We are making a rocket mass heater for our small (10X20 ft) greenhouse. Our plan is to have the exhaust flow through one side of a row of cinder blocks before turning 180 (using bricks) and flowing back the other side before exiting to an outside chimney.

My first question is how well would the cinder blocks hold up as our exhaust chamber when covered in our sand/clay mixture? We are building our burn chamber and riser chimney out of used fire bricks which will then be covered by a 33 gal steel drum. We plan on burning as small of fires as possible, as we are in the south and only using this as supplemental heat to keep temps a little warmer than they would stay overnight otherwise.

Second question: We will be using the mass heater as a bench to hold plants. Do we want straw in our mix? Do we need it? Our bench (covering the cinder block exhaust channel) will be around 3' wide, and about 18" to 24" high. We will be adding 4 part sand to 1 part sticky clay.

Can we dry the bench out by building very small fires?

Here is what I would do:

Take the 33 gal. drum, fill it with water, set it on top of the rocket heater and let it be the mass that does the heating. Get it up to near boiling at sundown and it will keep the greenhouse plenty warm through the night.

I have a 8'x24' greenhouse, and three 5 gal. buckets of water from the water heater (~120F) keeps it warm down to 20F.

Here in the South, you don't have to add a whole lot of heat to maintain a greenhouse like you do in the North. Our soil temperature rarely goes below 50F, so there is always some heat coming up from below, unlike in places where they actually have "frost depth". I think you will find that a full-blown rocket mass heater with a bench will be overkill if all you want to do is keep plants from freezing. Now if you want to have a tropical type greenhouse through the winter, where you have Hawaiian plants in full flower, then yes, a RMH would be what you want.

Thanks for your reply, but yes, we are wanting to do more than keep things from freezing which is what we did last year. I did store a lot of water under the pallet tables I set up, and in fact added 130 degree water from a 55 gal buried in a hot compost pile, but this year we want our tropical plants to do more than just survive the winter.

So, back to my questions about the cinder block, straw, and drying the bench.

Ken Carman : A few quick answers to your questions, the Lime in the Cement in your Cinderblocks starts to 'heat stress' and fail about 400F, Famously, paper starts to burn at 451 F.
Stick with Fire bricks and all cob (and any old bricks) for the shell of the Burner part of your Rocket Mass Heater ! Only about the First 6 ' of your Thermal Bench where the horizontal
exhaust flow starts is in danger, the temperature should have dropped by then !

To have two trade offs an 8 '' system is too big, a 6 '' system would find it very hard to flow up and back again ~34' - 38'~ and this through regular smooth walled stove pipe, don't
forget you are going to need at least 3 Clean out "T"s, and an Elbow ! I don't think you can flow that far with the anything but smooth walls of Cinder blocks and Bricks !

The magic that allows the Horizontal flow of hot exhaust gases through your Thermal Mass requires that a certain amount of heat energy, or B.T.U.s be radiated off of the barrel to
create the Thermal differential Engine effect, Counter-intuitively, the 33 gal barrel must radiate off the same amount of heat energy as the Larger 55 gal barrel, to do that with less
surface area to radiate from, it must do so at a higher temperature ! For this reason and for your backs sake, I would make your Bench much taller, with a more massive bench and
an increase in surface area - and by burying your horizontal piping deep within your mass, you will radiate the heat latter while it is cooler, and at safer temps - And your back will
thank you a thousand times a day !

Yes you want some clay straw, start with 'painting everything with clay slip, then your clay sand cob, for the last 2 -4inches you want the extra strength of Structural Cob - sand,clay
and straw, this protects your bench against the knocks and thumps of daily life! then a water shedding finish coat for the top.Try to create a flattened muffin top with just a little pitch
this should have a sheet of plastic to guard against spills which could eventually cause the bench to slump !

Go to the Rocket stoves forum and find the forum/Thread _ ' Rocket Mass Heaters in Greenhouses - - - - - ', there is a lot of practical information you will find useful there !Also at the
bottom of this page check out the Similar Threads, the computer thinks that these threads are - well - similar to your topic !

You can dry with small fires, it will take a long time ! for the Good of the Craft !

Think like fire, flow like a gas, Don't be the Marshmallow ! As always comments and questions are solicited and welcome ! yours in haste ! Big AL

Great info! I appreciate this very much. I am, however, not sure that I clearly understand all that you wrote about the cinder block portion. It seems that you are saying that we need to use fire brick for the first six feet of exhaust leaving the bottom of the outer barrel, and the rest could be cinder block, right?
Are you saying then that we should only utilize a straight run if using the block? The only reason I was going to do the 180 was because of all I had read about the heat going through too quickly, and not enough being absorbed. What if we had it only run a few feet, and then turn back? We can do that, or run the full 17' or so with no turn. What do you recommend?

Oh, and I'm confused about how making the bench high(mass greater) will save my back. I understand what you said about the 33 not radiating as much heat as a 55, but didn't get the connect to bench with greater mass. Wouldn't a portion of the success of the draw have to do with the chimney outside?

Again, many thanks!

Ken Carmin : O.K. let me try again,. I am more worried about any use of cinder blocks within that part of the Rocket Mass Heater R.M.H., that exists prior to the Thermal Mass
Bench, all use of cinderblocks are certainly likely to fail there, any cinder block within th first 6 feet of the horizontal piping in your thermal bench is also likely to suffer from the
high temperatures experienced! This could all be made of just Cob, but, this is in no way an endorsement of the use of cinderblocks Do to their rough , jagged internal walls they
will rapidly slow the flow of gases through your horizontal pipe, I tried to send you to "rocket mass heaters in Greenhouses - - - " where this subject has been considered,
It is possible, but I know of no working model with cinderblocks!

Both barrels will HAVE TO radiate exactly the same amount of heat, the Smaller barrel must by the laws of Thermo-dynamics 'release' or radiate the same amount of heat as the
larger barrel, and is constrained by its lesser surface area to do it at a HIGHER Temperature !

This is why when your 3-year old wants to climb up in your lap they are such a great snuggle, their Surface Area to Weight, or Total Skin Area to Mass means that they have to
radiate this heat out at a higher Temperature !

Again, everything else being equal, A larger mass will radiate just as much heat energy as the smaller mass, doing so at a lower temperature, if the horizontal piping is buried
deep inside the Thermal Mass, and the Thermal Mass is much larger, the increase in surface area will mean that you will radiate exactly the same amount of heat energy to warm
the surrounding area as a smaller bench, but at a lower. safer temperature, and do it much latter at night/ in the morning as compared to a smaller bench !

With a larger/higher Thermal Bench you will be bending over not as far and the back that you save will be your own !

Many years ago in the building trades we used to say " A good coat of paint is like a whores blanket, it covers a multiple of sins !" Think of the vertical chimney as assuming the
same purpose, ( adding suction where needed !) 15 feet of vertical pipe will correct for 10 horizontal feet of bad/ inadequate piping ! Consider each elbow or 'T' as being equal
to 5 feet of straight smooth walled pipe ! Again if any part of this is not clear come back often your fellow members will try to help ! For the good of the Crafts !

Think like Fire, flow like a gas, Don't be the Marshmallow ! As always, your comments and questions are solicited and Welcome ! Big Al ! PYRO - Magically Big AL !

We are working through our list of available materials (without having to purchase). We would now like to use 10" ID X 1/4" steel pipe for our exhaust run. Would this cause a significant problem if we build our "J" burn chamber entirely out of firebrick (including internal chimney), which would give us about 6 1/2" X 6 1/2" square inside the "J" In other words, what happens if our exhaust run is bigger than the run inside our outer barrel? Would it help if we condensed back to 6 1/2' square before heading up our exterior final chimney? Would we be better off using the 10" metal pipe for our internal chimney too?

How important is it to utilize stone in our bench for heat storage? We have unlimited sand (and clay), but would have to locate stone (as we live near the ocean with no native stone).

Ken Carman :We have wasted a weeks worth of your time that we will work very hard to give back, in order for you to understand clearly the importance of keeping a Constant
Cross Sectional Area in the Feed Tube, Burn Tunnel, the Heat Riser, and at the top and sides of the barrel, and match the size of your horizontal and vertical chimneys to the
size of every thing upstream you Need to go to rocketstoves.com to get a PDF Copy $15. of Evans' book Rocket Mass Heaters , there is STILL No other book in any
language with more rocket mass heater R.M.H. information! (I don't make a dime ) rather than spoon feeding you the information you need Now, and do it question by question
you need to have this information at your finger tips to save you time and money !

I am afraid that a over large 10 inch diameter pipe would cause the flow of hot exhaust gases created earlier to become very sluggish, wasting the R.M.H.s ability to flow them
horizontally, any changes that we would have to make to use you 10'' pipe would require building a system much much to big for the size of your green house !

Any mass that is heavy will do for thermal mass, People often make nuisance dumps of construction materials at the end of one way roads, old pieces of broken up concrete can
be included, a phone call or two will find you a contractor who is redoing concrete driveways, side walks and steps who has run out of places to dump his spore, Make sure you
can use what you get, hauling it yourself over finding that 4 large dump trucks are sitting in your drive way, and a man is asking where you want all this fill !

Often within monolithic pores of concrete there is a center core of packed stones and gravel, I would call this a cheat to save on cement but was a common technique during and
after WW11, finding one of those might give you a glimpse of your regions history and a stone source !

All fill that is not cob can be planned for in advance, saving them for decoration after final placement of pipes has been found that worked for you during your out door test fire,
first everything gets coated with a layer of clay slip, followed by Cob, then your stones and broken pieces of concrete, presoaked in water and thin coated in clay slip to prevent
the trapping of air in your cob and finishing with stones and old brick from job sites/ an old school, the end result should be both functional and beautiful !

Your best option so far would be to make your burn chamber and Heat Riser out of brick, stacking your fire brick on its side to promote height over thickness and then making an
insulative coat of clay slip and perlite outside of that. Be sure to give yourself at least 2'' of air space between the outside of your insulated Heat Riser, and the inside of your
barrel all around !

You may be able to find where old road materials were quarried from shell piles from ancient sea beds, still containing large pieces of ancient corral, this material a potter calls Grog,
and it makes for strong durable cob indeed ! Please get your copy of the book and come back here often to work towards your future R.M.H. for the Crafts ! Big AL !

Well . . . been having problems. We get fairly decent draw at first, but as it heats up, the draw diminishes, and fuel starts burning upward as much as down the burn tunnel.
Here's what we have; fuel chamber 7" X 9", burn tunnel 7 X 7 1/2 (52.5sq")and 15" long, chimney 7 1/2 X 7 1/2 and about 38" high above the roof of the burn tunnel. {All fire brick, as suggested}
We have about 4" gap above chimney to barrel, and at least 3" (average) gap around and between barrel and outside of chimney. Perlite/slip mix makes the square chimney round, but very thin at corners (about 2" thick in centers). The barrel is in fact off center, pushed toward the exhaust side, with the opposite side resting against the perlite so that all air is flowing down one side which has a gap entering our cleanout of about 70 sq ". This cleanout area is probably over 100sq" at it's widest spot. Our primary heat exchanger is a 3/8" thich 10" pipe that runs 40", then a 90 elbow, then about 20' more before transitioning into a temporary cinderblock chimney that is 7 blocks high.

So, why has our draft been so weak?

1. Is our feed/burn chamber too big?
2. Is our tunnel too long at 15"?
3. Do we have too much space between the top of chimney and the barrel? (at least 4")
4. Is our cleanout chamber too big? (at least double the cross section of burn tunnel)
5. Is our primary heat exchange pipe too big ? ( at 10" diameter)
6. Is our temporary chimney inadequate?
7. Is our fuel inadequate? {we have been burning split up pine pallet pieces, wanting too save our oak for needed use}

BTW, yes, we did purchase the pdf book.

Ken Carman : Please look at the answer I gave Galadriel Freden in her Rocket Forum/Thread 'having a little trouble with my mock up' Yours in Haste, BIG AL !
this would also eliminate #5 as a problem in your test bed ! A.L.

Al,
Thanks for your quick response, but I am a bit confused about the answer. I read the post and your anser, and it sounded like they were having a very similar problem, and you addressed the vertical exhaust chimney.
A. I'm confused how that eliminates #5 (10" heat exchange run) as the problem.

B. Does it matter how big any cross sections are past the burn tunnel & chimney, as long as they aren't smaller than the burn tunnel cross section? We were thinking that maybe we have too much space above and/or around the barrel . . . or in the larger "cavern" where the cleanout is, just before entering the 10" run. Can those cross sections be too big, or do they make no difference?

C. Is our only problem our vertical final chimney?

Thanks!

Ken Carmen : Hopefully by now you will have had time to try my suggestion, This test hookup eliminates any problems downstream from the Transitional Area Of your RMH.

The 10'' pipe is oversized and will actually slow down the flow of the gases through your system, you went from white water rafting to being stuck on a lake with its fast
flowing outlet miles away ! Put it an other way, you might have great water pressure in a garden hose that allows you to squirt water onto your roof to help clear your
gutters but, if you tried to run the same water through a 3'' fire hose the pressure would drop so sharply that you would be lucky if the water flowed out of your pipe with
a vertical column of 1 inch! Anecdotal evidence says that you can decrease the diameter of the Horizontal piping going through your Cob Thermal Mass near its end, or
even at the base of your vertical chimney! On a first build is not the best time to experiment, I have not done this trick yet and may never do so !

When I sent you the quick note this morning, I was trying hard to be helpful by not over-diagnosing! What I suggested would have gone a long ways in telling you where to
look for your problem next ! I hope I was clearer this time when I try to give information quickly,in a condensed manner it always causes confusion ! Good Luck ! Big AL