Rocket mass heater design performance analysis

Is anyone doing detailed testing of RMHs? I think that if the data was collected and shared supporting RMHs many more people would jump into the innovation cycle. The cycle should be performance and user based. Many thanks to many people for uploading videos of rocket stoves designs, problems, and re-designes.
Here is an infrared of a short burn test of my Proto RMH.

- o.k. you've got My attention degrees in centigrade,Right! more info please!

I am sorry the temperature readings are in Fahrenheit. The detailed reports are PDF files as soon as i can get them to upload i will. The small pictures are hard to interpret.

The short burn tests:

Rocket stove: 1/2 pound oak log split into 8 parts burned in 25 minutes. Picture taken at 30 minutes. Little or no thermal mass stove exhaust designed to fit into picture to show exhaust heat
Barrel was tuned to a light rocket sound with no smoke in exhaust. exhaust temperature averaged 80 degrees F. Outside air temp 38 degrees F (stove is outisde)

Buck stove: 1/2 pound oak log split into 8 parts burned over 45 minutes. Picture taken at 30 minutes. After ignition, dampers tuned till the exhaust was around 80 degrees. very little smoke leaving exhaust. auto blower disabled to minimize heat leaving metal shell of stove. Outside air temp 38 degrees F (stove is inside 62 degrees F)


It is hard to compare the two images. It looks like the Buck stove got hotter in the flu and the rocket stove is really hot in the burn chamber and dissapates that heat to the large barrel fairly evenly in a short burn.
Anyone have any ideas? I can keep trying to get better images of longer burns if it could help.

- i'm a little surprised that the Rocket stove did that well not having a chance to run on prewarmed room air, but you at least proved that some one could go away/come back to a cold house and the stove would still light, can you report on your lighting technique ?

allen lumley wrote:- i'm a little surprised that the Rocket stove did that well not having a chance to run on prewarmed room air, but you at least proved that some one could go away/come back to a cold house and the stove would still light, can you report on your lighting technique ?

This rocket stove is for a greenhouse. Because of efficiency reasons the rocket stove will burn outside air. Burning inside air and exhausting it causes cold outside air to come in to replace the exhausted air.

Lighting: I make a small pile of DRY wood varying in size from minute twigs to 1/2 inch splinters about the size of the fire box(kindling). I fill the bottom of the fire box with DRY crumpled paper and cardboard. Light the paper, wait till it really catches then blow hard toward the J tube. After the flames steadily head that direction, I drop a pile of mixed size kindling in the chamber and away it goes. The colder the outside air the faster the rocket drafts if it is built right. The draft is better because the difference between the barrel inside temp and outside temp is greater (rapid heat dissipation).

This is my lessons learned drawing:

Josef : is your Heat riser at least three Xs the height of your wood feed!?!, So you have outside air but you are carrying your wood in from outside to feed Your Rocket ? using wood stored inside green house cuts down on green house room and is problematic for keeping your rocket going during operation, and not a good thing for starting your rocket when you want the dryest wood possible !

If you are worried now about condensate (due to high humidity in outside air or wet soil) I leave to your imagination what will happen with the combination of water and the lye in your wood ashes making a LYE Slurry -brick mortar and stove pipe ! G'luck Allen L.

Yeah it is about 3x taller. I flow air through my grate so i don't need the burn chamber to be designed like other RMHs. I would like the riser be as tall as humanly possible, but i was lazy. Ideally Dry Wood is stored outside under a shed roof so that we don't have to open the greenhouse doors to feed the stove. I don't see any wood ash where we have heavy condensate. It is dry everywhere in the fire chamber and all the way untill the air begins to condensate in the ducts under the thermal mass. I put drains everywhere! I now have to make a small roof to protect the fire chamber from snow/rain though.

I would like to throw out there that a regular buck stove could give you similar if not better results if it were design right. although i chose the rocket stove because i could make it for free, others might have the money to spend on a better system.

Upon a cold start up, naturally the burn tunnel mass is cold, and I think your infrared pictures prove that, it would be interesting to see the burn tunnel mass infrared picture after the RMH has been run for 48 continuous hours (I would expect the mass temp. to rise, but I think most of your heat being consumed to the massive cooling effect of the earth the RMH is cut into & built upon).

If you could temporarily uncover the mass that is covering the burn tunnel roof, that might show you were most of the RMH heat is concentrated. The sides of the burn tunnel also absorb heat, but mostly I'm expecting in the roof bricks...if your simply looking for were the high temperatures are.

I think the heat riser is not even fully warmed up in a short 30 min. run & the burn tunnel mass is absorbing most of the heat, as it should, but cannot be seen because of the earth/ mass that covers it. But because your unit is outdoors, overcoming natural ground temperatures is an overwhelming task (nature provides more continuous freeze & moisture, than your fire can overcome). So it would be interesting to see thru the use of your infrared picture, just how far into the ground/mass does the heat penetrate, after a prolonged hard run?

Nice pictures!

James Beam

I'm actually surprised how cool the barrel is in the picture. My IR thermometer maxes out at 500F about 3/4" up the barrel and it's around 300 at the base. With my pellet feeder, it's up to temperature in about 15 minutes.

Looking at your drawing, I think you're going to run into problems drawing all your air through the grate....it will get clogged with coals and ash...been there, done that. If your thermo image is based on this design, it may explain when it's running so cool.

I know your diagram isn't to scale, but IMHO, your burn tunnel looks too long.

josef: when i type this I want to stick on a couple of e's at the end and don't know why !

Do you now have a way to close off your air supply through the grate and do you have one for the Feed Tube ?

I Know you said your picture was not to scale, but what I see looks like you should be having thermo-syphoning problems!

With air supply through your grate and Feed Tube , unless you have a way to shut them off I'm afraid that right after your burn is complete your cold outside air will continue to travel through the system picking up heat energy and blowing it outdoors !

The J-Bend Feed tube coupled to the Heat Riser alone, by itself is supposed to work like the water trap under your sink, stopping thermo-syphoning when the fire goes out !

That probably will work if the outside air to/through your grate is closed off.

Every knowledgeable person I talk to tells me that these two locations are the only where any kind off a damper to close off air flow should ever be used. I'm passing that on as a 'General Knowledge ' Pyro-AL

I have been playing with shut offs on both the feeder(a washtub) and on the air intake(an old damper). Surprisingly the grate never gets plugged. As the ash burns it gets smaller and falls through. Eventually it would plug but it is easy to rake it clear with a hoe when you load the feed tube. You all are right the diagram is not to scale and the outside air will infiltrate the system after the fire dies.

Based on this input I am going to take it apart and redesign with full thermal mass. After that I will burn for 5 hrs and retake a thermal image. I know I can get 500 degrees in 5hrs. We then can see how much heat is absorbed into the mass. Then I will take pictures every hour to show how slowly the heat dissipates into my WV clay earth.

Installing this into the 2000 dollar sollex greenhouse is nerve racking. Move over chickens here comes a rocket stove!

Josef,

Wondering if you have any pictures of the stove. It's on the outside of the greenhouse? I don't understand that.

I am writing a book on energy-efficient greenhouse design and looking for more examples of rocket stoves. So would love to get in touch.

Thanks
Lindsey

I am sorry we have demolished the rocket stove. It took up to much space but wasn't much value in our operation. The simple diagram above shows that the air intake is from outside but the barrel was inside. I know its hard to see but the lines on the sides represent the greenhouse enclosure. Many people don't understand that if your intake air is from inside the greenhouse the air that replaces that intake air will be outside air temp say 35 degrees unless your exhaust enters the greenhouse (which is really dangerous). Ultimately your stove should not make a negative non pressurized greenhouse.

If anyone has a real science based article that discusses rocket mass vs other systems please let me know. From what I see and experience an old fashioned buck stove with a tall exhaust pipe inside the greenhouse seems to do much better that the RMH.

I have ultimately found that a wood fire hydronic system is the best for me. I like the simplicity of the wood fired boiler paired with the versatility of water heat storage and delivery. I state that solar water pump systems are not low tech, cheap, or easy. Water mass systems in greenhouse just work better. I also like having a way to use my well water for cooling and heat storage. A greenhouse that is 75 degrees during the day can store a lot in water to release on a 35 degree night and that is all I need.
Unfortunately I am in the process of a rebuild and I hope the new system cost will have an good return on investment. I am hoping to use DC solar pumps and aquaculture combined in my thermal mass. I should have some pictures of that by Spring meanwhile here is a simple diagram without the wood fired boiler and PV solar pumps.

Readers should note that the description of this system shows several departures from standard tested RMH design, such as the greatly varying cross section through the length of the system, and the lower air intake instead of the feed tube regulating the draft, not to mention the exterior/interior layout. Thus, I don't believe it is an accurate test of the standard RMH design.

Josef,

Thanks much for the information. I understand your concern about the air infiltration and think it's wise to avoid that. Why was it not a value? Not enough heat given off because of the cold air intake? I posted that question in a another thread to see if people have had other experiences. I have seen many rocket mass heaters in greenhouses but don't know the specifics on their intake or performance.

On the solar hot water system, very intrigued. To clarify, your water barrels are housed in an insulated outbuilding? I've also been researching solar hot water, but like the idea of using the actual grow beds soil underground as the mass. Without an aquaponics set up that is a lot of storage space for water -- seems too space intensive. Complicated control system too (controls in the water tank, greenhouse and fish tanks). It seems you need a third heat exchanger coil to go to the aquaponics set up. Water can't be directly circulated with the sludgy fish waste water.

Would love to continue the conversation. I don't know if this thread is the best. I am doing my own research, I think I mentioned, on sustainable greenhouse heating systems for a book. Hope we can swap more ideas.

Lindsey

I don't have hard numbers on hand, but have read about the relative air quantities for feeding a RMH vs. ordinary infiltration and healthful air changes per hour, and it appears that an ordinary RMH would use less fresh air than what you would normally expect in a house - a greenhouse would probably be leakier unless specially built.

1st Anyone reading this thread ask yourself this question: Do I want to spend any money? No, then learn to make a working rocket stove. Its worth every penny. Watch out! RMH proponents are now trying to make money off this "ok" option. So be weary.

Everyone else spend the money on any of the better alternatives out there. I have always just asked for good testing with some science, We augmented the rocket stove to every internet guru's specs and found no real change except burn right or bad burn.

So here is the truth i didn't want to offend my fellow permies with until now:


If you are burning and it sounds like a low rocket sound you got it.

No smoke is coming out the exhaust you got it.

High heat in fire box low heat in exhaust you got it.

The thermal mass absorbs all you heat and you are happy with that you got it. No guru needed.

Take a look at the thermal images provided and decide for yourself. BTUs (heat) are going through my rocket stove system, Heat is going into the thermal mass, little heat is exiting the exhaust and i was impressed but not warm.
BTUs (heat) are going through my Wood stove, Heat is going into the air, heat is exiting the exhaust i was not impressed but I was warm.

"We augmented the rocket stove to every internet guru's specs"

Which "internet gurus" did you follow? There are many theorists on the internet and youtube who are giving advice known by the major innovators (with hundreds of working examples to their credit) to have issues. Did you test a plain vanilla RMH built to specs published by Evans & Jackson, or Ernie and Erica Wisner?

I haven't seen where you describe the way your RMH thermal storage was set up, though from comments it sounded like it was under the greenhouse floor. What kind of isolation from the ground did you have? Trying to heat the whole earth is not going to work well.