newbie with problems..

i just finished my rocket stove heater yesterday to heat my shop. it is made out of 4'' 1/4'' wall pipe. the riser is 21'' long and has a 1.5'' gap befor it meets the barrel at the top. the fire tube is 12'' long and the feed tube is about 10'' long. i place a cap onto the feed and regulate air via the open end of the fire tube. ive tried everything i can think of except cutting down the riser to try to get a reburn. No luck with anything ive done. ive got the riser wrapped wit fiberglass insulation about 3'' thick. do i need more insulation? the max temp ive seen off of the barrel is about 425 degrees. im assuming the reason im not getting any hotter is because of no reburn? any info will be much appreciated. im about to scrap the idea and build a regular wood stove with a heat ex-changer.

try 1:2:4 ratio, insulate the burn tunnel to, and avoid metal

Thanks shilo. So a 10" long feed tube. Then a 20" long burn chamber and a 40" riser?. I really cannot make it out of brick or other materials because of space limits. I need it to be as compact as possible

The stove does "rocket" but it is just that much air being sucked into the burn chamber..I hear nothing in the barrel

A 4" system for a RMH is considered to be an advanced project - that small tends to be very tricky to make work well. A 6" system is the smallest recommended for a first build.

Cutting down the riser would make it less effective, not more. At small sizes, the ratios may need to be adjusted - maybe try adding 10-20" to the riser. Don't add to the burn tunnel. Is your size constraint horizontal or vertical? By the time you get enough insulation on the burn tunnel and riser to be safe and effective, you might as well use brick. If you get your rocket burning well, it will destroy the metal fairly soon; if it doesn't corrode the metal, you are not getting hot enough for complete combustion.

What are the dimensions of your barrel, and what kind/size/shape of mass are you trying to use? What clearances do you have in all directions? And what are your chimney arrangements? Pictures would help us advise on the best solution.

As for hearing anything in the barrel, the combustion pretty much all happens in the burn tunnel and heat riser. Especially in a small system, you are not likely to have conditions for additional combustion, and the combustible gases should all be used up in the riser anyway.


"i place a cap onto the feed and regulate air via the open end of the fire tube."
Be very careful with this. You may get away with it in a small system, but what you are making is a reservoir for hot unburned gases which will rise into the top of the feed tube. When you remove the cap to feed it, you are giving them a shot of fresh oxygen, and if they are hot enough they will make a nice fireball. If you are lucky you will only lose your eyebrows. This is a major part of the reason the air supply is supposed to go down the feed tube, to keep the wood from all pyrolizing at once and making an overburden of dangerously hot combustibles.

The barrel is 12" dia. And about 24" long. I have 2" of vermiculite completly surrounding the burn chamber and riser then used fiberglass ins. Around that only on the bottom and on burn chamber inside the barrel

The insulation in the pic has been changed. Read post

hi ben, nice set-up you have there. I would try for non-fiberglass insulation in the base of the unit as shown in this picture. If you used cob you could mold it to allow the exhaust port to remain wide open. If that increases your temperature, you might think about adding more cob in the same area, building it up around the riser & wall of the barrel a couple inches & retest, making sure your exhaust port is wide open.

I'm not sure your fiberglass is doing 'enough', but you might try a 6" or maybe an 8" thin wall stove pipe over your existing heat riser tube, & fill it with cob, or ash, or clay...some people use vermiculite...to replace your fiberglass wrapping.

If the unit is primarily 'stationary' then I wouldn't hesitate to use the heavier insulation materials, if it is portable, using powdered clay might work-out, because damp clay will easily form & set where you put it. And after your done using it & disassemble to move it, a couple sharp raps to the side of the heat riser, the clay powder will crack & fall out the bottom as you pull the 8" thin wall stove pipe upward.

james beam

Metal is doomed, and so are angled feeds!

thanks james. ive actually got a 10'' 1/8'' wall pipe on the heat riser and burn chamber on the inside the barrel. that gives me close to 3'' of vermiculite around everything. and im still not getting any big fire seems mostly lazy.

Satamax, I've seen that phrased tossed out a few times now and am wondering why exactly. I mean, it rings true, I like doom metal... But like what, It just oxidizes and crumbles or what? Sorry to jump in on your thread with nothing to offer ben, but I am curious.

Landon Sunrich wrote:Satamax, I've seen that phrased tossed out a few times now and am wondering why exactly. I mean, it rings true, I like doom metal... But like what, It just oxidizes and crumbles or what? Sorry to jump in on your thread with nothing to offer ben, but I am curious.

Watch this!



And, Ben, metal, even if thick will fail quickly!

I've made puff pastry out of 1/4 thick hydraulic tubing! In about 80 hours of burning.

hey ben, well ya, you got the whole 'metal is doomed' thing going, which is one problem, and RMH's are not well known for high performance with a 4" burn tunnel size, and I think the other problem might be in the mostly metal structure is radiating most of your heat too quickly. As you probably know, RMH's are specified to have firebrick for the burn tunnel, and for important reason. The firebrick absorbs the heat & holds the heat much differently than steel.

Most RMH's seem to always encapsulate the firebrick burn tunnel, & the feed tube areas with a thick application of cob, or whatever...which further slows down the ability for the firebrick of the burn tunnel to quickly radiate heat. Instead the combination of the firebrick & the thick cob overlay concentrates the heat in the burn tunnel & heat riser for a longer time as compared to metal. This concentration/insulation of heat in the burn tunnel & heat riser forces higher temperatures further down the line till it is allowed to exit the heat riser at maximum heat obtainable. The sudden transition from the 4" heat riser to the 12" barrel causes a change in the flow rate, slowing the hot air down somewhat.

You already got 450F out of it, which seems adequate for a shop heater, but for fun, I would like to see if you can get it near 650F. Remember the higher temperatures are what erode the burn tunnel faster, than lower temperatures...at 450F, that little stove should last for years, just like it is.

Just for fun, you could try dumping 2 or 3 big wheelbarrow loads of dirt around the support legs of the thing, mound the dirt pile up around the base, & sides covering the exposed metal lower parts of the unit. I would bring dirt & mound it up high, till it was to nearly 1" below the feed tube opening, therefore the whole bottom half of your unit would be embedded in dirt, including the exh. pipe area if ya want, this dirt pile might slow down your heat radiation in the lower parts of the unit, which is where you need to concentrate heat. This is just an experiment suggestion you might try. If you decide it is concentrating too much heat, some of the dirt mound could be easily pushed away...after all-- all you want to do is heat the shop, it is not necessary that you melt the burn tunnel out of it prematurely.

I think as your picture is now, it is your exposed metal pieces that are radiating the heat too fast, both internally & externally. I think the main goal of your style shop heater should be low chimney temperatures, therefore proving that you have extracted & used as much heat from the fuel you fed it. Generally if your chimney temperature is over 300, then more mass in/around your stove would help capture some of that excess heat.

james beam

Well ive had the stove about 450. But have yet been able to duplicate it. It usally stays right around 350. It is not burning all the fuel tho. Quite a bit of ash. I took your advice and put vermiculite around the entire burn chamber and riser. No noticable differences. Im thinking your right about the metal cooling too much. I did another burn today and the top of the barrel was350 but at the bottom it was 120 max. Sooo if I extend the riser to say 40" overall that should produce more draft and pull more air in. Possibly producing a hotter fire? Or should I try some 8"dia pipe at 21" length. That in theory is 2x the area I have now.?

And I have never seen exh. Temps that high. Maximum recorded was 120. But that was 2" away from the exit of the barrel

"Or should I try some 8"dia pipe at 21" length. That in theory is 2x the area I have now.? "

A drastically larger cross section will not help combustion, it will let the flames be lazy and probably not mix thoroughly. Also, 8" diameter is not twice your 4" square, but more than 3 times (would be 4 times if you were comparing the same shape).

hey ben, no I think I would stay with the 4" inside heat riser pipe + the 3" of vermiculite + the 10" outside heat riser pipe as it is now about 21" long, --if that is what ya have--, I wouldn't change that part, it looks decent proportional-wise too me, but I could be wrong. On your 4" unit, I really wouldn't expect the flames from such a small fire to lick any further than your 21" heat riser. I don't think a 40" riser is going to improve 'draft' or increase temperatures, but I don't know. You might try building up your internal vermiculite in the base, even more than you already have, I would want at least 3" deep and 6" deep if I could get it...of vermiculite covering over the burn tunnel & extend that vermiculite to cover most of the heat riser. I've never used vermiculite before, but they say it insulates well, I just use clay dirt. You can probably fabricate a piece of sheet metal to make a 'mini-bulkhead/barrier' to keep the vermiculite in place and not allow it to wash & fill into the exh. port. You may have to make some kind of thin sheet metal 'form' to hold the vermiculite build-up in place, till you can get your barrel back on the thing. I used a sheet metal form on mine to raise the dirt up higher inside the barrel...kind of a rinky-dink way of doing things, not really a standard RMH practice (see picture), but I was able to increase the insulation inside the barrel & still get the barrel back on the thing....hahaha hope that makes sense.

~~~I probably ought to tell ya, I ain't no RMH expert~~~ but I have had a little fun experimenting from time to time. All my suggestions should be taken with a dose of salt.

Anyway, I think it matters if you can get more insulation covering over the burn tunnel, how ya do it is up to you. I'm guessing you got rid of the fiberglass blanket set-up, which should be a good thing, dirt or vermiculite should insulate better than fiberglass in this situation. It also matters that your fuel wood, better be really dry wood, wood just from the outdoors soaks up all kinds of humidity & it burns colder, than dry wood...so there is that.

I don't think, it matters much that filling in more insulation (dirt) covering over the burn tunnel...main thing is keep your exh. port wide open, you may have to make another metal form or something to hold the insulation back away from the exh. port. Also you want to look at the "J" of your burn tunnel/heat riser, that elbow should be covered with your best insulation, to separate that intense heat from that elbow, radiating heat directly to the adjacent exh. port. I like how you got your exh. port attached to the side of the base, you want it insulated in that area, but cannot restrict the exh. port flow. I had to use a kind of funnel looking thing to collect as much exh. flow (inside the barrel) & bring it unrestricted to the chimney pipe (outside the barrel).

james beam

Good morning. I'm new to rocket mass heaters as well. I just finished my first build and rebuild. My system is similar to Ben's, except that my core and riser are made of fire brick. My stove features the traditional J tube, but like Ben's is a 4 inch system. My feed tube is 6", burn tunnel is 12" and riser is 32" (all measured at their longest points). The whole thing is inside 2 16 gallon barrels put together.

I am also concerned about temperature. My highest so far has been 375F. It usually hovers between 300 and 350. I have some ideas as to what causes this.

First, my riser is insulated with fiberglass. I have an 8" metal pipe around the riser, and the fiberglass is in there. My burn tunnel is not insulated. Would covering the burn tunnel and riser in perlite help?

Second, I have a small port in the front of the feed tube. This serves as a clean out. I have a small brick that fits in the port. After my mortar cured, the gap around the plug increased a bit. I can slide paper around 3 sides of it. Is it possible this is causing problems? Seems to draft well enough.

Third, my exhaust duct ties into an existing chimney. I have about 8' of 4" duct, then an adapter to 6" where the old chimney was. Could this create too much draft and suck a lot of the heat out? After an hour's burn, the exhaust is too hot to touch, but there is only steam coming out from the stack outside.

My last target is the exhaust port in the stove. Is it possible that much of the heat is just blowing through to the outside? I feel like that may be the case, which might be caused by the change in duct size. Either that, or I need some sort of baffle over the exhaust port to slow the flow.

This stove lights and burns pretty well, I just wish I knew how to make it burn hotter. I have seen where other folks made sub 6" and claimed to have good results. Any help is appreciated. Of course, I will report back any findings to help someone else who may be in the same boat.

Jesse Neal wrote:

Would covering the burn tunnel and riser in perlite help?

Yes. The burn chamber (tunnel) performs better when insulated. So too the fire brick around the fire riser. The feed tube is less critical, I believe, although you do need to keep the temps high enough to burn, and pre-warming the intake is also a good thing. That said, if there is any part of the fire box not to insulate, it would be the feed tube. Everything else, from the opening of the burn chamber all the way to the top of the fire riser really needs to be well insulated for optimal performance.

Jesse Neal wrote:

Second, I have a small port in the front of the feed tube. This serves as a clean out. I have a small brick that fits in the port. After my mortar cured, the gap around the plug increased a bit. I can slide paper around 3 sides of it. Is it possible this is causing problems? Seems to draft well enough.

I doubt that is a significant problem. To test it, seal the gap with a little fire clay your next burn and see if it increases your temperature. I doubt it will, but then you'll know, and if it makes no difference the fire clay is easy to remove (wetting the clay along the seam, giving it a few minutes to soak in, and then using a slight tap with a hammer from inside the feed tube ought to do the trick, I would think).

Jesse Neal wrote:

Third, my exhaust duct ties into an existing chimney. I have about 8' of 4" duct, then an adapter to 6" where the old chimney was. Could this create too much draft and suck a lot of the heat out? After an hour's burn, the exhaust is too hot to touch, but there is only steam coming out from the stack outside.

Maybe. This is a test/prototype, so there is no thermal mass, is that correct? If the exhaust pipe leaving your "barrel" is exposed to the room air, and not surrounded by cob or other thermal mass, it will get hot and radiate some heat into the room, but with only 8-feet of pipe, I'm guessing that is not enough to take out the majority of the heat, so yes, if that is all true, a lot of the heat may be going out the chimney.

I don't think too much draft is the problem however. I think it is too little thermal mass.

I have a similar situation in the 6" system I am testing/prototyping in my living room. But at this stage I am not trying to optimize thermal storage (in fact, I am ignoring that for the time being), rather I am trying to get the burn characteristics worked out, draft, feeding and general care of the "dragon" in the living room. Later this year I'll worry about the thermal mass after the winter passes, when I make the permanent build on the (to be) enclosed porch (and then open the existing wall between the porch and living room).

I would *guess* that about 20-feet of cob bench would be in the ball park for a 4-inch system. Less 5-foot per 90-degree turn, and adding maybe 15-feet if you have a good chimney (which it sounds like you do), so I'd take a guess and say that you'd be good for up to 20 feet of ducting in a cob bench. If your floor can take the weight, and the heat, you could create an air gap, and then stack some block and/or brick around your pipe and see if that makes any difference (creating physical contact, and filling in air gaps, using clay or cob, etc, is even better).

I just estimate the length of pipe in the cob bench by multiplying the system size by 5: 4*5=20; 6*5=30; 8*5=40. These are only ball park guesses. The Wisner's, for example, have observed 8-inch systems can push up to 50-feet of ducting, and sometimes a little more. It depends on a lot of site-specific variables. But just from what I've read others doing, it seem like the 5 multiplier gets you in the right ball park, and then subtract 5-feet per 90-degree elbow (that's minus 10-feet for an 180-degree turn in a bench) and adding 15-feet or so for a good strong chimney draft (all from what I recall of the Wisner's suggesting as good rules of thumb).

But please look into this. I really am *guessing* that 20-feet might work for you. I've not messed around much with a 4" system, and most of what I have paid attention to when reading is either a 6" or 8" system. What I am trying to say, is the rate of change may *not* be linear, it may be geometric. Which is a long winded way of saying, that maybe a 4-inch system will only support 10 feet or so of cob bench.

The area enclosed by a circle changes very quickly. Going from a 6" to a 4" circle results in a lot of loss area (CSA). Therefore, with such a large drop in air movement, you may not be able to push 20-feet of ducting. My best *guess* is that is getting close to an optimal situation, which you may or may not have.

For all of those reasons, I'd be inclined to just cob in the 8-foot of pipe you have set up (after correcting the manifold - see below), and see if that helps.

Jesse Neal wrote:

My last target is the exhaust port in the stove. Is it possible that much of the heat is just blowing through to the outside? I feel like that may be the case, which might be caused by the change in duct size. Either that, or I need some sort of baffle over the exhaust port to slow the flow.

By "exhaust port" do you mean "manifold"?

The transition from the bottom of the barrel into the ducting needs to be rather large. Making this too small is said to be a very common error. And if you are going from the bottom of your barrel directly into 4-inch duct/pipe, then you are making this error, and constricting the flow of air. That leads to poor combustion and lower temperatures, I would assume.

If you have directly attached a 4-inch pipe to the bottom of your barrel, you can test this by adding a manifold, or plenum. Any relatively large (air tight) cavity (that can take the heat) should work. You could get a transition piece, like you did to connect your 4" pipe to the 6" chimney pipe, doing basically the same thing, but in reverse. Step *up* from the 4" pipe and connect to the bottom of the barrel with a larger size pipe. You can't make this too large. Going from 4" to 6" might be enough (and would be an improvement at any rate) but you may be better off going larger, or connecting two step-ups in series (going from 4" to 6" and then from 6" to 8" actually fitting the 8" to the barrel - which will require a larger opening in the barrel, of course).

Of course, your description is that of a very strong draft. Maybe your chimney is tall enough that it is correcting for a theoretically too-tight manifold?

In any event, those are my thoughts. If I understood you correctly.

Thanks Erik. I am replying via the mobile site, so forgive me if my responses are off.

I was unable to get any perlite, but did find vermiculite. Is this applied dry and loose or do I mix with fireclay? I have some in powder form.

You are correct; this is a prototype with no mass yet. What do you think of increasing the length of duct without any mass? I plan to add mass later.

By "exhaust port", I suppose I do mean "manifold.' I have used a 4" starter collar, inserted directly into the back of the barrel. It sits about an inch lower than the burn tunnel. If I understand correctly, you're suggesting that I remove this and use a 6" starter instead? Would I then continue with 6" duct out to the chimney? Alternatively, what about the plenum or manifold you mentioned? I'm not quite grasping how that would be installed.

Thanks again for your input. I plan on adding mass in the spring, so I will have to report on that then.

You used a 4" to 6" reducer connecting your 4" duct to the 6" chimney, right? Use the same thing, or even an 8" to 4", or an 8" to 6" plus a 6" to 4", connecting the barrel to the 4" duct.

Ah, I get it now. I think I have another adapter somewhere.

As for the vermiculite insulation, how is that applied? I have heard some people combine with clay as a wet mix, while others just pack it tight and dry.

Jesse Neal wrote:

I was unable to get any perlite, but did find vermiculite. Is this applied dry and loose or do I mix with fireclay? I have some in powder form.

Did you try a brick seller? That's where I get my Perlite. You might also try roofing contractors, as it is sometimes used in commercial roofing applications. And you could try nurseries, but generally speaking, that doubles the price for the same material, so I wouldn't expect a good price from a nursery. But you might try larger agricultural suppliers who wholesale to farmers and ranchers.

You can use vermiculite. Many people do, although Perlite works a little better. In either case, you can just pour them in, but there are problems when doing so. First the particles are not good to breathe, and secondly, if the containment breaks they will pour out. For both reasons, most people mix them with at least enough clay to keep them holding together, snow ball like. When you are using them structurally, to build on top of, or inside a fire riser, mixing a little more clay into the mix is I think better than mixing in too little clay, because if there is too little clay it will crumble apart easily, whereas if there is too much clay it will hold together but have a somewhat less effective insulating value.

Mixing is always a feel it in your hands kind of thing. You learn what's right by doing it. But to get started you might try something like 1:3 or 1:6 (clay:perlite; or clay:vermiculite). And as little water as needed.

Jesse Neal wrote:

You are correct; this is a prototype with no mass yet. What do you think of increasing the length of duct without any mass? I plan to add mass later.

I do not think I would bother. But you can if you wish. You could try making a big upside down U-shape in your stove pipe between where it leaves the barrel and enters the chimney. I have seen some people do this with their wood burning stoves, trying to get more heat into the room. But if you want to get more heat to radiate from the exposed pipe, you would be better off clamping "fins" onto it. Just get any sheet metal you can work with -aluminum flashing is a popular choice- and cut it into rectangles, bend them, cut a couple slots a the bend (one each end) to pass a hose clamp through, and clamp them onto your stove pipe.

But for a temporary build, I don't think I'd bother with all that. Unless you were going to re-use the fins in your permanent build. Some people have speculated as to whether having fins attached to the pipe and then cobbing them over in the bench, makes it transfer heat to the cob better. I don't know if anyone has actually tried this yet.

Jesse Neal wrote:

By "exhaust port", I suppose I do mean "manifold.' I have used a 4" starter collar, inserted directly into the back of the barrel. It sits about an inch lower than the burn tunnel. If I understand correctly, you're suggesting that I remove this and use a 6" starter instead? Would I then continue with 6" duct out to the chimney? Alternatively, what about the plenum or manifold you mentioned? I'm not quite grasping how that would be installed.

Well, that's two different things. Just running the 4" pipe out the bottom of the barrel suggests a binding of the air flow, and is not recommended. I can't seem to recall what they call the piece that transitions from one pipe size to another. As I see it you have at least three options:

1. Just hook up a 6" pipe all the way from the bottom of the barrel to the chimney (replacing the existing 4" pipe).

2. Do the same thing you did to transition from the 4" pipe to the 6" chimney, on both ends, so that the 4" pipe steps up in size before attaching to the barrel.

3. Build a manifold. This is often just bricked in and then cobbed over. You start by cutting a fairly large hole at the back-bottom of the barrel, where the 4" pipe is now exiting. Then you build a brick box (usually with a clean out too) so that it covers that hole, and at on the opposite side you cob in the exit point for the stove pipe, and connect it to the chimney (usually by first routing it through a thermal mass cob bench).


On a totally different line of questioning....

If you have a 6-inch chimney, and you want more heat output, why not build a 6-inch system? They are said to be a lot easier to build and get tuned right than a 4-inch system, which is tempermental from what most folks say, and not recommended for a first build. Seems like a lot of reasons to build a 6-inch system. Why are you building a 4-inch system, and then planning to go through all the work of making the thermal mass for it?

Thanks Erik. I'm going to dump in my vermiculite this evening. I think I will go with the 6" to 4" adapter on both ends and see how it goes.

I decided on a 4 inch for a few reasons. 1st, was space. I had enclosed my back porch and installed a traditional woodstove out there. It took up entirely too much room. The porch is roughly 6' deep, and my stove took up nearly half that no matter how I positioned it. My 2nd reason was availability of materials. My barrels were found, and I was on a budget when it came to the brick. The 3rd reason was to prove to the wife that RMHs are a great idea. So now I have to get this 4" system working right. I don't think she would be too keen on me tearing the whole thing down again and sourcing all new material.

I will report back once I make the changes discussed.

Well, if you get grief because the 4-inch system either is not working right, or is not putting out enough heat, remind her it is a prototype/proof of concept build, and really on the small end of the usable spectrum, and difficult to get running right on top of that. Which is to say it may take a while to get it running optimally. Just keep at it and think of it as a long-term project not a quick 30-day or even one-winter kind of project.

How much square footage are you hoping to heat?

That's the approach I was going for. Hopefully I can get this working well enough to convince her to build a 6 or 8" system in the living room where there is more space. Either that, or reconfigure the room it is in to better accommodate a bigger system (which would require moving the chimney).

Right now I am looking for supplemental heat from my system with the added comfort of a thermal mass. I would like to help heat our 1st floor, which is roughly 700 square feet. We generally just leave the heat off upstairs until bed time, then we turn it on.

Results...

I'm sure the temp would have kept going had I not run out of fuel. Proof positive that a sub 6" system not only works, but can generate a significant amount of heat. This was off of a 26" scrap of base trim.

Thank you so much for the help. I can post pics of how I implemented suggestions if anyone would like to see.

Yes I would like to see what you did to get it to work.

Jeff

I first put a 6" starter in the bottom of my barrel. Then there was a 6" to 4" adapter to connect to 4" duct.

To insulate the burn tunnel, I used vermiculite dumped in loose. To keep it out of the exhaust, I made a little divider out of stove pipe. Once the one side of the barrel was full, I filled the pipe around the heat riser with vermiculite.

It took only a few minutes to get to 300F, then from there it was a steady climb to 550.