Epic Greenhouse Rocket-Mass Heater

This is an April 2016 Blog Post on how we built a Thermal Mass Rocket Stove at Jordan Rubin's 'Heal the Planet Farm'. 

Bill Wilson

We call it the Epic Greenhouse Rocket-Mass Heater because of it’s sheer size and multiple modifications we designed into it. The whole stove is built around an 8″ flue system that exits outside of the greenhouse below floor level, under the greenhouse end-wall, and then turns towards the sky.

Designed by Bill Wilson (MWP) and Kevin Kepplinger (HTP Farm) Construction and Design Assistance from Heal the Planet Farm Team (All are Midwest Permaculture PDC graduates)

The key modifications we made to this stove that are not usually found on more traditional rocket mass heaters are:

  1. The feed chamber is very large capable of holding full-sized firewood logs so it can be loaded to burn for up to 4 hours at a time and produce a lot of heat.
  2. The thermal battery or thermal mass is below grade and insulated so the heat will not wick into the soil surrounding it but instead radiate up to keep fish (aquaponics system) and bedding plants warm even in the dead of winter.
  3. We installed a multiple-speed flue fan and a flue damper near the end of the exhaust pipe to give ourselves greater control over the speed of the exhaust and even the burn. We can slow the exhaust down to hold the heat in the thermal mass longer when it’s hot or speed it up when we need a stronger pull to get it started. We know this is unorthodox and a bit like cheating, but it’s very helpful and the fan is variable speed using very little electricity.

Continue to slowly scroll down to view over 40 pictures of the actual build & workings of the stove
Click here for our conclusions at the bottom of the page.

Why should you build a rocket stove?

When is it appropriate to build a rocket stove?

We cover the why and when of rocket stove building in our Permaculture Design Certificate (PDC) courses.

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Picture Summary of the building of a Greenhouse Rocket-Mass Heater Build and its use from January-March of 2016

We started the rocket stove build on December 30th. The high was in the 30’s, lows below freezing. There was plenty of frost in the greenhouse come morning.

Kevin and the farm-design team had poured a 4′ by 6′ concrete pad for us to build upon. The first step was to dry-brick the stove together to get a feel for how it all might fit.  It didn’t take long before we had our basic design.

Here are two Sketches of the Basic Design

While we started to form the fire chamber, other members of the team began to dig the trench which was to become the thermal mass sink.  The backhoe saved us about 20 hours of digging.

Located near the north wall, the heat rises, rolls across the top, drops as it cools across the south face of the greenhouse and moves gently across the floor to be heated once again by the stove and thermal-mass trench.

The fire chamber takes shape and the double-walled, insulated, stainless-steel riser is set into position.

The opening to the fire chamber is just under 6″ and 9″. It is the same sized opening as an 8″ circular flue pipe. The idea is to maintain this 50 square-inch opening throughout the entire stove.


Now that we know where each brick goes, we disassemble and reassemble but this time using refractory cement for some of the fire bricks while also filling some of the 8″ x 8″ cement blocks with a simple cob mixture of clay and gravel to help provide even-more thermal mass. The bricks and clay absorb heat while there is a fire and then slowly release it into the greenhouse when the fire dies down, typically at night.

While digging the trench we came across a good seam of clay, perfect for making cob.

Also on the farm there is an area near the river that naturally provides this gravelly mixture. It is an aggregate of 1-2 inch stones all the way down to some sand and even clay particles. Since we are not doing any finishing cob work this mixture will work well for the rough work we do have.  All we had to do was hand pick the larger stones out.

To make a quick cob, we pulled out a small pile of clay, covered it with the gravel mixture and some water…

…and then ran back and forth over it with the tractor to mix it.  Some straw was the last item we added.

The shovel crew re-piled the mixture several times for the tractor to get the consistency right.

Here is a 45 second video of how effectively this large tractor mixes the cob.

Back to the trench. It has been fully dug and now the river gravel is being shoved out of the bucket of the backhoe and layered 3-4 inches thick at the bottom. The thermal mass is built up from this.

One side of the 8″x 8″x 16″ concrete bricks are laid end-to- end against the insulated wall and then filled in with gravel to hold them in place and increase the thermal mass.

Then, a second layer of bricks is added.

The other side is done the same way with gravel filling in the bricks. Next goes the insulation on the bottom between the rows, then 3-4 inches of gravel, then the 8″ exhaust flue, then more gravel and a third row of bricks.

Here is a cross-section of how the thermal-mass trench is constructed.

This is the back of the stove with all of the firebrick cobbed together with the cement block to form a single mass. Notice the cement slab the stove is resting on and the cement bricks forming the trench.

Jim (a 1-year work/study permaculture designer at the farm) has cobbed the riser to the fire chamber while another pair of hands lays down the base for the 55-gallon drum to sit upon.

The radiant barrel is nestled into the cob on top for its first seating. You can also see how the firebrick has been laid out in the stove to protect the feed chamber and create the opening to the fire chamber because concrete brick cannot withstand the high temperatures generated in these stoves. On the front face of the fuel box one brick was turned to make the opening for the fresh-air intake.

With the radiant barrel in place you can begin to see where the exhaust flue will mesh. The barrel extends a bit beyond the concrete slab foundation to accommodate this.

The next morning the team assembles to design the exhaust system that will connect the barrel to the underground exhaust pipe.

A “T” is spliced in near the top so that once the pipe is buried in gravel, one can still access the interior for visual inspection or to vacuum out ash.  We back-filled in around the pipe with brick to support it and for additional thermal mass. The elbow at the bottom will connect directly to the long exhaust pipe running through the thermal mass.

To maintain the integrity of the air-flow, all of the hot-gas/exhaust coming into the barrel must be funneled into the exhaust pipe. We don’t want smoke leaking out of the stove and into the greenhouse, so cob is used to seal around the entire flue pipe and the drum. With the drum removed one can also see how it is seated into the cob.

The drum is put back on, everything is cobbed into place, and the exhaust pipe connected.

Speaking of the exhaust pipe, the 2-inch insulation board has be laid in on the floor of the thermal-mass trench, 3-4 inches of gravel added, and now the pipe has been assembled with a single screw holding each 2-foot section of pipe together. Remember, we are maintaining the 8″ flue pipe dimension (50+ square inches) throughout the entire system.

Note that the third layer of bricks has now been added to the trench walls and the thermal-mass bed is now ready to be filled to the top with gravel.

With the system mostly in place we couldn’t resist building a small fire to test it out. Even without a lid, once the fire was hot enough to heat the riser the stove began to rocket, pulling all smoke into the fire chamber. It Works…!!!

While tending the fire we started to fill the trench with gravel and observed how the cob was starting to dry out.

During lunch we let the fire burn out. You can tell from the burn pattern around the opening to the fire chamber how much hotter it is right there. From the picture you can also get a good feel for the size of the fire chamber. How many pieces of firewood like the one in the foreground can be added into the chamber at one time?

Shortly after the stove was complete, the farm had a local machine shop craft a steel plate with hinges for the top of the feed chamber. The shorter section was bolted down to keep the plate secure and in place for frequent opening and closing.

On the opposite end of the stove, a variable-speed fan was installed. It does use a bit of electricity and as far we know  this is not a common feature of rocket mass heaters. It might even be considered to be a bit like cheating  for if one builds a stove “correctly” it should not be necessary to add this feature.  But we were not certain how the stove would perform due to the changes we made and we also wanted to have more control over the air flow. The goal is to only allow lukewarm air to leave the greenhouse for if it is hotter than this it means that the exhaust is moving through the gravel pit thermal-mass too quickly for the heat to  be absorbed.

To help slow down the air flow a flue damper was was also installed just before the fan. So now we have the ability to slow down the exhaust flow or, speed it up with the fan.  The fan is also wonderful to have when starting the stove to get the smoke pulling immediately into the fire chamber. Bottom-line… it has given the greenhouse team greater control which they appreciate.

8 Weeks Later…

8 weeks later the farm team had all of the greens they could eat.  The stove kept the greenhouse warm enough for good growth through January, February and into March.

I wanted to show you this picture of the north wall of the greenhouse from the exit end of the thermal-mass trench. You can see the sand that was used to level the top of the trench, the grow beds that were built on top, and one of the fish tanks resting on the mass at the far end. The drum of the rocket stove is just visible. You can also see a fan that is mounted on the greenhouse that blows some of the warm air radiating from the stove (the steel lid and drum get hot) into the larger space.

Jim has become expert at loading the stove for a good burn. Here he is getting ready to light a ‘small’ fire in the evening before a cool night.

The fire at night.

The next morning the fire is out but the greenhouse is 20 degrees warmer than the greenhouse next to it that is only now getting prepared for planting (early March).

Greenhouse #2 (without a stove) being prepared for planting in early March.

Fresh radishes and greens in February thanks to the Epic Greenhouse Rocket-Mass Heater…!!!


  1. The system works to keep the greenhouse well above freezing.
  2. The first cob mixture had too much clay and not enough sand and aggregate, so it shrank and cracked, compromising the full integrity of the stove. We took the radiant barrel and riser off during our next PDC course so our students could see how the inner workings of the stove held up. It took about 2-hours to remix some cob and put it all back together.
  3. It was a fairly large investment (about $3,000 in materials plus labor) but to keep the greenhouse above freezing for just one winter using natural gas or LP, we estimated a cost of about $3,000 as well. So, we figure the stove paid for itself in two seasons (one year for materials, one year for labor).
  4. Could the stove function well without the damper and fan?  Maybe, but having these options gives us greater control especially when starting the fire.
  5. For our next stove we are going to look for ways to create some kind of turbulence in the underground portion of the exhaust pipe to increase heat absorption in the surrounding gravel.
  6. As the fire cools and the draft slows down, the exit pipe outside cools quickly as well thus slowing the flow of exhaust gasses even more.  Insulating the exhaust pipe should significantly improve air flow.
  7. A re-design of the fuel chamber door will be considered as well. When the lid is opened to check the fire or to add more wood, too much smoke escapes, filling the greenhouse with smoke. A front loading system is more difficult to load and clean, but it’s much more difficult for smoke to escape. Here is a picture of a unit our friend Michael Egan built a with large, front loading fuel box.
Mike Egan designed rocket mass heater
  1. Final note: This is a good time to remind the reader that 1/3 of the 4,000 acre farm is oak forest. There is plenty of wood available on site for the taking–a natural resource. This is why we are looking at wood burning technologies for various energy needs of the overall permaculture design of the farm. If this farm was on the prairie with few trees, we’d be looking more at various techniques to better insulate the greenhouse or possibly even switching to Earth Air Tubes to keep the greenhouse above freezing all winter with no supplemental heating at all. 

2020 Update!

We have removed the entire stove system. 
As surprising as this might appear it is a good lesson in non-attachment.

When we built the stove we had a small team of permaculture course graduates living on the farm and enjoying the greenhouse production throughout the winter months.

But plans change. For the past two years there has been no one living in this area of the farm so the greenhouse has been unused. At the same time our priorities for growing perennial-plant starts has loomed up with our plans to plant-out hundreds of acres into perennial production and food forests. So our need for non-heated nursery space has soared.

We decided that the greenhouse nursery space was more valuable to us than the rocket mass heater so… we took it apart and pulled it out.

Yes, this was hard to do but we learned so much from this project that we are grateful for the opportunity to build and operate such an epic rocket-mass stove.

This has been a good reminder to us to appreciate the work in front of us while doing our best at creating workable and compassionate, long-term solutions to the design challenges before us. Yes, we want good results, but we do our best and then come to accept whatever results. This is spiritual work I think to desire everything, need nothing, and understand what is showing up.

We are glad we could share this project with you and appreciate your interest and feedback. We will continue to take questions on this post and will answer to the best of our ability should questions follow.

And we will continue to post on our other projects should we think there is something we have learned that might add to the growing body of work focusing on how to leave the planet in better condition than when we arrived on it.

Be well… Keep in touch… Bill (March 19, 2020)

Note: We do take a good look into rocket stove theory and application at every one of our PDC courses.
Schedule of Upcoming PDC Courses Here.

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This free 2-hour session offers a profound and impactful introduction to permaculture and a glimpse of how we deliver the Permaculture Design Certificate (PDC) course. With almost 20-years of experience and a determination to create more effective methods of teaching, we have developed one of the most meaningful and results-oriented permaculture learning experiences available anywhere in the world. Our goal is to make this course not just informative and enjoyable, but also easy to navigate and revisit. 

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84 thoughts on “Epic Greenhouse Rocket-Mass Heater”

  1. Hello, just starting out plans here and wondering if there’s a specific advantage to the vertical stack being external to the greenhouse and I know you said if you guys were to redo the project you would insulate it but could brining it inside reduce the need for that insulation? Many thanks for all the information on here, it is superb.

    1. It’s easier to pass the stack through an endwall rather than having the main arch of the hoop being irregular. In our design we actually had the pipe going underground and we could bypass the endwall completely. Hope that helps and good luck with your build!

  2. PS, I have a top loading wood stove, and know the amount of smoke that can escape when loading.
    With a large firebox like the one shown, side loading may be a better option than end loading.
    Another thought, CO2 is good for plants, so if the CO2 from the flue could be cooled, and drawn into the greenhouse, without the particulate, its free fertiliser.

    1. Hey Nik. Good idea regarding the small fire at the end of the run to support the now cooler air in rising out of the exhaust column. Definitely could be added as an optional control tool. And the side loading firebox makes good sense too based on the dimensions in this heater.
      Have not seen anyone who captured the final exhaust and routed it back into the greenhouse. Have you? Good idea however. Thanks for your thoughts Nik.

  3. It’s bad practice to put a damper in the flue.
    When wood burns, it releases a prodigious amount of gas. At least twice as much heat produced is from burning wood gas, than the solid matter. This burning gas needs plenty of space to expand into. If the flue is nearly blocked by a damper, the stove can backfire and all the gasses will end up inside the greenhouse.
    It’s far better/safer to control the air inlet.
    One possible way to get the gasses moving, would be to light a small fire at the exit end, to start the air column moving vertically, by drawing air along the exhaust flue. Once the air/gas column is moving, its momentum should keep it moving.

  4. Beautiful work! I am on the Oregon coast, and have been researching how to warm my green houses as well…. The thought of the rocket stove in the greenhouse is a great one! Did you insulate below the main pipe as well? was the foam sufficient? I could see insulation along the sides.

    I am also looking to build a dome shape greenhouse to better the circulation and airflow, I imagine the rocket stove doing well in that environment too?

    1. Erik. There is a cross-cut illustration of where the insulation is located. The trench is not 100% surrounded by insulation as we did not want the concrete blocks to be sitting on it, but what we ended up doing seemed to work pretty well. A dome is an EXCELLENT structure for a greenhouse and some sort of mass heater. The thermal battery is worth it’s weight in gold as it extends the heating window allowing you to sleep all night without having to get up to feed a fire. The thermal mass of an inside aquaponices system does a lot for this as well as water holds a lot of heat. Cheers.

  5. What a great undertaking! My hat is off to you folks for this idea.
    Would it help to have the whole exhaust pipe in a channel with a grate over it and not surround it with rock? It seems the direct heat transfer to air would be more efficient. Just an opinion.

    1. With a grate James you would get more heat during the time of the burn but then nothing beyond that. The system would cool down immediately when the fire goes out. The purpose of the thermal mass is that it slowly gains and stores excess heat which is released later so that one does not have to have a fire burning 24/7.

      1. Built one almost the same as yours in length but with a front load door on my uncles farm. To make the heat spread out more we used steel recycled drainage grates on the bottom and on top of the pipe that is buried with stone in one green house but not the other. The one with the grates heated the stone up far greater and is a lot warmer in the mornings. Note the grates you do want them in direct contact with the pipe. Also cast iron grates work better than steel. They take more heat and spreads it out faster than plain steel grates on a future build. Yes it is much better to insulate the flue pipe for draft and you could end up needing to clean it more from sap build up if it is to cold.

  6. Hi Bill. I started my greenhouse and RMH about a year and a half ago and trying to pick up any tidbits of info as I go. As you suggested, I am going to modify the RMH by taking off the top lid and modifying the burn chamber by placing a wood stove in there sideways and cutting a hole in the side to tie into the rest of the system and riser. The burn chamber will be 48” so the riser needs to be 96” or 8’. My plan is to fire brick 5’ and use insulated S/S stove pipe for 3’. Do you think that would work or would the heat still be that high 5’ from the ground and disintegrate the pipe?

    1. Oh boy Andrew… there are too many variables in here for me personally to know what would happen and how it would burn, or if the 3′ insulated pipe would hold up. The only thing I would suggest is to find a way to build it so that you can run tests and then make changes if/when you needed to. Very creative. Please posting something as you learn from your project and send us a link! Good Luck.

      1. Hi Bill. Still plugging away at my greenhouse RMH in Ontario. Riser question: the riser is supposed to be double the length of the burn chamber. Is yours or can you get away with a shorter bell?

        1. Probably not much shorter if any Andrew. The goal is to get the perfect pull in your stove so that you never get back flow (smoke in the house) and so that it burns hot enough to burn clean. You may get a short bell to work but will it become a ‘problem’ stove? Trail assemblies is the best way to go. Good Luck.

  7. I really like your idea – and the detail in the article was great and was very easy to follow.
    I have several comments and questions and hope you will abide with me.
    First, have you made any changes to your greenhouse RMH setup?

    We would like to build this style of heater under a 30′ by 50′ house that is sitting on solid rock. We are going to insulate the foundation walls with 12″ of rice hulls and 2″ of rigid insulation. The height of the foundation wall varies from 3’6″ to 6’6″. The RMH is probably going to be 4′ x 6′ with a 7′ long day bed on the first floor near the middle of the house. It includes an 8″ exhaust pipe running north for 23 feet where it u-turns and the exhaust runs 50 ft south to chimney with a fan like you used . Hoping to be able to run it for 15 minutes and shut it off. we will lay exhaust between concrete blocks, with soil, etc., as you did – but the other side of the blocks will be air in crawl space (not in soil). insulated exterior walls and wooden floor above to be warmed by exhaust run.

    Do you anticipate problems with any part of our plan? Would preheating incoming air with exhaust air using a heat exchanger be of significant benefit? Replacing stove pipe at some point in the future is a concern. What do you think of surrounding metal stove pipe with chicken wire coated with ferrocement? If metal pipe rusts out would the ferrocement surface be too rough for decent air flow?

    Thank you so much for your input!

    1. Hi TJ. Feeling out of my league here since you are working in a residential situation so I’m just giving you my thoughts…not advice. I think you’ll need to connect with people like Ernie and Erica who have a lot of experience building in homes. http://www.ernieanderica.info/rocketstoves

      Love what you are attempting to do. Here are the unknowns or ‘red flags’ for me.
      1. The exhaust run of 73 feets seems like it may be too long. The exhaust fan should only be an assist and I’m concerned if your system will pull at that length. The rule of thumb is a 50′ run for an 8″ system MINUS 5′ for each 90 degree elbow. I’m estimating 4 elbows in your system which gives you 30′ of run remaining. I think this is conservative but your 73′ is really stretching it.
      2. Be sure the upright exhaust at the end is insulated so the air does not cool at this crtical point and get to heavy.
      3. The way I’m reading this is that you will not insulate completely around the exhaust run. Insulating on the 2 sides and bottom are a must or you will lose too much heat I think.
      4. Does the top of the exhaust run touch the wood floor? I’m assuming you know that this is a no-no. You need a thermal mass here to protect the floor, absorb heat and release it slowly. Could you have a tile strip running down the floor here with the wood flooring on either side?
      5. Wrapping a metal exhause pipe with chicken wire and ferro cement… hummmm…. I think the pipe and the chickenwire will corrode over time as you expect. What about laying in terricota chimney liners and building in access at both ends of the run? Then every 15 years or so you can slide in a new metal (flexible pipe) chimney liner. You can create an insulated channel to lay the liners in and now the wood floor can be laid close to the terricota since there is an air space now between the exhaust pipe and the terricota and another slight space between it and the wood floor.

      Bottomline… build and experiement with the system before closing it in permanently. We won’t be building a topload system again as too much exhaust (smoke) escapes when refueling. It will be a front load system for us from here on out. Also…no metal top to the firewood combustion chamber – way too hot!! You could build a stone and cob top to the firebox and then build a cob oven on top of this! And just leave it open if not cooking anything – more thermal mass.

      We built out system with a large firewood combustion chamber to be able to load it up in the evening and have it burn most of the night. It worked well.

      Good luck TJ and be safe… Bill

  8. Here is a comment we received via email that we would like to share and answered in the forum here….just in case some of you also made this observation. Good catch Justin.

    Hey guys, nice site.
    One little thing I noticed, maybe you have a reason for doing this but
    the 2100 degree stainless steel packed pipe on the flue in some of your
    photos is installed upside down. This allows creosote to drain back into
    the mineral fiber between the inner and outer layers of steel pipe,
    increasing fire danger.
    Probably never be a problem but just thought you should know.

    1. So, Justin is correct. These insulated chimney pipes are designed to stack with the rounded end up and the cupped lip down. As Justin observed, this is to allow creosote to drip all the way back to the stove without filling the space between the pipes.

      To explore this aspect first, the nice thing about a rocket mass heater riser is that the temperatures get so high that there just is not any creosote to deal with. It’s just not there. Now, the exhaust pipe at the far end that is outside the greenhouse and 14′ tall… that exhaust does cool down significantly and during start up and cool down creosote does start to form and is something we have to design for.

      But the reason we turned the pipe up-side-down in the riser was completely unrelated. When we set the ‘round’ insulated riser down on top of the ‘square’ firebrick riser the rounded tube did not quite cover the corners of the square riser. As such, the hot gases would have been able to circulate around inside the cupped lip and we were concerned that there could be damage to the inside of the stainless-steel riser. So… we turned it upside down and cobbed the heck out of the joint.

      This brings up a common experience I have had with our permaculture design projects — no matter how carefully we design we are always met with unforeseen details when we go into the implementation phase. So now we have to design for or around a situation right on the spot. Sometimes we make a ‘good’ decision and sometimes not. But regardless, every situation like this brings us the opportunity to learn and thus increases our chances of making better decisions in the future.

      I love this process. And now that we have been doing this for well over a decade, I know for sure that I will never learn it all. There is just so much detail and mystery contained within every opportunity. As such, I have learned to really appreciate what knowledge and experience others have and to value their input.

      I see permaculture as just an attempt to live with integrity as we explore life and our relationship with it as humans.

      Cheers… Bill

  9. question:

    If the combustion in a rocket oven/heater is so efficient that the only exhaust is really steam and CO2 like people claim, why not just exhaust to a greenhouse?

    Free CO2 and humidity for the plants.

    Anyone else have this thought?

    1. Very insightful question Steven. I don’t have a hard ‘yes’ or ‘no’ on this.

      What I do know is that there is still a good amount of smoke/particulate/pollution at start-up and again at cool-down when the ‘rocket’ is not burning at high temps so there would have to be venting for these times. But even if we could throw a lever once everything is rocketing and put the exhaust in the greenhouse we still have the challenge of a closed-loop system if the greenhouse is tight. It would be like idling your car in a closed garage. Eventually we would burn up all of the oxygen and the fire would slowly die. (People too)

      Now… could we pull fresh air in through a dedicated air intake pipe that come in from outside? Yes we could, but in a closed system, if the greenhouse was tight, the stove would stop drawing. Air has to escape the greenhouse in order for new fresh air to be pulled into it.

      So, maybe we could allow 1/2 of the exhaust gas into the greenhouse and still maintain a good draft by having the dedicated inflow pipe and an exhaust pipe going out of the greenhouse. But still, this would have to be carefully monitored for any people who end up working in this space. Too much CO2 is toxic to humans.

      Bottomline… I’m not sure the benefits would out weight all of the challenges and risks. There would have to be a lot of careful monitoring and adjusting going on here.

      Anyone have some other thoughts on this? Thanks Steven

    2. I had the same thought. I’ve been toying with this idea for a future greenhouse I want to construct. Just exhaust all the literal greenhouse gasses you can into there and pull either fresh outside air into the stove, or the low-lying oxygen the plants pull out of the carbon oxides.

      1. Love this project and concept. Have built a very similar system on a much smaller scale. Garden of terraced house in the uk. 25’*12’. It seems too good a possibility to pass up. Use carbon neutral fuel in a super efficient burn, extract all the heat to extend the growing season and increase the yield by using the same plants to capture the co2 exhaust. Apparently damp wood fire ash will readily absorb co2 (bbc 39 ways to save the planet) and in the process turns into refractory aggregate that can be mixed with clay to make cob or fire cement.

        Keep the rmh as is. Take the flue off altogether. Continue the trench outside the greenhouse. Existing exhaust/thermal mass allows cool co2 to fill the new trench. Co2 is heavier that air so if cool it will simply collect there. The trench can be populated with plants and could then be dug in a U shape back into the greenhouse?

        1. Unfortunately the RMH will not consistently produce Co2 and Water. I think the times it is not burning at peak you’ll end up smoking out your plants.

    3. I was thinking the same… I wonder about monitoring the amount of CO2, and if it would become a chore to make sure you dont over saturate the life inside? Maybe thats an invalid thought…

  10. If you switch to cermaic refractory board, the riser will last much longer. Thanks for sharing your project, 4 hour burns is awesome.

    1. Sounds like a good idea Mart. After two long heating seasons the insulated stainless steel riser was just about toast. We will not be using the greenhouse this winter as we have a smaller team at the farm presently but with our slowly developing nursery the heated greeenhouse will be coming in handy again, probably next winter. I’ll post whatevery riser changes we end up making. Thanks for the tip.

  11. Would you ever consider placing the fire box outside the greenhouse to make it easier to fuel the fire and take up less space?

    1. I’m guessing here Robert…but with a good design and protection from rain and blustery winds it could work. I would also probably insulate what was exposed outside to keep from losing radiant heat. It does seem like a lot of work to save a few square feet of space but the idea has some possibilities. I like the idea that the stove would be pulling air from outside. Let us know if you try this and how it works Robert. Good Luck. Bill

  12. Bill, I did a test run on my RMH without any mortar or lid.
    Some of the Flames burned sideways but a lot burned straight up. Does the lid solve that?
    Once I am confident that it is going to work I think I will mortar everything. I am using repurposed cinder blocks so there would be a lot of gaps.

    1. Right Andrew…when building a dry-brick RMH the bricks need to be near perfect in size or shape to each other for it to pull well. Any gaps lessen the draw and makes the unit perform very poorly. I’ve built dry-brick stoves with used material that were less than satisfactory in their performance but after applying cob (sand, clay, straw) around the entire unit, thus preventing air infiltration, the stoves burned beautifully. Encasing the stove in cob has the added benefit of holding the entire stove together (the clay contracts) without the use of mortar. This is of course for a temporary stove …and… it has to be protected from rain if you don’t what to have to keep repairing it. Hope this helps. Have fun…!!!

      1. Thanks Bill!
        I am now sourcing metal for the lid.
        This seems to be the expensive part. Can you tell me what gauge metal you got and ballpark price?

        1. Andrew. I would not recommend putting a steel lid on top as a fuel loading door. See my updated comments at the end of the blog in the “Conclusions” box. I’ve added a picture there too.

  13. Hi, I love the design! I have two quick questions. One, did you have to provide an outside source of combustion air? It does not appear so in the photos, but I am curious as to the possibility of cold air being drawn in to replenish air loss and causing aforementioned crop damage. Two, did you keep track of how much fuel wood was consumed over the duration of use so far.

    1. Great questions. Yes… we quickly found that we needed to pull in fresh air from a dedicated supply. We ended up using a 4″ PVC with elbow to create a U-shape that automatically pulled air in from outside, down under ground to get under the greenhouse wall, and then back up into the firebox through the conventional opening in a concrete block. We did not calucate the exact amount of firewood as we were pulling from old piles of dead oak trees that were in various stages of decay. Through a dedicated winter (stoking the fire 2-3 times/day) I would guess we burned 4 cords.

      1. Thanks. Would you consider selling a copy of your plans? I bought the book on Rocket Mass Heaters but you have a proven design for a greenhouse and that is what I am working with.

        I might not build exactly the same but yours would definitely be helpful.


        1. Hi Andrew. There are no ‘plans’ per se. Everything we thought about and applied is right here on the page. Nothing else is written down. Help yourself to all of these ideas and let us know what you come up with as well. Cheers…

          1. Did you use mortar with the big cinder blocks? It does not look so. I am almost at the point of mortaring cinder blocks and using refractory cement for firebricks.
            Also, for the base of the bell, is there a continous surface of firebrick or are the regular bricks exposed to the heat travelling downward front the riser?
            One last thing, the riser is supposed to be twice the length of the burn chamber (per Ernie and Erica). Is that a problem for this design?

            Appreciate your feedback!

          2. No mortar on cement blocks… limited amount of refractory cement on the firebrick. Our system was designed to be able to take apart and modify if and when needed. It’s holding up just fine as is.
            There was not much consideration for materials at the bottom of the bell. The temperatures are so much lower here it was of little concern for us. Firebrick, brick and cob are all exposed to the downdraft. Holding up nicely.
            Even though the fuel box is large, the burn chamber is about 40% shorter than the riser…or… the riser is 2.5 times larger than the burn chamber.
            Good Luck Andrew

  14. Hi,
    Just wondering how the stainless steel insulated riser has held up and what temperatures have been recorded for the burn. I keep hearing that metal is bad for the riser and will burn out FAST.
    Any feedback would be appreciated.

    1. Right you are Katie. In our case, the stainless steel insulated riser sits on top of a 12-inch column of fire brick above the back elbow of the fire chamber. The seventh picture from the top in the summary shows this pretty clearly. The hottest part of the fire chamber is before this elbow so the temperatures are cooler by the time they reach the riser (maybe 700 instead of 1200 degrees). We’ve gotten 3 winters out of this one riser but it will soon need to be replaced as we can feel the insulation starting to shift inside. In the long run it will be cheaper and easier to just build the column with firebrick which we will probably do. Good observation Katie.

  15. Hi,
    I would like to know what parts of the system you have to clean and how do you eventually access? We are heating our house in france with a conventional wood-stove and have to clean the tubes regularly…
    Tank you in advance,

    1. Hi Dennis.
      95% of the ash settles in the burn chamber where it can be shoveled out between burns. We’ve installed slide out drawers to the side for this purpose as well for easier removal. The other 5% of ash ends up sitting on either the top of the riser or down at the bottom of the outside barrel where the interface with the exhaust pipe begins. We install a clean-out here too when we build the stove. It’s a good way to check for creosote build up as well.
      Speaking of creosote, with a properly operating rocket stove the temperatures in the burn chamber are so high that almost all particulate matter that creates the creosote is burned up and turned into heat before it ever reaches the exhaust part of the stove. Yes, the inside of the pipes turn black but the build up is almost non-existent. Our friends have a rocket stove that they have used for 5 winters and they have yet to use a flue brush. The only time there is a chance of creosote build up is when starting the stove or when it burns out…ie…when there is smoke. Otherwise, this is the cleanest way to burn wood I have ever seen. Becky and I have a high efficiency (clean burning) wood stove we could find when we started burning wood (Vermont Castings) and still, I have to disassemble the exhaust pipes each fall before the season begins to clean out the 1/8 inch creosote build up. Our friends stove of 5 years has yet to get any noticeable build-up. I hope this helps. Build it well. Keep it hot. Dry wood. No creosote. Cheers… Bill

      1. Hi Bill,
        thank you so much for your answer. We will well plan it and make it as good as possible!

  16. Bill, I find everything about this project to be fascinating. I have one question. With all of the discussion around heat loss due to conduction into the soil and the need for ideas in the area of insulation. What would be the possibility of raising the entire thermal mass structure above ground? To me this would produce 100% radiant heat. In effect giving off a better btu to atmosphere ratio. If so you could burn slower and longer would it not result in less fuel and/or more time between loading the stove. I was behind the idea of heating the ground until it became obvious the ground will take most of the heat generated thru conduction. To me this leaves me with one conclusion. I would love to hear your thoughts.

    1. Great idea Travis. I think it would work well and be more efficient. What ideas might you have for building this above ground? This is what I love about permaculture…there is always another way. Thanks Travis.

  17. Benoit, Emilie and Guillaume

    We are based in Namur, Belgium, and we would like to thank you for all your wise advices! We are starting already to build our very own greenhouse and will be respecting all the steps above to make it as great as you did with yours! A thousand thanks to your entire team for all your help and detailed explanations. God bless you! 😀

  18. Anders K Brandt

    Dear all
    Thanks a lot for sharing with us. Very nice and instructive.

    I am planing to make a greenhouse free of frost all year round, using a rocket stove and to put the chimney underground as you do.
    We consider the whole ground as a mass store of heat, and will insulate the whole block of soil outside the foundation.
    Then I hope to put the chimney almost one meter under level to get enough mass for storing heat.

    My plans started when I saw a greenhouse heated in the summertime by letting the hot air going through plastic pipes in the ground and storing the heat in the soil. In the wintertime the heat is leaving to heat up the air in the greenhouse.

    My plan is to do as you have done, and in the summertime let the extra heat pass through the underground chimney and lift the temperature of the soil and store heat for winter. Then later in the winter when more heat are needed starting the rocket stove. Your electric fan at the end of the chimney will in the summertime work as a regulater of the temperature of the greenhouse. It can be automatic.

    I will use a concrete pipe in the soil for smoke and hot air in the summertime. It will live longer in contact with moist soil and I do not need to put cement blocks round the chimney pipe.

    I also consider to place containers for fish and aquaponic systems. I plan to use ferrocement for that and place the tanks in the ground not to take to much space. Also to get a better heating of the water. The water container will also act as a mass store for heat.

    We will put an air-air heating pump to heat when we are not around in winter. We have 6 kw solar panels and store at the grid.

    We are living in Denmark and we have often freezing conditions for to-three month a year. Our greenhouse is about 4 by 4 meter, so it is not a big challenge as yours. We will insulate as much as possible. We got some two layer glass window free of cost and recycle them to the greenhouse as we do with a lot of materials for building.

    I hope to get response at our plans and to inspire others to use the ideas.

    1. I like the sounds of your design Anders. It should work well. Please let us know how your first season turns out. You will learn so much your first year it will blow your mind. 🙂 All this stuff looks so simple but there is a good bit of nuance that much be learned as one goes along. Good Luck and have fun!

      1. Anders K Brandt

        Thank you for your comment. I know that the first season will show a lot about the design. I hope very much that concrete pipes are good as underground pipe.
        I is in my mind, if it is necessary to add isolating layer under the pipes so heat is not lost to deep underground.
        I will let you know when I get started on the project. It might be next year, because other projects are asked for by my wife.

  19. LeAnn M Haggard

    Beautiful! I’ve been thinking for years of how to build a garden/backyard smoker that could also be used for heating a small greenhouse or other enclosed space in colder weather. I think this could be an excellent solution with just a few modifications. Always better to make the maximum use of our resources. The fire/ smoke should be usable cure/cook before running the thermal mass flue and heating a space. And a metal radiant top can be used for a stove top cooking surface. Perhaps even a closed chamber oven above the fire box! So much more use out of one burn!

  20. How warm can this type of rocket mass heater keep a 12×20 greenhouse? I live in zone 8b. I am wanting to use my greenhouse for tropicals that can be damaged in temperatures under 55 degrees. I was toying with the idea of trying to use a hot compost system but I’m not sure it would work to keep it warm enough. Usually we have warm winters but we have been known to get down in the high teens for a couple days. I will be growing in raised beds and large pots (25gallon). Could you please advise as to what temperatures to expect? Thank you very much.

    1. A rocket stove mass heater is definitely appropriate for this type of thing. I should add that you should have at least one (or more) other ways to heat the space, preferably automatic (like when the temp hits 60 a propane heater comes on). You’re also right about the compost not being quite up to the task (but it would help on a cold night with other systems).

      The temperatures will depend on how much wood you burn, the exhaust temperature after the thermal battery (and therefore the size and details of the construction), and the size and type of greenhouse you have. Typically the barrel can get 600+ degrees (at peak performance the wood burns at 2000 degrees), the mass say 80-90 some degrees, and the exhaust say 80 degrees. Of course that all depends on how you build it.

  21. I’m wondering if it would be effective to weld some fins at the top of the steel barrel, partially enclose that, then use heating duct and maybe another flue fan to push heated air throughout the greenhouse?

    1. That would probably be helpful Randy but in a greenhouse this large I’m not sure there would be much of a measurable difference in the overall temp of the greenhouse. We do however put a good sized fan above and to the side of the barrel and blow some of the hot air off of it anyway.
      Good thinking…!

  22. I had to see more when i found the photo of you greenhouse with a long thermal mass & how it works.
    I did not think 40 feet would work, until I read fan, I want to do fish & fruit with a rocket thermal heater. But here in South Carolina our low can get as low 10 F, this year we had 13 F on Sat. then 80 F on Thursday of the same seven days. So I should be able to pull it off in this zone 8a.

  23. Actually there is no need to insulate the stove nor the pipe it would work without. The soil will act as an insulator and you will get a much larger geothermal storage mass that way. Use the insulation in a vertical trench around the greenhouse instead. Just insure there is no moving groundwater thru the storage mass.

  24. Great work and great post! In my humble opinion it might be most effective to insulate the soil inside the greenhouse from the soil outside the greenhouse like a frost-protected-shallow-foundation. Also if you provide a 8″ diameter tube from outside to the fresh air intake of the rocket stove you will maintain your warm air inside the greenhouse instead of letting it go out the exhaust. Any air that leaves the exhaust must be replaced with air from somewhere else. Good luck and have fun!

    1. Hi Garrett,

      We have since extended the intake tube. It now goes underground and outside the greenhouse to pull air in from the cold outside instead of using the precious warm inside air. We have also considered insulating the soil, or moving to raised beds. We are also thinking of installing “climate battery” tubing. We use the tubing for a vermicomposting system in the same greenhouse to great effect. A solar fan pulls air in from the top of the greenhouse and warms the worm bed, keeping it above 50 F consistently.

      Thank you for your input; we welcome all ideas.

      “There is something I do not know, the knowing of which could change everything.” -Unkown
      -Jim P.

  25. Great post! I have a 10 x 20 hobby greenhouse with 8 55 gallon plastic drums along the south wall sitting on 2″ foam board. They’ve been full for the past 10 years, no appreciable compression of the foam when I removed 2 this past spring. That’s 440 lbs on a 22″ diameter circle. Hope this helps on the foam board worries! I’ll be building my first RMH this fall in a 24 x 30 greenhouse. Raised bed instead of in ground, with perlite for insulation under the firebox and first 5 ft of the thermal mass. I’m in southeastern Ohio, zone 6a. We usually have a week or so of single digits, so your data make me pretty confident that my Japanese maple grafts will be happy! Thanks so much for the performance data!

    1. Hi Gean,

      I believe that you will be happy with your results in the greenhouse. Yours is slightly smaller than the one we are using, which only assists in the heating results. We are zone 6a here too in the Ozarks, so we should have similar findings. Out of curiosity, is your greenhouse a hoophouse (basically clear-plastic tent) or an actual framed greenhouse with hard plastic siding? I would love to know the difference that the two designs offer in terms of insulation.

      All the Best,
      -Jim P.

      1. Hi Jim!
        It’ll be single layer clear poly over the arches, corrugated polycarb on the ends. I’m going to use it to graft Japanese Maples, dogwoods, redbuds and various fruit trees. I’d really like to compare our wood consumption once I get her going- I’m going with the raised bed to prevent heat loss into the gound, so that all the stored heat will radiate into the house. My target temp is 38-42 nighttime, with days in the sixties or above so I can get some growth . Mostly it’s freeze protection for the new grafts that I’m after, but the understock has to be in active growth to be grafted, and then continue for the graft to take. I looked at the Chinese greenhouses- what are they using for thermal blankets? That would be really useful on the north side of the greenhouse to prevent heat loss. I’ll be putting in a 100′ house soon, and would love to be able to find the trusses they used! Thanks so much for all the good info! Happy Growing! g.

  26. Thank you for this, I am getting ready to build a rocket heater in my greenhouse.
    .Can you tell me what is the average low temp for Jan and Feb where you are? When you burned at night, it lasted about 4 hours, what was the usual temp inside the greenhouse in the morning?

    thank you so much for your answers!

    1. Hi Susan,

      This is Jim from Heal the Planet Farm. This past winter was mild, temperatures rarely dropped below 0F. Most of the time, we were in the low teens. Our indoor temperatures in the morning (5 am – 6 am) were usually in the high 30s. We did not have an accurate weather station at the time our stove was fully assembled and our tinkering/fine-tuning complete. We will have an accurate weather tracking device for this winter to properly document temperatures and the effects of the stove.
      Our burn schedule was something like this:

      6AM: Light and completely stock with wood (sometimes there were still coals)
      10AM-12PM: Check and add wood if necessary (usually the sun was on the greenhouse at this time, so it was warmed by passive solar and thermal mass heat)
      4PM: Restock half with wood if necessary (the sun is no longer directly on the greenhouse)
      8-10PM: Stock full with wood
      *2AM: Check (we often had a “fireman” check in the middle of the night, usually the fire was still burning well)

      I am hoping this coming winter to experiment with 24/24 hour split. Burning for a full day, and doing nothing the next. I feel we may have been timid about letting the fire die out. We also have a much larger thermal mass via the aquaponics, though it may be well insulated enough to not be greatly affected.

  27. I am located in Nova Scotia and our winters can be very much below freezing for long periods. What were your external night time temps? What could you maintain in the greenhouse with one fire burning at night? Thank you for the great post. I really like all of the details.

    1. Hey Shane,

      This is Jim (pictured above). I wrote a response to your questions a while ago, but apparently forgot to hit “post.”

      Anyway, if you look at my response to Susan above, your questions have been answered. External temps rarely dipped below -12 C. We were able to maintain 3 C for the most part.

      I’m assuming that you already have a greenhouse/hoophouse structure built. One thing to be sure of is that there are no leaks in the structure itself, as the heat will be sucked out. We did lose some crops near a seam that was not sealed well.

      If you do not already have a structure in place, I would point you to researching Chinese Greenhouses and Passive Solar. A rocket mass heater could easily be an added benefit to to these systems; where you are, you can use all the heating help you can get!

  28. Our Blogger, Robert Aram, PE, took the time to email me privately to further clarify his knowledge on the subject of heat transfer as it relates to the thermal mass battery we built and buried below floor level. It so clearly explains the process that I asked him if I could share his email here. He graciously agreed.

    Thanks Robert! Here you go:

    Hi Bill,
    I want to reiterate a point I made in my first posted comments. This relates to the following statement:

    The thermal battery or thermal mass is below grade and insulated so the heat will not wick into the soil surrounding it but instead radiate up to keep fish (aquaponics system) and bedding plants warm even in the dead of winter.

    This statement reflects common misunderstandings of heat transfer. Heat is transferred by three mechanisms: conduction , convection, and radiation. In a solid, such as soil, the only heat transfer mechanism is conduction. In conduction there is no up or down. Heat flows from warm to cool. In your heated tube, heat will flow up into the greenhouse or aquaponics system and down into the earth below. Specifically, the statement above contains the following misunderstandings:

    · “the heat will not wick (conduct) into the soil” Yes it will. Conduction flows in any direction, moving from warm to cool.

    · “but instead radiate up” No, it won’t radiate, neither will it convect up. It will conduct in all directions including down into the earth

    · “the thermal battery or thermal mass is … insulated” No, it is only partially insulated. This is like building a house with well insulated walls but leaving a window open in the winter. The heat will escape through the window because that is the easiest path for it to follow. Your insulated wall has a window open. Another analogy is carrying water in a bucket with a hole in the bottom. However good the bucket may be, the water will eventually all leak out through the hole. Your thermal “bucket” has a hole.

    You can force the heat to move up into the greenhouse by providing an insulation barrier to heat transfer in unwanted directions. But this insulation needs to be continuous, i.e. no gaps. The current design has gaps, there is no insulation under the concrete blocks.

    Robert Aram, PE, LEED-AP

    Now… A few more thoughts from me (Bill)…

    This was our first build of something like this and quite literally, some of the designing we did we did from the hip… on the spot. And I remember standing there with Kevin, the project manager, discussing this very point about laying the insulation across the entire floor so the side insulation would rest on top of base insulation thus encasing the entire thermal battery and minimizing conductive heat loss.

    Kevin’s response “I’m concerned that the weight of the blocks will crush the insulation over time and compromise the integrity of the entire wall. I could easily see his concern especially if we ended up resting large fish tanks right on to the walls as we were considering for an aquaponics system. What if the walls sank in those areas and not others?

    So there we are and the guys are standing there, ready to cut the insulation and start building up the thermal mass. We both agreed to place the blocks directly on to the tamped gravel base, thus creating this thermal bridge for heat loss. Bummer.

    But what would you (the reader and larger public) recommend we could have done?

    I’d love to hear from some of you on how to design this with the concerns we had of too much weight on the insulating foam.

    Robert gave a suggestion in another comment he posted that I really liked and will re-post here:
    “1) Insulate under the firebox. This could be a pre-cast concrete slab, 2-3 inches thick resting on spacers perhaps one inch above the base slab. This would break the thermal path from the bottom of the firebox into the soil below.”
    “2) In the trench, extend the insulation on the trench floor under the concrete blocks, clear out to the side wall insulation. This would form a continuous barrier of insulation. Currently,there is a thermal path through the bottoms of the blocks into the soil. Don’t assume “Heat always(and only)rises.” This applies to convection only, not conduction. The heat transfer mechanism here is conduction through the blocks and into the soil below.”

    I love the idea of an air-space below a concrete slab. That would stop a lot of conductivity. But we still have the blocks eventually resting on insulation foam board resting on a concrete slab.

    Would enjoy hearing from some of you. Thanks again Robert.

    Toward the greater good… Bill

    1. You are burning wood. Its more expensive to insulate than to burn more wood.

      Any conduction heats the ground, which is beneficial if it is above frost line. The goal is to build a thermal mass through the summer that is sustainable through the winter. Double walled poly, bermed outside, thermal break at outside wall with insulation to prevent outside frost to transfer to inside (below ground), more ducting in the ground… All these create passive ways to keep the ground warm.

      Your labor costs are going to kill the project. The night fire may be unnecessary if the ground is warm, certainly checking it at midnight is unsustainable for the average farmer.

      1. Perhaps I am misunderstanding your comments, but I think what you are saying is to build up enough btu’s in the soil during summer months to last throughout the winter? It is impossible to have enough btu’s stored under a greenhouse in order to heat your greenhouse throughout a winter. A 24’x72′ passive greenhouse with R1.63 double wall poly and R30 south wall can lose up to 14,000 btu/hr during the coldest nights in Montana. There is no way I can store millions of btu’s in the soil to compensate for such heat loss. Thermal mass needs consistent regeneration of btu’s inside mass in order to keep ambient temps above freezing.

  29. Did the location of the aquaponic tanks over the trench cause the water temperature to vary much or were you able to maintain a reasonably constant temperature?

    1. Rick… the aquaponics system was brought on board in early March when the stove did have to be run often so we don’t have good temperature data yet. But what I can tell you is that the water in the fish tank is constantly replaced with the cooler water coming from the floating bed tank that is nestled into the ground. We don’t expect the water temps to get much above 65 degrees all winter but we shall see. Please check-back next January…!!! 🙂

  30. Useful blog post ! I Appreciate the specifics ! Does anyone know if my assistant could get a blank a form version to use ?

  31. Robert Aram, PE

    More on Conclusion/Speculation #4: I don’t think trying to create turbulence in the long horizontal pipe is a good idea. Any device placed in the pipe for this purpose would:
    1) Greatly reduce the draft
    2) Increase the settling of ash in the pipe, which will eventually clog the pipe and necessitate cleaning
    3) Make it impossible or very difficult to clean ash out of the pipe
    Even with the current design, you are going to need a cleanout at the chimney end of the horizontal pipe. This should be at the level of the horizontal pipe and straight inline with the pipe. This would require a pit and access box under the chimney.

  32. Robert Aram, PE

    Conclusion #4. Could the stove function well without the damper and fan? The answer is no. Draft in the chimney is needed to pull the exhaust down from the top of the barrel to the pipe below. When starting, the chimney will be cold and there will be no draft. Smoke will come out the loading door and not start through the pipe to the chimney. A bypass damper which could be opened to provide a path from the firebox into the pipe might work to get the fire started. The damper would be closed as soon as the chimney was hot enough to provide the necessary draft. I think your draft blower is a better approach. It may not be needed after the fire is established and the chimney gets hot. If the pipe in the ground extracts almost all the heat, so the flue gas is cool when it reaches the chimney, then the blower would need to run continuously.

  33. Robert Aram, PE

    I appreciate this design but suggest that there is still a lot of heat going into the soil which will never be recovered. Suggestions for improvement:
    1) Insulate under the firebox. This could be a pre-cast concrete slab, 2-3 inches thick resting on spacers perhaps one inch above the base slab. This would break the thermal path from the bottom of the firebox into the soil below.
    2) In the trench, extend the insulation on the trench floor under the concrete blocks, clear out to the side wall insulation. This would form a continuous barrier of insulation. Currently,there is a thermal path through the bottoms of the blocks into the soil. Don’t assume “Heat always(and only)rises.” This applies to convection only, not conduction. The heat transfer mechanism here is conduction through the blocks and into the soil below.