The normal seasonal consumption of wood for my house and shop (combined) is between 4 and 5 cords. The wood heat is stored in water--presently 1500 gallons but the new design will be for a lesser amount. The present furnace exhaust flue is electronically controlled at 500 degrees F and has a 3/4 hp induction blower, which I see as inefficient in terms of wasted electricity and heat.
My requirements are that the furnace be long-lasting, not need constant tending, and be reasonably efficient--would like to get the exhaust flue temperature down to around 200 degrees. The Professor Richard C. Hill stick furnace has reasonably good efficiency and a combustion chamber of adequate or nearly adequate size (which might be able to be made taller) for my requirements, which are to be able to load 80-100 lbs of wood per burn. What I am thinking of is to marry the Hill furnace combustion chamber to the rocket stove.
The Hill combustion chamber is 14 inches ID and would be used with a 25.5" burn tunnel (8" diameter) with a riser length of between 51 and 58 inches. The heat exchanger (55 gallon barrel in most installations) will be a propane tank, that with one end cut off is 60" tall and about 23 inches in diameter. This heat riser will be surrounded by another propane tank that is 39 " in diameter. The height of this tank will be determined by the headroom available to drain and lift these tanks in order to check and maintain the riser. I would likely install a draft fan to be used for starting the system and am not sure if the combustion blower would be needed in this situation or could be smaller than what Hill used.
There are many questions I'd like to ask but will stop here while others digest this info and hopefully submit their thoughts on the feasibility and pitfalls of this project. The Hill furnace can be found here: http://www.vtwoodsmoke.org/pdf/hill-79.pdf
Thanks in advance for your comments
One, he only measured co2 and based everything from that number. His CO number was not very good, equivalent to 2100 ppm. Who knows when he took that number. You may run into problem getting your construction approved for emissions depending on where you live.
He computes 80% efficiency based on molecular weight etc. and acknowledging he is leaving out several other factors. This is an old paper and test methods were primitive, so I would would not assume that his numbers are really that accurate in real life.
Storing heat in water is less than optimal. You may want to consider a batch burn masonry style heating and use other materials for thermal storage.
You also supplied us with the names of Three wood stove manufactories, that can be researched, we may get lucky and find a Parts listing
that includes an exploded diagram !
Really, Really lucky would be finding one in the back of some members work shop, while I am not crazy about the need for fans to increase
efficiencies this should be shared with the rest of the Wood Stovers ! For the good of the Craft ! Big al
* I am wondering if you could try taking your original copy to a 'photo shop' and ask them how much improvement their equipment can make
in copying and sending, If a simple testpage sent to you is sharper and clearer perhaps you could get them to send us a better copy -
Where I live, it would be a 35 mile drive to where they could do that for me - I know nothing about your circumstances, this may not be possible
at all, Just thought I would Ask !
As always, your comments/questions are solicited and Welcome. Think like fire, flow like gas, Don't be the Marshmallow PYRO - Big Al !
Late Note : A quick google search found a Wikipedia article on the Jetstream Furnace (Tempest wood boiler ) Allen L.
The main reason for choosing his design as a starting point is because it holds more fuel than most rocket stoves and I want to get away from steel—and gain efficiency. Am sure that even a poorly designed refractory rocket will burn hotter than my current version. If someone has a design that will hold 80-100# of 3-5” diameter wood, or even larger diameter sticks, I’d love to learn about it.
As far as my heat storage medium, I’ve experimented with sand, which was a disaster (my design, anyway), paraffin, which is GREAT but expensive, and water. Will have to stick with water because my shop and house (80 feet from the shop) both have water-heated slabs.
Hey Big Al: I ran across this design on the internet probably 5 years ago. Unfortunately what you see is what I have. This document is on several websites but they are all the same and I am unaware of any original copies.
Have been able to approximate most of the dimensions of what he designed by printing out the drawings and measuring the known dimensions and using that data to get a pretty good guess of what is undocumented.
Several years ago I explored the three companies mentioned and found pictures of at least one of the commercial models that were built, and if memory serves, the retail price was in excess of $5K per copy. To the best of my knowledge, there is little info available on these commercial versions and they are no longer manufactured. In a way, I can see why they may not have been popular. Imagine spending that much money for a batch type unit. I am unaware if the commercial models automatically shut off when the water reached a certain temperature, along with the fact that it may of have to be reloaded several times per day in cold weather which probably isn’t a big deal to stove enthusiasts but many people aren’t.
My current furnace can be loaded and started and will shut off when 1500 gallons of water reach 200 degrees. During cold weather it runs every 1 to 3 days. Kind of a lazy way on my part but even though I’m retired, I don’t want to have to tend the thing several times per day. This probably isn’t the time to discuss this but if I can build the kind of furnace I described, there will likely be further experimentation with an auto feeding system and maybe even auto start. I know that most of the members here have no interest in these types of things but I love technology.
and /or Paraffin you will find a boat load of fellow members like myself who would love to talk to you about Any Successful Build !
Wait for it, I need to brace myself before I say it-
If you go to You Tube land, and then enter web4deb into the you tube search engine you will end up at web4deb(s) channel, you then want to scroll down to his rocket stove video
Collection, caution he is very techno savvy, and even things you Think you are not interested in are fascinating !
For the good of the craft, As always, your comments and questions are solicited and welcome ! Think like fire, flow like a gas, Don't be the Marshmallow ! PYRO - Logically BIG AL !
Thanks for the info and I did watch several of the videos and share a lot of common ground with many of them. Posted my experience with paraffin and my heat battery--hope it is with the right group.
This is my wood fired water heater based on the design of Professor Richard Hill. I constructed it myself and it works very well. The device shown has had the benefit of several modifications and I am looking forward to it having a long service life.
There is plenty of information displayed on this web site: https://sites.google.com/site/mywoodfiredhydronicboiler/
When I started this thread, I had hoped others would comment on the water jacket above the combustion chamber used to prevent preliminary ignition of the fuel. This, according to what others post, is a problem with rocket stoves and thought this might be a solution for those who wanted to use a larger fuel load.
I cast the sections of my burn chamber using Densecrete 145 Castable which I bought from the Australia supplier at this site: http://www.darleyfirebrick.com.au/castables.html
For refractory insulation I used Litecrete 1280 from the same supplier. Of course you will not be buying from this supplier but you could look up the site to get an idea of the specifications of the products I used.
I cast the burn chamber in five pieces because each piece was small enough that I could bake it in our home oven. Bigger pieces would not have fitted. I went further than that and took the baked pieces to have them professionally fired but the operator declined to do the job in case he did it incorrectly.
I then went ahead with the assembly on the basis that the pieces would be fired when the heater was being used. I took the precaution of lighting a couple of small fires before doing a full burn.
I would certainly recommend casting the burn chamber in a number of pieces as a monolithic structure will have no give and take. I butted each piece against the next piece and used kiln cement in the joins that I obtained from the Big Ceramic Store USA.
I would recommend making molds for each piece with very smooth surfaces. I followed the manufactures instructions in mixing the refractory and I vibrated the mixture in the mold. I allowed the pieces to cure for a couple of days and then put them in the sun to dry out before baking them for about eight
I would recommend building the burn and secondary chambers inside a steel jacket to hold everything together. The steel jacket will also prevent leakage of gas in the event that you do have some cracking. I used about one and half inches of refractory insulation to fill the gap between the refractory
pieces and the steel jacket.
I would recommend that you build your heater in modules so that if one component fails it can be replaced or repaired without having to rebuild the whole heater. Extensive use of isolating valves for the storage water is useful as having to drain hundreds of gallons of treated water is a pain.
The burn tube in my heater has walls about one inch thick. The structure if strengthened by the refractory insulation and steel jacket that surrounds it.
I think you will have a lot of fun and satisfaction in building your own heater. I know I did.
It is nice to see a video of what someone has built and I watched all three of yours. Amazing that the small amount of wood preheats your chimney enough to get it to draw. Am wondering about your experiences with vertical feeds. Are you using the same size burn tunnel and riser size that you had with the vertical feeds? And what were the sizes of the vertical feed tubes? Is the main difference between your present furnace and the vertical feeds the size of the combustion area or is there more to it than that?
good questions, I'll try and answer/comment and speculate on them. The original vertical feed I built 3 years ago had a traditional ceramic flue liner which cracked and fell apart in a couple of weeks. The next one had all firebrick which held up well for 2 heating seasons. The riser fit inside the 55 gallon barrel, I think it was about 7" x 7" square. The firebox/burn chamber was small, based on Ianto Evan's book for an 8" flue (I read the book multiple times and still refer to it). The draw was fair, slow to heat because of the firebrick I think. The side loader you saw in the video has the 8in by 36 in tall double wall insulated stainless steel riser which I think helps get the draw going faster but the firebox/burn chamber is still regular firebrick (vermiculite insulation) which takes probably 20-30 minutes to get really hot and have a draw. The video you saw where the big logs were igniting as soon as I put them in shows the extremely high temperature in the firebox but that was after it was running for an hour or so. At that point I can put in big, long logs every couple to 3 hours which, along with the long exhaust, builds up heat storage. It's basically a masonry wood stove with the rocket barrel and long exhaust. I'm not against the vertical feed but I couldn't get it to put out the btu's I needed. I also had safety issues with the vertical when the fire sometimes climbed up the feed tube and either smoked or flamed into the shop.
Regarding your comment about starting the stove, I have start a little fire in a tray about 6 ft from the vertical chimney; (I have a video of it, let me know if you want to see it and I'll post it on youtube) it's quick and easy, takes about 2-4 minutes and warms up the chimney. Then, when I start the fire in the firebox with kindling and sticks I still get a very small amount of smoke leaking out of the seal at the top of the barrel for a couple of minutes, then it stops. I can't smell the smoke and have a CO detector right next to the stove so I know it's a small amount; my guess is that as the whole path heats up the draw actually starts sucking more than enough to be pulling tiny amounts of air into the seal as opposed to the initial fire pushing out of it. I think I have so many 90 degree turns that it slows the draw but with the 28 ft chimney it counteracts it and after a few minutes the draw is fine. If I did it from scratch i would do straighter runs but I had already built the chimney before I knew about RMHeaters.
The most significant improvement for me would be a castable firebox followed by a higher and castable riser and then a higher barrel. The only precaution would be the higher heat might melt the barrel top so the need for some kind of cast ceramic just below the top on the inside to buffer that might be good.
Your project sounds interesting; with your experience combined with the RMH concepts you'll have something very efficient, lots of people will be wanting to learn the details. Let me know if you have any more questions and keep posting here so I can follow you.
Am definitely planning on a modular design, using at least 4 separate pieces: A combustion chamber, burn tunnel, ell and support (for the riser), and riser. I really like the rocket stove idea and hope that the combustion and draft inducer blowers can be eliminated with this design. I do have a Tjernlund draft inducer and will probably incorporate it to be used when a fire is first started and if necessary, when fuel is loaded during a burn. The main question in my mind is whether or not a larger diameter and taller combustion chamber can be used with this design, hence the modular design where the chamber can be swapped for different designs.
Had not considered a multi piece combustion chamber until seeing yours but really like the idea and am working on a design. The height of the chamber would probably be around 18 inches with a steel ring at the top that could support interchangeable feed tubes. The ideal feed tube would be tall enough to allow multiple pieces of wood to be stacked end to end. The tube would probably be divided internally(vertically) into maybe 3 sections so that smaller sticks of wood would not bind and could drop freely. The top would have an airtight lid and I'm pretty sure a water jacket, at least at the bottom would be required--the question being would this water jacket need to extend all the way to the top?? Noticed that you have a valve to regulate the water for your water jacket. Do you find that you need to move very much water to prevent pyrolysis and do you have a feeling for how much heat is necessary to remove: does it seem to get hot quickly if, during a burn, you open the valve fully to cool it as much as possible, then close the valve?
Had first thought about casting the tunnel and riser using an 8 inch sheet metal (HVAC type) duct inside a 12 inch one but am not sure if a 2 inch wall is necessary. Am also uncertain if it is a good idea to cast the length of each of these in one pour. Wonder if casting these in sections and somehow butting them together would have a better result. The riser will be about 57 inches long and inside a 20 inch pipe filled with vermiculite and clay but 57 inches at hopefully >2000 degrees F could cause a lot of movement.
Any thoughts or comments very much welcomed.
Thanks for the info on your furnace. Had suspected that you were using the original 8 inch riser and this makes me think that one might be able to obtain more heat (than most are realizing with an 8 inch feed) with a larger combustion chamber, horizontal as yours is or maybe vertical as I'd like. I am probably mistakenly using the words combustion chamber interchangeably with the words feed tube. With your present furnace, is the combustion chamber connected directly to the riser or is there also an 8 inch burn tunnel? Am impressed by the size of the firewood you are using (after your furnace gets hot) and would like to be able to use a similar size. Much of my information also comes from the Ianto Evans book which I have read several times.
My description of the steel box that will contain my furnace and heat exchanger will likely sound like overkill to many but when the weight of the heat exchanger is added to the weight of the refractory cement, my estimate is >1300 lbs. In order to install the heat exchanger, the unit will have to be moved on pipe rollers to a spot below the peak of the roof in my shop where a piece of the ceiling tin will be removed in order to set up a winch to lift the heat exchanger high enough to clear the riser. The peak of the roof trusses will give me about 15 feet with little to spare.
The next steps in my plan are to cast the pieces, assemble them in the box (I think without adding the vermiculite insulation at first) and rolling the assembly outside for testing and curing. The top of the riser should be just under 75 inches from the floor and fit under the roll-up door. Think the next step would be to insulate the riser and tunnel, roll it back outside and retest. Am looking for a couple of scrap water heaters that have a tank OD of about 20 inches. Will put these around the riser and fill with vermiculite/clay insulation.
If all is well will then install the heat exchanger. but it won't go outside again. Total weight, filled with water should be about 4385 pounds. One of my concerns is the exiting flue temperature after the heat exchanger. I have no way to calculate that. The heat exchanger will be about 34.5 square feet and will have water on the other side which should be more efficient than air at capturing heat. Am hoping for 200 degrees or less going up the chimney. Maybe Ian will comment on the square area of his heat exchangers and his exiting flue temperature.
To answer a couple of your queries the surface area of my heat exchanger totals about 2124 square inches and the stack temperature is generally in the range of 230 to 284 degrees F.
However, in Professor Hill's report in Section II titled 'The Problem of Burning Wood' he details all the variables and difficulties one encounters when burning wood. He then says:-
"Given this complexity, the only design approach is to cut and try. The only design rule is to keep the combustion zone hot and turbulent for a sufficient time to complete the reaction."
In another section of his report at V(2) he discusses experiments he conducted to eliminate the requirement for a forced draft fan.
He said when this fan was eliminated the fire burned in a traditional wood stove mode-
Yellow smokey fire, heat exchanger deposits etc. The high velocity stirring of the forced draft fan seems essential.
In my case I followed Professor's design fairly closely as I had already had two failures in trying to create heat with a reverse cycle heat pump and then with a coil inserted into a slow combustion heater.
I'm still getting used to the terms introduced by Ianto Evans so I might be confusing my description as I use old terms with his. My firebox in the video is basically a long, masonry box stove insulated with vermiculite. the riser (8in double wall insulated) sits directly on the firebox at the end and goes up 3ft to around 3in below the top of the barrel. That's it. There is no burn tunnel.
Regarding vertical feed vs horizontal I'll be very interested in how you go with this: I spent lots of time building, sitting and watching, re-reading Ianto's book, tweaking and adjusting the size of the feed tube, scratching my head. I really wanted the vertical feed to work but changed to horizontal because I couldn't get the btu's I get now. There is so much physics and chemistry involved and I don't know how to separate and measure the variables but my working impression at this point is that the vertical feed requires a small, constricted feed tube (door) and burn tunnel (firebox). The velocity which gives the rocket sound is higher in the vertical feed than the horizontal with the bigger firebox. The challenge for the vertical feed is to make it bigger without losing the velocity; the taller riser should help but I don't know the physics and I haven't seen formulas or tables which I think we all would appreciate or maybe they're out there and I've missed them. I know I spent a lot more time babysitting my vertical feed stoves than my horizontal stove. The side feeder doesn't roar but it puts out heat, draws well and burns clean.
Also, your plans for casting sound good with doing it in sections and I'll be interested to hear what you use for the recipe as I'm sure many will. I've followed Matt's (braudio) work with castable cores and Ian appears to have knowledge of many details you're putting together. I haven't seen much here on use of water storage and transfer so you add an important dimension, especially because you have many years of experience. Plus, you appear to be doing lots of prep work/planning which bodes well for your build.
With the understanding that the castings will heat fairly rapidly, the following questions would seem to me to be important. These questions mainly refer to MORCOLITE 30 SF O and MORCOLITE 60-2500 because whichever of these I choose will be used in the burn tunnel and riser, the hottest areas of the furnace. If these castings are round, the internal diameter will be about 8-1/2 inches and the OD will be about 12.
While rectangular castings may be easier to create, do you think that round castings might last longer because the lateral stresses of expansion and contraction might be more uniform? Lateral stress and expansion will be the same in rectangular and cylinder shapes. Rectangle shapes will want to crack at the 45 degree edges where this is not possible with a cylinder.
The length of the burn tunnel is estimated at 29 inches and the riser at 57 inches. I realize that there will be stress laterally, around the circumference as well as in a lengthwise direction. Might it be prudent to cast these tubes in shorter sections? Liteweight materials will compress during expansion because they are not strong. I would not worry about the circumference and allow an 1/8 on the 29 length for expansion and ¼ on the 57 length.
If the tubes are to be in shorter sections, can this product be drilled so that stainless pins could be placed in the ends of the sections to be used for alignment? yes Can these castings be cut with a concrete cutting blade? yes
Casting thickness: This part is very confusing. Have read that 30cm is fine and others say 3 inches. Think I told you 2 inches on the telephone. It appears that, because of forms available, if the tubes are to be round, the thickness will be in the area of 1.4-1.6 inches. Would appreciate comment. The thicker the walls the better to eliminate cracking.
Hope this will be helpful to someone.
I had some time today to re-read and consider the postings in your thread and I am interested to see a graphical representation of your design.
It is an interesting concept to use a part of Professor Hill's design and adapt it to work with principles from a rocket heater. However, by not using a forced draft inlet fan you are dramatically moving away from the Professor's design.
It would seem to me that before you spend a lot of money that you need to build a prototype to test your design and theories.
Not needing electric powered fans is certainly good if your power supply is interrupted but of course you will still need pumps to circulate the water that you heat.
After understanding why rocket stoves have rather a strong "draw" and can actually "push" out flue gasses, I began to wonder if this strategy could be used with the "Hill" combustion chamber: my thinking being that the rocket stove combustion air, in many cases, appears to be tolerant to a broad range of air to fuel settings while seemingly burning very hot and clean. Please understand that I realize that fine tuning can improve performance. For my application, most of the rocket stoves/rocket mass heaters just don't have the btu output that I need for my application. With the thought that if the insulation of the burn tube and riser are adequate and the fuel to combustion ratios are in the ball park, why couldn't one substitute Prof. Hill's combustion chamber? In my design, I am first going to modify Prof. Hill's design by making it a little smaller, 12.5" diameter and not use his forced air design (at least in the first version).
My idea for the combustion chamber/feed tube is, starting at the bottom, to have a 2-1/2 inch thick round pad, 12-1/2" dia, surrounded on the outside by a ring of at least 4 castings with their tops standing about 4 inches or so above the pad. Laying on top of this first ring of castings, an 1"-1-1/2" tall steel combustion air "manifold" (two rectancular sheets of steel separated by 1 to 1-1/2" steel flats welded vertically to direct even airflow) This is wider than the combustion chamber by several inches, and with a round cutout in the center of 12-1/2", the same size as the ID of the combustion chamber. Atop of the manifold will be another ring of castings, about 10 inches high. Above that will be a feed tube surrounded by a water jacket with an airtight lid at the top. If this design doesn't work, I'll try Prof. Hill's.
It rained this afternoon so I spent several hours trying to make some drawings that I can't seem to upload--will try later.
Are you still trying to upload your illustration?
This is a short but interesting video about the worth of injecting air into a rocket stove.
Food for thought maybe?
Bill Fox wrote:Just can't seem to get these .bmp drawings to load. Am trying to attach them and keep getting the same message after they upload:www.permies.com is currently very busy and can't respond to all requests or is undergoing maintenance. Please try again later. Thank you. This makes propably 35 tries--any suggestions??
How big are those .bmp images in terms of the disk size? Too big perhaps for the upload that it times out during the upload process?
Ian: Wonder if you might have any input about the distance between the floor of the combustion chamber and the bottom of the combustion air manifold or the top of the manifold vs the top of the burn tunnel. I've guessed that the top of this manifold should be about even with the top of the burn tunnel. Will keep in mind the idea about forced air entering the manifold.
Michael: Don't know if you noticed but Matt Walker has a website with photos of his commercial rocket woodstove that is in ways similar to what you have. The website is: www.walkerstoves.com He was kind enough to share some of his info with me.
I have looked at your drawings with interest. Whilst I don't want to be negative I am concerned that your design neither follows Professor Richard C. Hill's design or that of a rocket heater.
Of course I may be wrong in my assessment and I hope I am.
A rocket heater's layout and dimensions are critical to get the necessary high draft of air and efficiency. Your water jacketed feeder tube is not necessary in a rocket design as the burning should occur in the burn tunnel and not the feed tube. In contrast in Professor Hill's design the burning of the timber takes place at the bottom of the feed tube with the assistance of the injection of forced air. The resulting gases are passed through a tunnel into the secondary chamber where burning is completed.
I can understand your fascination with rocket heaters and they have their place in the scope of things. However, they are not designed to burn large heavy pieces of timber like Professor Hill's design.
In the circumstances I think it would be prudent to build a prototype of your design using simple materials and test your theories.
The burn tube, riser, and inside part of the heat exchanger are pretty much by the book but could be a problem. The tests will first be without the heat exchanger, then with it in place without the water jacket, then with the water jacket. Smoke, temperature, and wood consumption observations will be recorded.
I don't expect this to reach completion for some time and have the patience to work with it. Fortunately, my old furnace still works and when this new project gets its water jacket, it will be connected to the main hot water storage tank. Will be able to play with it all winter if necessary. Any and all comments appreciated.