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Sm Greenhouse Mass Heater using steel burner and cement block exhaust

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I built this rocket stove for our greenhouse and then discovered this forum once the welding was done. I used all scrap stuff I had laying around from building stoves in the past. After reading that steel will not hold up to the heat I have looked high and low for cheap fire brick and the only ones I have found are $17 each, which I would have to order. I have fired up my build and the draft seems fine. The main fire pot is a old propane tank which I wanted to have more area for the wood so it might burn longer and the stack is thick walled 8" which I plan on using cob and perlite to insulate it. I have decided to install it in our greenhouse anyways since the temps are dropping down to 40 deg at night and in the 50's during the day. I guess using steel is not the best option but it's what I have and the exhaust is going to flow through cement block which is laid along the back wall on insulated panels and sand base. I have the block on a slight slope and the exhaust will flow through the two holes in the block which is 4 3/4" square holes. I'm using a 8" stack that's welded to the stove and a 30 gal barrel instead of 55 gal and a 5' rise on the 8" chimney out of the wall at the top of the greenhouse. My exhaust will run about 25' total with 4 elbows. My question is which is, is just sticking the pipe through the wall ok, or should I add another elbow and have the stack above the roof? My test run to see if the flow works through the block will be this weekend. Hopefully all goes well and not many modifications will need to be made. Here is a link to a YouTube video I made last night when I fired it up to check it out. Any advice is welcome.
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Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Korey Klug ! Welcome to Permies.com, our sister site Ricchsoil.com, and a BIG Welcome To the Permies Rocket and Wood stoves Forum Threads !

With over 35,00 Fellow Members world-wide, you have a place to come to 24 / 7 to talk to people who want to talk about what you want to talk about !

With Metal Working skills like you have we can understand why you attempted an all steel Rocket Mass Heater !

There is a Whole boatload of stinking Crap videos on U-Tube ! Finding a good D.I.Y. Builder to trust is a lot like finding a poker player you would want to
invite into your house !-

Check the comments section on a video, does he show up regularly and is friendly and answers questions, Can he be bluffed/B.S.d is he a B.S.er, does he
know the mechanics of the game, or will he try to draw to an inside straight !

Each Video is like a card- so if you are going to partner with him he needs to have at least 3 of a kind - for you that would be 1-2 videos showing
''this is how I build it'', and 1-2 videos showing it running well and any improvements added, additional cards are more Rocket videos showing repairs or
changes to the system ?

If Our D.I.Y. Builder has a couple of other videos of well made products thats like a full house 3 of a kind (or better) backed by a pair !

O.K., Have you been to rocketstoves.com to download your copy of The brand-new 3rd Edition of Rocket Mass Heaters- The download will
put it into your hands now to print out copies as needed! no worries if you trash a page or two !

By the time your hot exhaust gases turn at right angles to flow horizontally through your Thermal Mass The temperatures will have dropped to a safe
temperature for the single length of Black 'Wood Stove' stovepipe, after that we have a new problem with exhaust temps in the 600ºƒ range the portland
cement found in most cinderblocks found in North America will eventually fall as the Portland reverts to its individual components !

The next problem is the Cinder blocks themselves, they are more insulating than Heat absorbing, and there rough interior walls will slow down the flow of
the exhaust gases, stalling your draft !

As soon as the Barrel goes over the Heat Riser- you must have a final vertical chimney, this will allow you to test the Rocket Burner, so that later after you
have Added the Thermal mass if you run into a problem you already KNOW the problem is not in the Rocket Burner- the Feed Tube, Burn Tunnel, Heat Riser,
Gap at top of the Heat riser or in the transitional zone as the gases flow horizontal ! Always check this part ! ( often any problems found are leaks)

The final vertical chimney should be placed on the downwind or Lee-side of your structure and be 5' above the peak of the tallest object bigger than a TV

Fire brick is where you find it, you did not give us a location so we cant provide a dealer near you ! Here is one link


This will keep you busy for a while- 'WE' will follow up with a few more links !


I know that this will seem like drinking from a fire house for a while, I promise it gets easier and you will end up with a working Rocket Mass Heater RMH !

For the good of the Crafts ! Big AL
Posts: 3124
Location: Upstate NY, zone 5
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Your all-steel core may not last for the long term, but since you have it and it appears to work you may as well use it through this winter.

You give sizes for some of your elements, but not the burn tunnel. As that is the smallest cross section, it will determine the effective system size. The feed tube is more of a chamber than a tube, and with even its top opening much larger than the burn tunnel you are very likely to get smokeback when the core is connected to a system.

There is a reason the recommendation is to keep a relatively constant cross section through the core: airspeed in the feed tube is what keeps the fire from creeping up the sticks and making a reverse flow or at least burning/smoking out the top. The base intake and cover may seem like a good idea, but they will assure that the sticks are burning top to bottom at the same time and make it unsafe to ever open the lid while burning.

You don't give the height of the riser, but it looks like the proportions of "feed tube" to burn tunnel to riser are around 1:1:2 instead of the recommended 1:2:4. This means that you get maybe half the power you would get from a properly proportioned core to push gases through the mass. Your riser can and should be about 2" shorter than the height of your 30 gallon barrel (from wherever you mount the base of the barrel) to get maximum power.

A 30 gallon barrel may not be wide enough to have a reasonable amount of insulation around your 8" pipe. Perlite/clay is recommended to be at least 2" thick in this case. Ceramic fiber blanket insulation can be 1" thick.

Your block mass idea can work as long as you use concrete block and not lightweight cinderblock. You also as Al mentioned need to have a metal liner for the first few feet until the temperature drops to a safe level for the block. The burn tunnel pipe looks to be 4 or 5 inches square, so the double 4+ inch block cores may work okay. You will be limited in how long you can make the run, and bends will further reduce this. We need more details before we can give more specific dimensions.
Korey Klug
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Hi thanks for all the advice, I am using cement block for the thermal mass air duct (exhaust) which has 2- 4 3/4" square holes for the exhaust. I had planed on having the cement block raise about 2-3" in about 7'. I'm using cutout foam insulated door pieces to lay down to keep the frost from cooling the block and mass. The height of the riser is 20" from the bottom of the burn tunnel. The burn tunnel is 1/4" thick 4" square tube which I questioned if it would have enough flow. The 20# propane tank is somewhat sealed and I can shut the flap on the intake causing the fire to smother out, when I open the lid I close the intake feed first and only a little puff of smoke comes out. Once opened you can see it draw in to the burn tunnel and hopefully putting it together doesn't change that. I had planned on lining the block at least the top and sides by bending a long strip of sheet metal into a channel for better flow. I planned on setting it up this week end and doing a trial run before I cob it in. If the flow isn't enough I can cut out another hole and weld another burn tunnel above the one I have which, i figured more smoke and gasses might enter the insulated riser just above the flame causing a secondary burn off of the gasses in the riser and 30 gal barrel. I have little investment into this project so far just some perlite and cement and mortar that I did not have already. I have a large portable tow behind cement mixer which will be handy with this build. The greenhouse is only 10' x 15' in size and I would like to grow in there all year if I could. I am adding some pictures of where I'm placing it in the greenhouse, having a limit on space I tried to keep it small. Being from Michigan and not wanting to feed a fire constantly I figured the propane tank would give me more space for wood (fuel) and possibly a more controlled burn. The pictures I am attaching should give you a better understanding of what I'm trying to do. I now am planning on getting the ceramic fiber blanket insulation for insulating the riser stack if I can find it before Sunday. Again Thanks for any advice.
[Thumbnail for DSCF7838.JPG]
Here is the lay out of the rocket stove with the blocks going down 7' and back creating a 180 flow director out of sheet metal for better flow and a 8" exhaust pipe from the block to go outside.
[Thumbnail for DSCF7839.JPG]
In this picture I plan on using 1/4" steel to make the flow director to channel from the 30 gal barrel into the block.
[Thumbnail for DSCF7841.JPG]
This is the materials I have collected or had before. The buckets are a cob mix I dug up and sifted to get the impurities out. It seems to have a good sand clay consistency for the cob.
Glenn Herbert
Posts: 3124
Location: Upstate NY, zone 5
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A 20" total height riser is not going to generate enough power to drive any amount of heat exchange ducting. I would at least double that. Also, a 4" square burn tunnel is too small to give much energy, and welding a second parallel one will only complicate matters and not be likely to work reliably. You will get smokeback when you connect the core to a whole system..

Given all these factors, I think you would get much better results with less effort by scrapping the whole steel contraption, and building a core with firebricks or even old soft red bricks (the kind that leave a red streak on concrete like sidewalk chalk). Make the whole core with uniform 6" x 6" dimensions or similar, and 12" feed tube (top to bottom), 24" overall burn tunnel, and 40 to 48" high riser (top to burn tunnel floor). Mortar the bricks together with a very thin mix of refractory cement or even fireclay and water, and wrap insulation around the core (ceramic fiber blanket or perlite/clay with a wire mesh wrapper). Make sure there is at least 2" of space all around between the riser insulation and the inside of your barrel.
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