Rocket Stove inside Polycarbonate Greenhouse

Greetings all, I've been following this forum for a while however, only recently do I have the opportunity to build something of my own on my recently acquired land.

I've just built a 4 x 3 meter tube greenhouse with 4mm poly-carbonate siding. My next project is to build a rocket stove inside it so that I can sleep in one corner if it's too cold to sleep in a tent and grow plants all along the southern edge.

I've attached mocks of what I'm trying to build. I'm planning on using found metal pipes of varying diameters, going from a 16,4cm diameter pipe to a 16cm pipe to a 13cm pipe to a 11,4cm pipe exhaust. I want to heat the greenhouse literally from the ground up. I'm using what I believe is a 55-gallon barrel (60cm diameter barrel x 86cm height). I want to build my stove as close to the wall, and away from the center as possible, and I have several questions:

How far away does the barrel have to be from the PC wall? I see that PC melts at 147ºC, however I can't find anything that shows how hot it is around the barrel.

Do I need a heatshield? If so, what are my options for making one out of scrap parts vs. buying one?

For the south-facing section of the pipe, where the plants will grow, I plan to place the pipe under the earth. Does it make sense to build a thermal mass section along that entire front, or will the thermal mass of the ground suffice? (The earth is clay/silty as is)

Any thoughts and advice is appreciated

Hi Maruf and welcome to permies. In a typical rocket the top of the barrel can easily exceed 300 C and the sides will be somewhat cooler. I would put a heat shield about half the distance from the barrel to the polycarbonate. A piece of sheet metal should suffice as long as there is airflow past it...no special materials or engineering should be required to do this.

I would recommend that you fit all of your flue pieces together above ground and test fire the rocket a few times before committing to a build. The tried and true design for rocket mass heaters keeps the same cross sectional area (CSA) throughout the entire system, with the only exception being the interior of the barrel and the manifold where the bottom of the barrel transitions to the flue (this area is bigger). Your decreasing CSA along the path could cause back pressure and restriction and lead to poor draft unless there is some other design feature to counteract this.

I think this is a cool idea Maruf. I want to see this!!

Thanks Phil, all very useful, especially about the CSA's being the same across the system. Seeing as I already have the pipes (and it is too expensive or troublesome to find pipes of the same diameter for the whole thing) I'm seriously considering opting for a fan device on the last exhaust pipe.

Anyways, I will flatten out the stove and do some test runs, and keep you updated on the progress:)

11.4 cm is about 4 1/2 inches, which is far too small for a robust RMH system. I am pretty sure that that would choke a core (combustion area) built to a 16.4 cm (6 1/2") scale, and a core in scale with the smallest duct is below the size that is easy to make function well (and even if it functioned, it would probably not have enough heat output to keep the space warm in cold weather). I believe that using the 11.4 cm pipe anywhere in the system will lead to failure, and the 13 cm pipe is iffy. If you build a 16 cm core, the 13 cm pipe as the final chimney might work.

6" (15cm) is the smallest system size that is known to work reliably when built by a careful beginner, and also the smallest that will reliably put out enough heat for a modest sized minimally insulated space (4 m x 3 m, 10' x 13').

Lithuania is on the Baltic Sea, but how close are you to it, and how cold does the climate get in the seasons when you want to heat? Is this for year round use?

I would advise you to scrap the 11.4 cm pipe, and the 13cm if you possibly can, and look for bigger material. Can you find old bricks, or any masonry material? Buried metal ducts in a damp greenhouse will be likely to rust out fairly soon. You can build channels with masonry (keeping the inside fairly smooth) and it will last forever. Using the 16cm duct as the vertical chimney would work well.

A batch box instead of a J-tube core can put out a lot more heat for a given system size, though that is a much more technical build which has to be done exactly right to work. The practicality of this depends on how good you are at masonry, or at figuring things out by yourself and reading.

What is the character of your soil? Sand, clay, something in between? (Edit, I see it is clay/silt.) If it is sandy and dry (no groundwater), you might be able to heat the ground effectively, but otherwise, most of the heat you put through the duct will try to heat the whole earth which will fail. You need to insulate the duct from the soil below. I think that your plan of a steel barrel around the heat riser will overwhelm the space, and make it too hot when the fire is burning, and maybe not hot enough at other times. I would advise, given the size of your space, to make a brick or masonry (even partly cob) box around your heat riser, as described at the batchrocket.eu website. This does not need to be much larger overall than the barrel, and it will be close enough to every point in the space to warm it all effectively, and keep radiating heat for hours after the fire is out. The masonry box, or "bell", can be used with a J-tube as well as with a batch box.

Hi Maruf;  Welcome to Permies!
I second everything Glenn/ Phil suggested. It is spot on!
I will mention if you use a brick bell rather than a barrel you will not need to protect your poly walls. And your heat holding will be much longer.
I also strongly suggest putting an insulator under your pipes and or masonry channels as suggested.  The earth will try to rob your heat and keep it for itself.

Glenn I took your notes to heart and I simplified the design to forego the 11.4 and 13cm pipes. Now I'm going with the 2.6 meter pipe connecting to a meter-long right-angle pipe which will exit outside, instead of a bending all around the greenhouse. Further, I plan to cover the entire barrel with cobb, as it is fairly rusted and has some holes. I dug out the area and built the stove without the barrel and without gluing the bricks together to give it a test run and so far so good. Anyways, I have some questions:

Cleaning – I see most if not all designs have an opening near the beginning of the pipe for easy access. What sort of build up should I expect? I can't find anything online about what the cleaning looks like. I have a T-form made of a light metal that's clearly not meant for heat. I'm considering covering it with cob and using it as the intake pipe. Is this a bad idea? (I've heard that kind of metal can melt, but will it matter if the cob is solidly formed around it?

Do I need a filter – I've seen some designs with a mesh going over the intake pipe from the barrel. How useful is this?

Ground Insulation – The well-diggers just dug up some very fine, dry sand. Will this work for ground insulation, or do I need coarser sand?

By "intake", do you mean the transition from the barrel to the buried duct run? If so, you do not need to worry about extreme heat there. It is recommended that the first few feet be black stovepipe which is sturdier than galvanized duct, but use what you have. What happens is that fly ash can get swept up the heat riser, and then settles out at the bottom of the barrel or in the beginning of the duct. It will be dry and powdery. You want to have access to clean this area periodically, usually only once a year or so (but check it more frequently at first to see how yours operates). You will need a metal cap for the cleanout.

A filter after the barrel is not something I have ever seen or heard recommended, and I don't think you need to worry about that.

Sand is not good insulation, but if it is all you can get, it is better than nothing. I think fine dry sand would be better insulation than coarse, as it is the tiny air spaces that do the insulating. If the sand gets damp it will lose all insulating value, so you need to keep it protected from groundwater. It would eventually dry out if kept hot.

Thanks Glenn that answers everything. I see in a manual it recommends an ash pit before the pipe begins where it'll be easy to catch it and clean it out. Otherwise, I'm considering laying down sand as a secondary insulator and building a stone/cob encasing around it. I'll let you know how it goes.

The more I read up on it, the more I'm starting to worry that my stove will leak too much heat to the ground. I went ahead and backfilled the bottom 20cm below where the pipes will be with very fine sand, however, I'm starting to think I should get some actual insulation. Can anyone recommend a sustainable material that's better than sand and commonly found in hardware stores worldwide?

Clay and straw makes an excellent insulator.

Make it mostly straw with just enough clay to bind it and make a mix that will be stiff when it dries out. Even if the straw rots or chars away later, the tiny cavities in the clay will be insulating.

I've made a video (where you can also see my progress in the background) however, I have two questions regarding building with stones and what types of straws are the most viable for cob building and insulation

Hi Maruf;   Straw is hollow and hay is not.  Therefore straw makes the better insulator.

If I understood your question about rock.  The more rock you use the less cob you will need to make.

Remember you want 4" of the straw filled cob under your pipes not over them.  After you lay your pipes down ,  you switch to just cob and carefully cover your pipes with that.   Then you add stone / cob/ stone /cob until you have as much mass as you want.

Thanks Thomas, that answers everything. I didn't realize there's a difference between straw and hay. In the video, that is in fact straw, not hay like I said it was. I'll have to check if the newly cut grasses are hollow. However, if it's hollow, I assume it's just as good?

One of the features of straw versus hay is that straw is just cellulose, with nutritious green materials and sugars gone. Thus it gives little for bugs to feed on. Hay is nutritious and can support decay organisms better.

Straw is the stems of cereal grains after they are harvested, and is usually stiff hollow golden cylinders. Hay is grasses and other plants harvested green as feed for animals.