while.  Steve's notes of caution are well founded.

Thank you for all your good input and the cool photo of your system! I had not heard of/seen that type of "Shepard's crook" before. Cool piece of tech. Will have to remember that if get into making a solar hw array later.

My transition property is grid tied at the moment and on a municipal water supply for potable water. I am running it through a filtration setup and then on through the house (single wide trailer) piping. My plan with this setup was to immerse a copper heat exchanger in one of the radiator tanks and run my potable water through the heat exchanger picking up heat from the tank before going into the hot water lines of the house.  This will be run at municipal water pressures on a separate loop from the Not-Boiler.

Something that might be a consideration is using the water as a mass and running it under the floor.

I have only read about this concept though I like it.

Anne Miller wrote:Something that might be a consideration is using the water as a mass and running it under the floor.

I have only read about this concept though I like it.

I have seen designs for an underfloor heating setup using hot water. They are very interesting. If I was doing a design for a permanent home I would definitely consider it.

This design is for use in a 1970's era single wide prefab home (trailer). It was a piece of property I acquired with the trailer on it that I am renovating to repair a bunch of mold issues and water damage, etc. and upgrading/replacing some of the systems. It's still a neat idea, thanks for sharing!

One important factor that I have not seen mentioned is that you are putting a heat exchanger around the burn tunnel/heat riser area. This is the opposite of correct rocket technology: it pulls heat away from the main combustion region cooling the fire, when what you want is the hottest possible combustion zone for complete combustion, FOLLOWED by a heat exchanger to harvest the fully combusted heat.

The water heating aspect is another matter, and there is a proven safe and effective method involving a circulating pressurized coil in a nonpressurized tank exposed to the heat.

Glenn Herbert wrote:One important factor that I have not seen mentioned is that you are putting a heat exchanger around the burn tunnel/heat riser area.

One of the many mistakes I learned of with this post. The original post was my first thought design based upon the video I referenced. The current iteration has the rocket engine heating an unpressurized tank of water with a "pressureized" heat exchanger loop of copper submerged in the "hot tank", much as you suggested. Thank you for helping to keep me from boomsquishing!

Room Air Temp = 65F
Floor Air Temp = 75F
Floor "slab" Temp = 85F
Pex Pipe Temp = 95F (1000ft containing 31galllons)
Circulating pump + Mixing Valve
Domestic Hot Water Tank at 140F, with pressure relieve

Now comes the hard part, how to extract heat from the RMH to heat the tank.
Wood Gasification Boiler: https://woodboilers.com/wood-furnace-heating-indoor-wood-boiler/froling-s3-turbo-wood-boiler/