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rocket mass heaters

RMH in my hoop house

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posted 5 years ago

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I'm planning on building an eight inch RMH and allowing the heat to drop below my heater. The pipes will then run about 12" under ground about thirty feet before venting. I would be using just the ground for the heat mass. Is there enough heat retention in the ground to keep a 20'x50' hoop house from freezing when the temperature is about 20F at night?

thomas rubino

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posted 5 years ago

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Hi Mark;
Highly recommended to insulate under those pipes.
You will lose a good portion of your heat if you do not.

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Mark Grindahl

Posts: 26

posted 5 years ago

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I guess my thought is the heat would just dissipate into the hoop house but it sounds like I need to contain it.
Is that the theory

thomas rubino

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Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Hi Mark;
Yes , you need to direct the heat. Having a mass of some kind over those pipes ,(other than the earth) will help as well. Solid stone retains heat longer than dirt.
Insulation of some kind under the pipes is most important. Insulating at the sides will help as well.

How tall and long is your hoop house ? Do you use double skin plastic?
20 all night is pretty cold. Are you hoping to heat all winter ? Or just extend the season on both ends ?

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Mark Grindahl

Posts: 26

posted 5 years ago

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I'm just planning on extending the seasons. I live in Central Minnesota and I have just one layer of poly.
I could put a layer of two inch foam on the bottom and sides. Would two inches of sand be enough thermal break between the pipe and the foam. I could then fill the trench with rock and sand. I have quite a bit of two to six inch rock.

thomas rubino

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Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Yes 2" foam on bottom and sides , with 2" of sand over it would work well.
Now for over top of the pipes. Cob (gooey mud) is much better than sand. Sand acts as an insulator. Cob holds the heat the rocks hold the heat longest.
Any kind of clay mud. It fills any air gaps. Just enough to seat the rocks.

Depending on the type of things you are growing, single skin plastic might not be enough.
You may need on the colder nights to throw something (blankets? ) over the plastic.
Hopefully not though. RMH's work amazingly well.

I have an 8" J tube RMH in a plastic greenhouse here in northern Montana. We heat all winter long. No fire all night. -10 F outside 40+ F inside! They really are Awesome!

Greenhouse-studio-RMH.jpg

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Mark Grindahl

Posts: 26

posted 5 years ago

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Nice looking unit you have there Thomas.btw thanks for all your help.
I don't have a real good source for clay so I'll have to do some checking around.
My hoop house is 20'wx50'lx11't so extending the season in the fall for some late raspberries we have in there and possibly starting our seeds in the spring would be the main use of the heater. We have a large garden so when my wife is starting her seeds in the attached garage I need to put up allot of lights for her.
I may consider building something over the bed of rocks for starting plants in the spring.
Any idea what size the rock bed would be? The trench will be about thirty feet long.
Also if I made a clay slip with fire clay, is that good enough to set the fire brick with?

thomas rubino

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Posts: 6525

Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Yes Fireclay as a slip with some sand in it is perfect. for firebricks.

Forest roads, creek bed, anyplace you see an excavator digging (building foundations) all are good spots to find clay. Asking the county road crew if they have a problem clay bank you could harvest from.
Worst case use wet dirt rather than sand.

Sand makes a good temporary mass but is lacking good long term heat holding capability.

You really won't need much if you use lots of rock.

EDIT) Rock bed should be as large as your space permits. More mass is always better than not enough.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Thomas, what are the dimensions of your greenhouse?
Is it one or two layers of poly?
What would a burn schedule look like for a week with the temp around twenty degrees?
Also, how often does wood need to be added to the fire?
Thanks again for all the info.

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Hey Mark;
12x20 x 16' tall. There is also a "mud room" entry and a wood shed on the far end. The whole structure is probably 40'.
The greenhouse portion is covered with a bendable product called Solexx. Expensive but really good stuff. Actually has an R-value (very low) Part is single pane glass that gets covered with plastic during the winter.
For our 8" J tube, with dry soft wood. We allow 45-60 minutes between loading's, after the initial startup and its gotten up to rocket levels. Coals could still be there at 1.5 hrs but don't count on it.
Side note) A hot RMH that went out , relights very easy.

Our greenhouse is also (mainly) an artist studio. We burn all day and depending on expected temps we stop around 9 pm. Next morning I will restart anywhere from 7 am -10 am.

Are you talking a low of 20 overnight with closer to freezing during daylight ?
I would guess that raspberry's can get chilly (above 32) without damage. So I would guess that a fire started mid afternoon and run into the evening would hopefully keep you above freezing all night.

The thing about an RMH, is the mass takes time to absorb the heat. If you run a RMH every day its easy. If you try to pick and choose when to run it, you will need to burn longer to fill the mass fully with heat.

Mud-room-studio.jpg

woodshed-studio.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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It looks like my biggest challenge will be keeping the heater fired up
If I'm away from home for nine hours at work I may not get enough burn time to heat it properly

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Hi Mark; Don't despair yet. There is another option.
There is another style of RMH, not a J tube but what is called a Batch box.
This style burns hotter than a J tube. Fire last 1 to 1.5 hrs, wood is larger and lays down horizontal.
You will get more heat charge in your mass quicker.
There is a small catch... of course... This is a technical build. dimensions , air channel must all be spot on.
There is vast amounts of free information available from master builder, Peter Berg. here is his website http://batchrocket.eu/en/
Another website strictly about RMH is our friend Kirk Mobert (AKA DONKEY) here is his web site http://donkey32.proboards.com/

It may be that a J tube build will work for you. The mass stays the same between both builds. The difference is the core burn unit.
You may consider building a J tube to try. If it is just to much effort to maintain temps with your schedule. Then tear down the J tube and turn it into a batch box!
The beauty of working with fire clay versus concrete is , fire clay easily knocks apart. Concrete does not.
Don't let my statement of a technical build worry you. Plenty of people have built this style and our cheerful crew of rocket scientists would be willing to help guide you thru the build.

Congratulations You are about to become a rocket scientist !

thJU1W6X2H.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Thanks again Thomas.
I had done some reading on those but I think the RMH is what I'm looking for.
Even though the lakes are frozen over here and it was 16 this morning, the ground in the hoop house is still workable so I've gotten a start on my heater.
I hope to get everything laid out and then assemble it in the spring.
Can I just use fire clay for sealing things up around the burn barrel or is cob preferred? I read something about cob not working so well in a damp environment.
Are larger rocks preferred in the mass and is there a general rule of how much mass to be on top of the pipe?

Mark

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

3394

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posted 5 years ago

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Hey Mark;
Yes , you can use fireclay in or as cob, just add sand. I can get 50# sacks of fireclay around $8.00 I have seen it as much as $17 .
Cob will soak up water if you expose it. I guess just don't spay the hose on it. Clay/ cob is all the same no matter if reg clay or fireclay. (fireclay is for high heat areas)
Your mass can be completely covered on top by bricks. Thus eliminating any water issues.

Large rocks mean less cob. Your cob in the mass is simply filler between the rocks and the pipes.
The only concern you need have is not allowing the rocks to crush the horizontal pipes.
I dealt with that by standing rocks along side the pipes, fill with cob and bridge across with a larger rock. After that bury away. Cob Lasagna , layer of rock ,layer of cob, No air spaces. The more mass you have, the longer it will take to heat up BUT.. the longer it will take to cool off. General rule of thumb with a mass, is bigger is better. Use as much room as you have.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

3394

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posted 5 years ago

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Hey again Mark;
Because of your limited time to build a fire, you may want to start with less mass on top so your heat moves thru faster. Experiment , more mass is better, but not if you can't heat it up.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Hey Thomas
The best price I have found so far locally is $15 for 50# but everything I saw on line is over twenty.
Is there a specific sand to clay ratio that people use for a cob like finish?
I've also been looking at possibly using wood pellets in my RMH .I think if I could get it to burn for a couple hours in the morning when I'm gone, I could fire it up again with regular wood when I get home.
I have seen some videos where there has been some success dong it so I think with a little trial and error I could get it to work.

Mark

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

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posted 5 years ago

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Hi Mark;
What part of the country are you in? Nearby large city's ?
Try looking for a masonry supply house.
I use (Mutual Materials) in Missoula, they are all over the north west.
If you can't locate one. Then try to locate a fireplace builder. Look in the yellow pages. They will know where to buy fireclay, or maybe offer to sell you a few bags.
Last ditch is the small rural hardware store. They may stock it for local contractors. Cost will be higher of course.

DSCN0616.JPG

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

3394

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posted 5 years ago

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Now let me answer your other questions...
Pure clay will crack. Add sand and it gives it thermal resistance.
Add too much sand and it wants to fall apart.
3 parts sand to 1 part clay is a good starting mix. Most of my cob was mixed at that ratio. Some of my crack repairs on the barrel are much heavier mix of sand, 5 to 1 maybe?
It depends on what use that particular batch is going to. Higher heat , thinner cob (on barrel) use more sand. Filling up your mass... use what you want.
One note about sand. Its not all equal. Beach sand from the ocean does not work well. Its to fine. River / creek sand can be fine. Sacks of building sand from the same place your fireclay comes from work the best (cleaner, very uniform size).

Now wood pellets. Were you thinking of a self feeder ? Those have been built successfully but they have been known to start fires where you DON'T want one.
You would not want to fill your feed tube past the burn tunnel roof or it would starve for air.
I don't think they would burn as long as regular firewood.

One thing that works for us. After your rocket is up to full burn, you can add full round's of wood. Add a few smaller pieces around the corners and go away.
It will burn hot and clean for a while and then the round will stay going by itself for much longer. Its not clean burning at that point, but you won't be home to notice.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Morning Thomas,
I bought the book the rocket mass heater builders guide a couple of weeks ago so I am getting some of my questions answered there. I could probably get all of them answered if I seriously read the book.
Does using a cement mixer work for mixing the clay and sand?
My idea with the wood pellets would be to use an auger and experiment with having them fall in to the burn area. If they fell into the back half of the burn chamber, it seems it would be similar to adding wood.

Mark

Mark Grindahl

Posts: 26

posted 5 years ago

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I forgot to mention the fire clay I found was from a masonry supply house

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

3394

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posted 5 years ago

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Yes, you can use a mixer. Although it might make for a heck of a gooey cleanup.
Myself I used a big blue tarp. One part dry clay to three parts sand. Add water, roll tarp , stomp, roll again stomp.Add water as necessary.
Was high summer when I did mine. I tanned a nice dk brown , lost at least #10 and built up my muscle mass. The wife liked to sit in the shade sipping tea and watch me... naughty girl...
Other folks are smart, they use a tractor. Dig a trench for the rear wheel , pour in dry clay and sand , add water ... drive back and forth to mix... very easy... if you have a tractor.
Other inventive folks use child labor... make it fun dancing on the cob tarp. Have hot dogs & pop for the little dears ... and work them till they go to sleep!
If you have teenagers the previous suggestion will not work...try cash!

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Hey Thomas, I've gotten my firebrick dry stacked on a slab and my heater mass form in place. I put a two foot wide piece of two inch foam on the bottom and the sides are also two inch foam two feet high.
So my mass will be 20"wide, 2 feet high and 28' long.
I'm still leaning toward trying a mixer so I can keep my clay/sand mix loose and pour it into my form in layers. Then I know there won't be any air gaps around the rocks. I may have to let the layers dry some before pouring another.
I was reading in the book that a 1/4"rise every ten feet in the vent pipe will work.
Mine is currently 2" every ten feet as it follows the floor of my hoop house. Do you see any issue with that?
Mark

thomas rubino

rocket scientist

Posts: 6525

Location: latitude 47 N.W. montana zone 6A

3394

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posted 5 years ago

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Hi Mark;
No problem with your rise. That heat was going up no matter what.
Whatever mixing method works for you with cob will be fine.
I can assure you that throwing sticky wads of cob down and then patting them in (just like patty cakes !) you will fill all air voids.
Pouring it in from a mixer sounds like it might be high moisture content. If so it will take WEEKS or more to fully dry and heat up. It takes quite a while no matter how wet or dry your cob goes in.
Size of your mass sounds good. I'm assuming a single pipe run , with a vertical ending ?
I'll mention that a dry stack core will not perform very well. Even for experimentation. Mix up some clay slip. Dip each brick in as you place it. The purpose is to seal air leaks, very thin is all you need.

I found a few old photos from my greenhouse mass build. Cob lasagna, layer of cob, layer of rock. Of course I had beautiful flat slate slabs to work with.

R-21_01.JPG

R-22_01.JPG

R-27_01.JPG

R-25_01.JPG

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Mark Grindahl

Posts: 26

posted 5 years ago

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Yes I'll have just a single pipe with a vertical run at the end. I'll put a T at the end for cleaning and pre heating the flue if necessary.
I do plan on using clay slip on my fire brick. It's stacked now just for layout and dimensions.
I'm sure the mixing of the clay will be a bit of trial and error. I do have a tractor I can use so that is still a possibility.

Mark Grindahl

Posts: 26

posted 5 years ago

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Trying to send a pic of progress so far

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