Rocket stove mass heaters...

Discussion in 'Off Grid Living' started by Witch Doctor 01, Dec 6, 2011.


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  1. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    No use wasting Good Room Air, to feed a fire that is heating an Enclosed Space. Very counter-productive....
     
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  2. Rabid

    Rabid Monkey

    Any wood stove uses air from within the room, combustion air is rarely brought in from outside except for some sealed fireplaces. If you have a wood burner now and it is working properly then a rocket stove mass heater will work also. I have a friend who had his house so air tight that he couldn't burn his wood stove unless he cracked a window open. Most houses just aren't that air tight and one that is may not be very healthy. If you wanted you could run an air duck to the fuel opening in the mass heater and it will draw from outside. I would put a butterfly damper in it though. If your concern is controlling the burn of the mass heater there is no controll, it burns flat out there is no control. The idea is to burn it hot, hot is clean, and get as much heat off of the burn as possible. If you don't have a large enough heat sink then your exhaust will be hotter and that is wasted heat. There is comming a day when the government will no longer allow those smoldering, smoking wood burners and people will be caught off guard.
     
    kellory likes this.
  3. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Apparently you haven't been around the technology innovations in the last two decades of Wood Stove design. Any Good Wood Stove will have an External FireBox Air Intake, so that it does NOT use Internal Air from the room. This does two things. First it keeps the Draft at a constant level, that doesn't change as the in-room environment, changes. (Like opening a door to another room in the cabin) Second, It doesn't allow an Air Pressure change, as the combustion sucks the Oxygen out of the room, and forces the CO2 up the chimney. The results of a Negative Air Pressure inside the cabin is that Outside Cold AIR is sucked thru every crack, door, and window frame, which just makes the place all that more harder to heat, and burns more BTUs to hold the same room Temps. If you lived in Cold Country, and heated with Wood, this would be self-evifdent. ..... YMMV.... but the Physics doesn't Lie......
     
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  4. gunbunny

    gunbunny Never Trust A Bunny

    Yup, an outside air source for the cumbustion is 10% more effecient than using the room air because the stove doesn't have to pull replacement air through the cracks in your walls. Here's some pictures of my pellet stove's air system.

    012 (903 x 1204). 011 (912 x 684). 010 (912 x 684).
    The top picture is the front of my stove, shown so you can understand the layout of the next two pictures. The middle picture shows the view down from the top of the stove, and the last picture is slightly to the side, looking down. The upper duct is fresh air, and the bottom duct is the exhaust. The picture makes everything look much dirtier than it actually is... ;)
     
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  5. Rabid

    Rabid Monkey

    I guess the reason I'm so hot on the mass heaters is that I got burned on the wood stoves. (puns intended) After I bought my wood burner the town put a ban on them because of the nuisence smoke. Yes, I took a loss but a few years later I moved out of town and bought a place that just happened to be down wind of a smoldering black box. On nights when we had an inversion layer I thought I would die. No wonder I got the house on my first offer. I have not given wood stoves much consideration since those early years. I can't say I even know much about them. Now that I am looking to move to a more remote location I don't want to send up smoke signals to let everyone know where I am. Since the rocket mass heater only smokes a very little bit on start up It has caught my attention and I have been working on trying to figure out my own design. One of the things I plan to include in my design is an outside combustion air source. Thanks guys for pointing that out. I also have an idea for a rocket stove mass heater/ oven. I guess i had better get on the stick and find a piece of land.
     
  6. Motomom34

    Motomom34 Monkey+++

    This is the most interesting thing. Due to fire danger I worry about sparks and plus the smoke would be an issue, also any time a neighbor lights their wood stove I get on high fire alert when I smell the smoke. This is something I think would be do'able. Occasionally there are no burn days but this rocket stove wouldn't really apply. Going to see if I can locate a book.
     
  7. Cruisin Sloth

    Cruisin Sloth Special & Slow

    smouldering black box is ???

    I live with an Airtight (wood burning fire ) , Mine is hard to see smoke.

    Sloth
     
  8. Motomom34

    Motomom34 Monkey+++

    I wonder how long the exhaust vent must be. I see that these seem to have long sitting areas but I wonder what the minimum would be.
     
  9. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    The issue is how well the Exhast Stack DRAFTS, when the firebox is burning at temp. The longer a horizontal run and the more 90s in the flue, the worse it will draw, and the efficiency goes way down. So any design needs a long straight vertical Flue, connected, after the thermal Mass and the surface area between that thermal Mass, and the flue Gases needs to be as much as possible without constricting the flow of the Gases, up the flue. These are the design trade offs.
     
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  10. Rabid

    Rabid Monkey

    The vertical flue doesn't seem to be an issue. I've seen many with only a two foot rise. There must be some kind of formula out there to figure the optimum run of stove pipe and amound of heas sink you would need.
    Here are a few vids that helped me understand rocket mass heaters

    Rocket Mass Heaters
    Rocket Stoves -- Home
     
  11. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Wait till after you build a few, then come back with some REAL World experience, on just how much flue, is going to be required, to draw appropriately.... It isn't as easy, as they make out on the Vids... ..... YMMV.....
     
  12. Motomom34

    Motomom34 Monkey+++

    The old saying to good to be true.... well this seemed like a very interesting and smart idea. I started researching. Though the flue issues/questions raised are very valid, this idea has been trashed. Done! In my research last night I came upon this article:
    rocketmassheaterpermitting - www.ErnieAndErica.info
    that basically said these people started this project in 2009 and in 2012 they were getting their permits. My eye started twitching. Years of permits, inspectors and county officials, no thank you. I googled locally to see if I could find anything and no way am I going to spend years teaching a bunch of yahoos with hard hats and clip boards that rocket mass heating is okay. Someone may have already blazed that trail but I like sanity. Just putting up a simple deck is a permitting nightmare.
     
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  13. Rabid

    Rabid Monkey

    I had not considered the permit aspect of the rocket stove mass heaters but luckily for me my area hasn't quite gotten to the permit and licnesed electrician to change a receptical insanity. There are a few Russian fireplaces and home built fireplaces that weren't professionally engeneered and permitted in the tri county area. All the same I think I will experiment outside before putting one inside.
     
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  14. Oddmar

    Oddmar Monkey+

    To answer BTPost's question about draft before you read this post... The 'chimney' is inside the stove...all the 'exhaust stack' does is get rid of the exhaust gasses...the draft is created inside the stove and not by air flowing over the roof, or by a super-hot wasteful chimney...read on.

    ++++++++++++++++++++++++++++++++++++++++++++++
    I built a prototype Rocket stove from doubled duct work and vermiculite, then started to build a rocket mass heater for a friend which ended up being just a good rocket stove barbecue. When i tried to rebuild that to make it lighter it fell apart.

    I'm the kind of fellow who peruses the internet and books for months doing research, spends alot of time making detailed plans, re-does the plans a couple times, then it never gets done because i can't afford the materials.
    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    I moved from central Illinois to central Arkansas back around Thanksgiving. Partly because on trips down here to go to a major 4-wheeling event with my friend i fell in love with the fresh air and forested countryside, partly to help my now-disabled friend build a 30x48 shop.

    They will be living in the back of it when the bank repos their double-wide trailer house in the next couple of days. We have totally run out of money but have gotten the roof on the new building and the walls halfway up...my friend says he and his brother n sister have lived in worse conditions for months in the past, so they'll survive until we can afford the tin to enclose the building.

    When we get it completed we'll open a 4x4 truck shop, do vinyl signs & graphics, aerial photography, and maybe sculpture, so money ought to start trickling in soon.

    I built my first 8" rocket mass heater over the last month. The thermal mass is a little short, so the exhaust stack gets hot enough i can only lay my hand on it for a couple seconds. But it stays cool enough for safety.

    I remember Matt Walker (Permsteading.com • Index page) saying he had to leave the windows and front door open for awhile because his first build was cooking him out of the house. I remember 'Fisherman's Daughter' on that same site saying "We have to lay on the floor to breathe". The one-room cabin I built after i moved down here is only 12'x12'. I've found out what y'all experienced firsthand. (Pant...Pant....Gasp)

    My friends and friends of theirs who've stopped by to help with the new shop construction are very impressed that 6 hours after i've had a short fire in the RMH, the cabin is 80°.

    001planning.

    003planning.

    004stucktogether.

    The feed chamber, burn tunnel, and heat riser are all built from 2"x4"x9" firebrick, cemented together with Rutland refractory cement. The feed is 8"x7" (oops) for a CSA (Cross Sectional Area) of 56 sq in. The burn tunnel ceiling is 8" down from the top lip of the feed chamber. The burn tunnel is 6"x7" for a CSA of 42 sq in, and 10 inches long. I formed a 'Peterburg' tripwire 4" in on the ceiling of the burn tunnel from refractory cement smeared to a sharp ridge. I've yet to build Peter Vandenburg's fresh air injection plate in the front of the feed chamber.

    The heat riser measures 7"x7" inside for a CSA of 49 sq in, close to the CSA of an 8" round pipe, 50.24 sq in. From the ceiling of the burn tunnel to the top lip of the heat riser it is 40". This is four times the length of the burn tunnel, and drives the system very well. The top of the heat riser is 2.5" or so from the 'top' of the drum. I don't have a pyrometer yet so i dunno how hot the drum is getting...but it turned a neat shade of blue near the top.

    005ceilinginsulated.

    006exhaustplenum.

    007exhaustplenum.

    I welded together a three-legged metal stand for the drum to sit on, and built the largest exhaust plenum i could. I'd heard other RMH experimenters warn about choking the system by making the exhaust hole from the drum too small. It is 18" wide, 3.5" high over the burn tunnel to 7" high by the back of the drum (more room). That descends 20" sideways to become 10"x10", then feeds into an 8" tee for the ash cleanout. The pipe inside the thermal mass is 8" and about 18' long before it exits the mass and turns upwards into the exhaust stack.. That's a little short for this unit but i didn't have any more room.

    008testfit.

    009testlayout.

    Since this picture was taken, i've filled the thermal mass bench with a mixture of clay-rich dirt, limestone shale, and alot of gravel. I placed broken concrete chunks and melon-sized rocks everywhere they wouldn't impede the flow of fill around the pipes. The tube nearest the room is barely covered and the one by the wall is half covered...that's how i've been running it so far. I'll dump in more mass when i have some spare time, but moving all that weight is exhausting work. I'll probably mix up a soupy batch of red clay and pour it over the last batch of gravel i put in, to fill the small gaps around the gravel chunks and thereby improve thermal conductivity.

    010testlayout.

    014heatshielding.

    The drum ended up being about 5" from the drywall, too close for comfort. So i put sheet metal up on the wall with a 1/2" air gap between it and the drywall. Air can get in the bottom and flow upwards to keep it cooler and help heat the room...like i need another source of heat...lol. I put tinfoil everywhere else i thought it might get hot for added safety.

    The T in the exhaust stack is there so i can burn a little paper in the stack to get the air moving in the system, to make it easier to start. I welded metal tabs to some expanded metal mesh, then ran self-tapping screws through the T into the tabs. The mesh sits level with the bottom of the cap...a platform to put crumpled paper on and keep it from falling down the pipe. Then i light the paper, quickly pop the cap in place, and light twisted paper to stick in the feed chamber. The air being pulled into the burn tunnel by the hot updraft in the exhaust stack sucks the flame through the kindling, starting the system with minimal flame in my face.

    015firstrun.

    Today i folded a 30"x10" piece of 'hardware cloth' (metal mesh with 1/2" holes) twice to make a heavy screen to lay over the feed chamber when i need to go up the hill to the construction site and can't stay to mind the fire. That should keep embers from popping out and setting anything ablaze. I try not to leave it while it's running but sometimes ya gotta.

    ++++++++++++++++++++++++++++++++++++++++++++++++++++

    Once again last night it dropped down to 39°F...i didn't Really need to run the heater, but i did cause it's neat.

    One load of wood, loosely filling a space 14"x7"x8" (the feed chamber), burned for about 40 minutes. I stayed up long enough to see it burn down to dark red coals...then i put a metal lid over the feed opening to keep it from drawing air all night and cooling the mass off by convection.

    It was comfortably warm in the cabin for 8 hours. It was still nice in the morning but i stepped outside and was shocked by how chilly it was. So i started it up again and heated up some water to make coffee and Ramen noodles for breakfast.

    Burning the stove a second time in less than 10 hours made it really toasty inside. I opened the window and after awhile it was comfortable again. The weather forecast says it'll be 38°F tonight. That will mean i'll have to burn a third shoebox-sized amount of wood to keep it 70°F till morning.

    By the way, i've opened the ash clean-out cap several times expecting to get at least a cupfull of ash out...but so far there's not enough to bother with. That's about three weeks total firing this stove and not even a handfull of waste ash...hmmmm. And using an inspection mirror to look down the pipe that runs through the thermal mass, nice bright shiny pipe with no ash accumulation in the bottom.

    If there are bright coals, toss a new stick of wood in and it will light right up...dark coals it may ignite or just smolder...then about 15 minutes from the last wood burning down to coals, to a mostly dark but still hot feed chamber...time to put the lid on.
     
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  15. troubleticket

    troubleticket Monkey+++

    I am gathering materials for a RMH. I would like to use the smaller(35 gallon) metal
    drum, with a smaller burn tunnel. If I use an 8" exhaust about 20 feet long, do you think it will run?
     
  16. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    has anyone written scale-able math for these? something you could plug in sizes for parts in advance like an engineer's planning model?o_O
     
  17. Oddmar

    Oddmar Monkey+

    To answer the last two questions...

    Find the 'cross sectional area' (CSA) of the feed chamber, the burn tunnel, and the heat riser. If they are square, 4x6=24 sq. in. So if you have a 4" high by 6" wide burn tunnel, you need a heat riser CSA the same size or bigger. 7x7=49...it would suck air through that burn tunnel like crazy. 4x7=28...it would run smoothly and not consume the wood nearly as fast.

    Good rule of thumb... the heat riser must be at least twice as tall as the burn tunnel is long. My burn tunnel is 10", and my heat riser is 40", measured from the top of the burn tunnel to the top lip of the heat riser. That is 4:1. Also my heat riser is 7x7=49 sq. in. CSA, while my burn tunnel is 6x7=42...it really sucks the air through. Make sure the pipe downstream is as big or bigger CSA as the heat riser... 8" dia. pipe... the radius squared times PI... 4x4x3.145=50.32 sq. in. of CSA... I'm ok.

    There are alot of factors that will affect performance. The easiest way is to download this book... Rocket Stoves -- Home That will provide you with 'scalable' designs.

    The way i did it is to read the forums for two years and build a couple stoves before i got it right.

    GTG eat supper. TTYL
     
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  18. cdnboy66

    cdnboy66 Monkey++

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  19. cdnboy66

    cdnboy66 Monkey++

    BTW, Oddmar....fantastic post, Thanks for that
     
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  20. cjsloane

    cjsloane Monkey

    Oddmar, where'd you get that drum? What is it made of?
    Anyone else have success with this?

    After 18 years it's time to replace my woodstove. We burn 2 to 2 1/2 cords per year depending on the wood and the weather. 2400 sq ft. R38 on 6 sides. I image we could get that down to a cord of wood with a RMH no problem. Setting up a coppice rotation should be a snap. Ideally along the driveway - it's long enough to provide all the wood we'd need.
     
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