I have been battling mightily with my first effort at building a RMH. A few weeks ago I wrote in and received some comments but I am many steps down the road now and still not having any success so I thought I would try again.
I have been building this heater in a shop for a friend (I mention that for a reason that will be referred to later). It is on a cement slab and backs up to a raised 18 inch area also concrete. Seemed like a good place since it would allow for more "mass" effect. The floor was uneven so I built a brick base using fire brick and mortor. I then built the heater roughly around Ianto's book (I say roughly because I had been studying the book before building and then at the critical time couldn't find it) and made a brick heat riser. The stove seemed to rocket fine so I continued with the build. Now the problems begin - I installed the 6 inch pipe (flu) which would be enclosed by the cob and also the barrel. At that point the stove no longer seemed to rocket. It was at this point that I wrote this forum. Since then I have shortened the flu pipe with no noticeable improvement. I then removed the barrel and removed the brick heat riser - installed a metal heat riser made from a piece of 6 inch pipe inside an 8 inch pipe with perlite/clay insulation in between. Once again it seemed to rocket very well so I reinstalled the barrel and lost the rocket (it still draws but nothing you would consider a rocket). At this point I decided that it must be the fact that we had no cob around the flu so we began cobbing the flu. My intention had been that we would only go half way down the bench so that we could still shorten the flu if necessary (there is 30 foot of pipe) but I was gone for a weekend and my friend finished the cobbing so now all the pipe is covered. Still no rocket. In the initial build I had added a clean out pit and door to the feed tube area and decided I might be getting enough air infiltrating to cause the feed chamber to lose downward draft so I rebuilt the feed tube and eliminated the clean out door.
Long lead in I know but here is my first question (sort of a hope without much faith) is there any chance that the draw will improve once the cob around the flu pipe dries out? I am able to build a fire that burns adequately as long as I put in small pieces of wood and keep the fuel low in the feed chamber. I also have narrowed the feed chamber and put a brick on top to narrow the area air comes through. This keeps the fire from coming out of the top of the feed chamber but if the wood is put in just a little wrong the fire chokes and starts smoking back into the room. We have directed air across the feed tube opening from a fan and the fire seems to burn pretty well - I guess we are adding the rocket by force feeding air.
A couple other comments and then I will stop. At one point I decided that I must have made the burn chamber (bottom of the "J") to large for the 6 inch system so I installed 1 inch thick bricks to narrow the chamber. I am now sure that the chamber is smaller than the 6 inch pipe.
Is it possible that I don't have enough insulation in the heat riser and should go to a larger outer pipe?
Any other advise will be greatly appreciated - I am at my frustration limit on this one. Thanks Jon
Jon Carter : The 1st place I would look is at the measured distance/space between the Heat Riser top and the bottom of the barrel.
The bottom of the barrel that rests on the bricks' that end' Is completely cut out making a 22 1/2 inch hole right ! Iran into a newb who tried to cut out a hole for the heat riser, and one for the gas discharge to the base where it turns 90degrees to the horizontal chimney run! Didn't work !
You need to show your math I'm afraid, your inside diameter of your heat riser is 6''. Its Cross Sectional Area , C.S.A., is pi Xs 1/2 diameter squared , 3.14 x (3x3) just under 50 '' all other Cross Sectional Areas , C.S.A.s should be very close to that at burn tunnel, combustion chamber and at the Rocket Stove Base. So if it worked before we added the barrel we have to look at what was added to the system when we added the barrel show your C.S.A.s and check and then recheck your horizontal pipe for obstructions now you know why we put-in so many clean outs! Pyro- Al
Success has a Thousand Fathers , Failure is an Orphan
Thanks for the reply. I did take the complete bottom of the barrel out and the edge of the barrel is resting on a brick circle I used for directing the gases into the horizontal flu. I did check the flu for obstructions and it is free and clear and when I build a fire it does burn sideways to a large degree but any wood that is above the edge of the burn chamber will eventually have flames climb up them and there is not enough draw to suck them down and into the burn chamber. If I put a brick lid on the feed chamber so I restrict the size of the hole at the top I get less of this problem but it isn't cured completely.
I am trying to figure out how to post pictures so I can better show you what I have been up to.
One other thing - if I just try to start the fire by placing paper at the opening of the burn chamber I get no draw but if I first start paper burning in the clean out at the end of the flu just before it goes up the wall and out I immediately get draw and things start to look like the videos I've seen of a burning RMH. Once the fire gets rolling though I start having the issues I had previously described.
Thanks Peter - it is looking like I may be having either a "man I missed that" moment or just a senior moment I'm not sure which. I left the barrel at full height (33") and brought the heat riser up to just under 2 inches from the top (31"). If I understand what is being said is that my barrel and heat riser are just too tall. Frankly that would be a lot simpler solution than other things I've worried about having to fix so other than feeling really stupid this might have a happy ending. Am I understanding correctly that my barrel should be no higher than 28" from the top of the burn chamber. If that isn't what is being said please correct me so I don't run off half cocked. If that is the case I am sure it was in the book and I just completely missed it.
Now that I think about it, if that isn't what is being said then please explain so I can get it - feeling a little thick at the moment - Jon
Jon, I am having similar problems with my 8" unit. Take a look, we may end up answering each others problems https://permies.com/t/17748/stoves/st-RMH-build Is your rmh in your basement? As far as a heat riser being to tall, I dont think that is possible (within reason). My unit has a riser height 3 times the length of the burn tunnel and 4 times the height of the feed tube. It should only make it draw harder relative to the exhaust exit and, as I am learning, the surrounding air pressure to exhaust gas density ratio.
Where is an Admin when you need a good formula? How about that, a numerical ratio of surrounding air pressure & density (@ feed tube) to flue gas density to outside air conditions? A solid set of parameters here to work backwards from could be quite groundbreaking!
Hi Jon, First thing's first. Don't start changing anything too quickly. There are two places we need to look first. Allen asked you about the CSA of your heat riser as it opens to your barrel. If you didn't make any errors in measurement and your barrel is nearly 2 inches above your combustion tube/heat riser this will give you sufficient csa (cross sectional area) at this zone. Here you would calculate the circumference of the riser at it's tallest point x the gap height. Circumference = 2x pi x radius or 2 x 3.14 x 3 = 18.84 cxheight 1.5" =28.26 (your csa for a 6" diameter pipe) so a 6 inch diameter pipe will require AT LEAST 1.5 inches gap between the heat riser and the bottom surface of the barrel lid. It is highly recommended that you design it with about 2" to insure that the gap is maintained under operation. The high heat can warp the bottom of the barrel and this could close the gap somewhat. Use a straight edge of sufficient length and measure the gap between the lip of the barrel and the center of the top. Compare this gap with the gap near the outside of the barrel to determine if your barrel is warped downward which could impinge the air flow. Also, it's a good time to double check that you were indeed measureing the height of the center of the inside surface of the barrel (not the outside lip which can be a difference of up to half an inch in my experience) when calculating your gap.
The next area of concern is the opening directing the combustion gases into your flue pipe. You described a brick circle with an opening into your flue. We need to know the cross sectional area of the tightest spot (think 3 dimensionally here since the gases are essentially turning 90 degrees from downward to horizontal). By any chance, in constructing this, is there a spot where the corners of the bricks are closer together than the opening to the duct? It could be inner corners of the bricks. This would act the same as a kink in a copper tubing, the openings are the same csa, but the csa at the kink drops to nearly zero.
It should be one of these two locations because the drafting issue appeared and disappeared when the barrel was added. Can you supply any photos or measurements of these two spots?
Hope this was helpful, it's sometimes difficult to describe these aspects in written language.
Good morning and thank you for the replies - Sorry Colin but I have already begun major changes. Your comments are still helpful though. I will make sure I don't have any tight or constricting spots in the transition chamber to the flu. My plan though is to completely rebuild the "J" tube portion of the stove. I have been modifying it enough that I have decided to stop with the "bandaid" approach and to just rebuild it using exact measurements from "the book". Figure this way I will have a better chance of it working properly. Besides it give the cob used as mass to dry out. Sure don't want to have to pull that all apart to shorten the flu pipe.
I do have a question though - I have been using fire clay/mortor to do the work including setting the bricks. I get lots of cracking and in some cases the bricks seem to be loose. Is there something I can add to make this better or some procedure I have missed. As I understand it I'm not suppose to add straw at this step. Let me know - thanks
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