Rocket Heater for 20’ x 42’ x 14’ with questions and comments

Hi Newbie here.

I need to replace my 15 yr old home built shop wood heater in North Central Wisconsin.  Heating season is ~ mid Oct to late April.  January temps ave high 20° F low 1°F  (-6 to -17° C).

Some information about the shop.
   1. Size 20’ x 42’ x 14’ (6.1 m x 12.8 m x 4.3 m) (840 sq ft,  11,760 cu ft or ~78 sq m, 333 cu m)
   2. R Value  Walls 19, Ceiling 32-35, no insulation under the floor, but 2 inch of foam board goes down 24 inches vertically around the exterior hopefully preventing frost from invading under the slab.
   3. 4 inch poured reinforced cement
   4. There are two 4 ft x 2 ft x 20 ft double plastic(7.5 mil) lined channels under the floor filled with fist sized field stones.  
       A. There are also 4- 6 foot lengths of 4 inch flexible drain tile scattered randomly in the channels to help move air through the rocks.
       B. On the exterior wall are two twelve inch tubes that reach within 18 inches of the ceiling.
       C. On interior side wall of the shop are two twelve inch fans drawing air through the channels from the ceiling.
           a. When outside temps are below 20 degrees F,(-17.8 C) running the fans will keep interior temp at above 42F (5.6 C) degrees.  Not running the fans drops the temp to 30F (-1C).
           b. This occurs when no supplemental heat has been present for at least 48 hours.
           c. Even a week of subzero temps, the shop stays at the 30F(-1C) without supplemental heat.
   5. Current heating system is a homemade box 24 x 24 x 32 in(61 x 61 x 81 cm, with a full frontal door.  
       A. It is above the floor by 13 inches (33 cm) for easy loading and ash removal.
       B. There are no firebricks but ash accumulates on the bottom with only an annual cleanout.
       C. The firebox is designed to block smoke from the top 4”(10 cm).
       D. It also has a 3” (7.6 cm) x 24” (61cm) close-able opening at the bottom of the door.
       E. The current flue is 9 feet(2.75 m)  of 6” (15.24 cm) well casing, running into 12 feet (3.66 m) of 6 inch double wall insulated pipe.
   6. Wood source are cutoffs from a kiln drying plant.  
       A. The cutoffs are not dry and are mixed with 25-40% sawdust from a bandsaw.
       B. The cutoffs  are collected in the spring and allowed to air dry after being sorted from the sawdust. ( A real pain, but better than paying for them or splitting wood)
   7. Five cubic feet(.14 m3) of cutoffs will raise the temp from 30 to 70 F (-1 to 21 C) in about 2 hours or less.  Another firing in 3 hours or less can warm it another 10 F (5.5 C).  The draft needs to be controlled otherwise the temp will rise too high.
       A. At this time or before the below floor fans are turned on to remove the heat from the ceiling to the floor to be released at a later time.
   8. Occupancy of space can be range from a few hours a day to forty or more per week. Usage can range from daily to missing a week or more at a time.

I have read the book by the Wisners, watched numerous videos and went through a number of threads on the Permies and Donkey32 boards.  I went through Peter’s Batchrocket.eu chapter and verse.

There are some good threads and well……….  One comment I would like to make although there are lots of very good pictures, drawings and diagrams, but universally lacking is labeling of the components.  Those who have had lots of experience with Rocket stoves understand them without the labels.  It took me about several days before I figured out what a port was.





Because of the various above  eight points, I am only interested in what would be termed as a batch rocket heater, with some heat retention.

Questions or Observations:
   1. In general firebricks can be use throughout a heater, but due to cost probably should be limited to the firebox and heat riser. Is this a correct assumption?
   2. In general the firebox and heat riser are located at floor level.
       A. When a firebox and heat riser are elevated, should firebrick be utilized to support these two structures?
   3. There are many types of building bricks available and many are mentioned in various threads. Because concrete breaks down, concrete blocks should be avoided due potential explosion, cracking and rapid deterioration. Are there any bricks that should be avoided? What about those with holes? Should paving bricks be avoided?  What about landscape bricks?
   4. When mortaring firebricks, it seems that general recommendation is to use a clay-sand mix ranging from 4-1 to 1-2.   That is quite a range, with reports indicating that the higher clay mixture will crack more quickly.  So where is the comfort zone? 5-3?
   5. Can and should the clay-slip be used outside of the firebox and heat riser? For instance fastening regular bricks in a bell or the support structure?  Should fireplace mortar be used in the firebox and riser?  The clay-slip apparently makes for much easier repair if needed. Should clay slip be used to mortar standard bricks or some kind of clay sand mortar maybe mixed with refractory  cement?
   6. When the goal is heating and not for heat storage, would adding height to the first bell barrel be a positive thing? Example two barrels.
   7. Clean-outs are rarely mentioned, but in mass rocket heaters clean-outs are often diagrammed in the ducted horizontal bells.  Where should clean-outs be located in a batch heater with only vertical bells?
   8. According to Peter, there are ratios for the dimensions of the firebox, port and riser.   It would seem a smaller flue would restrict the speed of the burn, but flow out of the flue would be very fast. Likewise a larger flue would maximize the burn, but flow out of the flue would be slower and could be problematic in windy conditions and when the outside and inside temps are nearly equal.  

See Flue Size Compared to Riser Diameter Chart:

   9.  How does the external flue (chimney) enter into any ratios?  Can a 6 inch riser be exhausted by a 4 or 5 or 7 or 8 inch flue? Or rather should it be?
   10. Most Rocket heaters seem to be exhausted from outside of the mass.
       A.  Is there a reason why the exhaust flue cannot draw the gases from the bottom of the last bell?  
       B. This may increase the exhaust temps a little because the flue would be warmed by the gases at the top of the bell.  
       C. This would reduce the foot print by 8-10 inches. This is important to me.
   11. A 6 inch well (thickness =5/8 inch /15.9 mm) casing weigh 17#  per foot (7.7 kg/304 mm).  This is a sizable mass able to store and to radiate heat.
       A. Support for such a flue would need to be substantial.
   12. Has any one tried to burn a sizable amount of sawdust, not shavings?  What I have comes from a commercial bandsaw and in small amount 10 gallons or less can be dried quickly with stirring.
   13. I do not wish to experiment, but would rather have a proven plan, but I do want as small of footprint that I can have to heat my shop with a six chimney flue.

Welcome Neil!

Thanks for all your research and reading before getting to this point.  You've got a daunting amount of detail here, and some of your questions may deserve their own threads.

I'm far from an expert, but let me see if I can answer some of the easy questions and let the experts tackle the more difficult ones (and correct me)!

I think many of your questions about brick are answered by clarifying that firebrick (and similar ceramic-board materials) are not only meant to withstand the temperatures without structural failure, but they are also insulative, preventing heat from traveling away from the burn chamber.  This means that once the temperatures drop after the bell, and on the other side of the firebricks, you actually want non-insulative, heat absorbing and heat-transmissive materials.

So ..
Brick Type - yes, firebrick/refractory brick or similar high-temperature materials are critical in the firebox, burn tunnel, riser and (I think ...) the base of the bell.  Once the gases exit the bell you have many more options on material.  Frequently the material used is less of an optimal choice and is based on whatever is around and convenient.

Raised Base ...you could absolutely use non-firebrick as a base. My rocket oven is built on a base of cement blocks.  The critical consideration is that the foundation MUST be very solid.

Building brick types -- once you're away from the heat its just a question of structure. I don't think there are any specific prohibitions, but some materials are better than others.

Mortar ratios - I think what you're seeing here is that the quality of materials varies.  Sand can be coarse or fine, clay has different levels of purity.  Getting the right mix is a thing of experience.

Clay slip - use it wherever.  the difference is that clay slip primarily seals while providing a weak structural bond.  Going with cementitious mortar provides much greater structural bond (and the cost of easy maintenance).  So if low and wide, slip is fine.  Tall and narrow ... consider mortar.

Barrel size - yes , a double barrel would increase the radiant surface area, providing more "instant" heat and providing less heat for storage in the mass.  IIRC, the Wheaton Labs shop/auditorium heater has a double barrel.

Ratios and such - you are right that changing anything here does have effects.  Allow me to paraphrase the experts ...DON'T CHANGE THE RATIOS on your first build.  If you want to build a test heater and rig it with sensors to see what happens, please do.  But the shop heater?  A LOT of time and energy has gone into deriving these ratios to produce a safe, reliable and effective heater - and a change risks all three outcomes.  So ... the system sizing is referencing the diameter  of the WHOLE system and its essential that the entry and exit are the same (although, obviously, the middle of the system may larger in effective diameter.  In general, going with a larger exhaust  than entry is less critical while a smaller exhaust than entry is pretty much guaranteed to reduce the performance of the system.

I can't imagine sawdust working well. It would need some sort of stirring/vibration mechanism to keep from forming a blanket with insufficient surface area and oxygenation.

Hi Neil;
Here is a photo of my shop (24 x 32) heater... with double barrels.  7" batch with an 8" riser.
I'll be back later to answer any questions that Eliot couldn't

Hi Neil;
Eliot did a good job already, but I'll see if I can expand on that.
1)Heavy and or insulated Firebrick are  used in the core box but ceramic blanket (5 minute riser) is becoming the new standard for a riser.
2) Most batches are located at floor level but they can be elevated. A plinth of red clay bricks and cob can support  a core unit. My shop stove is that way.
Others have used a metal frame to support a properly insulated core.
3)There are places concrete can be used in a bell or low in a mass. Most landscape brick is concrete.
Solid clay bricks are the preferred one.
4)  When using dry fireclay and bagged sharp sand . The mix is 3 parts sand to 1 part clay.   Lincoln 60 fireclay in #50 sacks and I like medium builders sand in #100 sacks.
5)Yes , Clay mortar should be used in the core unit and is highly recommended for the whole build.
Regular cement mortar mix can be used in a bell anyplace away from the core and riser. In the high heat area near the core and riser only refractory cement would hold up.
6) Yes two barrels extract a large amount into the shop, the brick bell helps hold that heat a long time.
7)With a bell it might be years before enough ash accumulated to need cleaning.  I used a standard masonry cast iron door as an access to my bell.
8)Peters measurements should be followed exactly on your core build. Box size and port dimensions have been dialed in by Peter and no modification will make an improvement.
9)The chimney needs to be the same as or larger than the riser size.
10-A-B-C) All bells exhaust from the bottom of the last bell.
11)Not sure here but you want to use well casing as a chimney?
You can if you want to but most just use regular black stove pipe and hvac pipe indoors.  Then insulated pipe outdoors.
12) Sawdust would explode if you tried to get any in a lit batchbox... they glow orange with a back wall temp around 2000 F. Maybe make a log with the sawdust and place it in before liughting.
13) With a 6" batch and a brick OR metal bell you can fit any footprint you have available.

That's enough for you to chew on for now.
Come on back anytime we will leave the light on for you!








   

Thanks to Thomas for adding the weight of experience to answer Neil's questions.  If when reading his answer and my answer you see a conflict ... take his answer.

I hadn't even though about adding sawdust to a lit batchbox, I was imagining putting some in and then lighting it - I agree that adding sawdust to a lit batchbox is a bad, bad idea.  Shavings might be ok, but sawdust can - and does!- explode.  So does wheat flour and so does , um, non-dairy creamer.  Yes, making logs could work in a pinch but since half the point of Neil's source is cheap (free?) and low labor, taking the time to convert sawdust into burnable logs may not make sense (a bakery in town with a wood fired oven uses such logs - they are pressed together with nothing more than water and they work very well and very consistently).

There's a possible side thread here, using something like a pellet stove auger feed mechanism to feed sawdust into a J tube.

Hi Neil;
1)  Yes if no solid bricks are available then filling the holes with clay mortar is good.
2) Well my shop is poorly insulated. None in the roof at all. So I am happy if its 20 outside and 40 inside in the morning. During the day with it 30 outside I can bring it up to 55-60
3)Yes extending the 6" box to 20" is fine.
4)Curious what you are burning , to burn thru a stainless chimney?  Ideally a brickbell exhausts around 200 F  Not burning thru thin wall hvac at that temperature.
5)Well my brick bell exhausts out the bottom , right next to the cleanout door.
6) I guess filling a cool stove 1/2 way with sawdust is ok. I can't see it lasting long though.  

I will add here that putting your questions in my narrative made it harder to find them.
A separate response is  much more reader friendly.

[size=12] Good Afternoon Thomas & Elliot

The best of Thanksgiving to you and your families.  While we have family nearby, we will be celebrating alone with each of family doing there thing.
A cup of coffee for you and a cuppa tea for me today.

Mea Culpa about embedding my questions within you answers.  I did try to us a blue font do delineate them.

   1. I am square on the bricks and mortaring.  
       A.  All bricks can be used, just fill in the hollow ones with best choice clay and sand.  
       B. Mortar can be used, just keep it below low the fire chamber and  riser.
           1. Above use clay and sand.
   2. Ugh, working in your shop would be like working in my shed with a salamander.  I like to work in my shop without a coat, plus I can get glues and paints to dry as quickly as in the summer. In addition the woods I work with are brought in a few days early to temper to indoor temperature.
   3. I believe that my stove was and is doing a secondary combustion in the chimney.  The opening on my door when partially open will almost suck a piece of paper in to the stove when the fire is burning full force.   A full load will last 2-3 hours and move the temperature 40 degrees. Reloading periodically, but with air damped down will maintain the temperature.
       A. With outside temps near zero I will burn 15 to 20 cu ft of cutoffs every 2 days+ and can reach interior temps of 80.
       B. In addition the well casing has a weight of 17# per foot compared to less than 2# for regular 6 in stove pipe.  In my case it should store the heat of 34 standard brick.  If a scratch build, I would use a standard flue pipe probably an 8 inch.  But I want to keep the well casing so I will build around it. If I had know about Rocket batch heaters I would have planned for an 8 inch riser, with a bigger masonry second bell.  But now I do not want to climb up on my 5/12 pitch steel roof to replace the chimney up there.
       C. A 6 inch with the double barrel should hopefully heat my shop above 75 with multiple firings.
           1. Many threads talk about firing in the morning several times, and then again in the PM. These are generally in residences.  Are there any that more or less keep a fire burning continuously and to what effect of the riser, firebox and clay mortar?
   4. In the thread https://permies.com/t/143274/Introducing-Minute-Riser you have a picture 20200629_135846.jpg, Riser set ,first barrel in place
       A. It would appear the barrel is offset to the center of the barrel.  Is centering a concern or not?
   5. In the thread https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket you were building an eight inch riser. Was that a ten inch stove pipe you were using? And is that how you arrived initially at needing a 31 inch blanket?  Pi times diameter.
       A. Ultimately you wound up with a 28 inch blanket would give you an 8.9 inch riser.   The 28 inchs is the circumference laid out.
       B. On another thread https://permies.com/t/92302/Ceramic-blanket-riser-board-core , https://permies.com/u/209538/Jesse-Baker  said “Another thing is that my riser with the one inch blanket inside the ten inch ss pipe measures about 7.5” ID.”
       C. Why the difference?  Did your blanket get stretched more?  If so then your blanket was just over a half inch thick.  I used https://www.omnicalculator.com/math/circumference to do the calculations.
           1. If you used a 10 inch flue, and installed a 1 inch batting then the new diameter should be 8 inches and the bat length then should have been 25.1 inches instead of the 28 inches.
           2. If Jesse-Baker got a 7.5 inch diameter than his one inch battinging was compressed to be actually be 1 ¼ inches.
       D. For a perfect six minute six inch riser, an eight inch flue pipe would be required.
           1.  Removing 2 inch from the diameter of the pipe for the width of the batting, would mean the batting should be cut at 18.84 inches (478 mm) for a perfect fit.  Perhaps 19 inch (482 mm) for a friction fit.  
           2. When you went from 28.5 batting to 28 inch batting it is a change of just 1.8% but was apparently enough for your ten inch.
           3. A reduction by ½ inch of an eight inch riser reduces flow by 6.7% , but a ½ inch reduction in a six inch riser reduces flow by  9.1%.  The difference in flow increases to now more than 40%.  It would seem that as the riser gets small exact sizing becomes more critical.
           4. Therefore it would seem that the riser is the most important component of the system, and if a six minute riser is slightly over sized it would be none critical but undersized would be critical especially with systems designed 6 inches and under.  
           5. If the six minute riser is slightly over sized to 6.5 inches it would require a nine inch flue pipe, with a one inch blanket that would need to be compressed so that the batting will be about 25 inches.  The question than becomes, will a 1 inch batting be stable when compressed into a 1 ¼ inch batting in a circle.  It can apparently be done, at least in the 10 inch as evidenced by the Jesse-Baker example.

Can you affirm these conclusions?
Neil[/size]

Hi Neil;  
Happy Turkey day to you! Enjoy your cuppa tea!
If you had been in my shop in years past you would think its great out there now!
I hear you about going up on your roof.  I strive, to not need to go up on mine 6/12...
Yes , Matt Walker for one & myself for another keep our rockets running all day in the winter.
No detriment to the mortar at all.  The 5 or 6 minute riser is a long term part.
The bricks in the firebox may crack at some point, as long as they stay in place just spear clay mortar in the crack.
Offset barrel to the riser is no problem. You wouldn't want it super close but a few inch clearance is plenty.
Yes a 10"  sort of round pipe and yes I used Pi to get 31" Knowing I would cut it down.
Morgan superwool is the stated thickness, 1" stays 1" it doesn't stretch, It also doesn't compress very much.  Imported ceramic blanket (hazardous) may..
Might be Jessi measured a 10" od not id.
Now I'll skip past your math its not my strong subject.
It is certainly OK to oversize the riser a bit.  Correct that undersizing would lead to issues.

Hi Thomas,  thanks for the cuppa.  Just got through with the turkey dinner and waiting a few minutes to begin  playing dominoes.
Thanks for the response.  Some day you will need to write the book with all the issues pulled together.  It may not make a lot of money however.
I am a retired dairy farmer and spent a good share of a winter day outside, nice or nasty dealing with equipment that needed to run every day even with temps -30 F,
so I like my comfort.  Electrical motors would groan to start equip with 90 weight oil in them.  

The math is fairly basic, if you envision that round tube being cut lengthwise.  The circumference would then become the width.
Next if the blanket is 1 inch thick, the diameter would be reduced by 2 inches.  When rolled into a circle than this becomes the circumference of
the inside.  To prevent a small gap from the inside to the outside diameter the blanket would need to be mitered so the outside measurement
would be greater than the inside measurement.   That would require some geometry and a jig to cut the blanket.

Any way I think I have all the answers I need.  Stay Healthy and neither be first nor last, most of the time.

Neil

The ceramic fiber blanket does not meet at an angle when rolled to fit a circle. The edges butt almost square. You would use the average diameter of the insulation to figure width; for an 8" riser in a 10" pipe, figure from 9" diameter, and add a tiny bit for a firm fit. I would probably make it 30" and be prepared to trim a hair if needed.

Glenn's description is spot on with my experience with using the cf blanket as well. Not complicated at all. Any small imperfections can usually be solved by slightly stretching or compressing the blanket and your good to go. The difference in diameters will be taken up by the blanket with small creases on the inside portion. I asked Matt Walker if this would affect performance at all and he said no.