Success story, 4" RMH for my workshop

Hello everyone. I thought I had an account here before but it seems not. Paul asked if I would stop by and share my design with the group. It's a scaled down Rocket Heater without the mass (yet). The primary goal is heat on demand for my new workshop rather than spread out over days. There still may be a thermal mass in the future that I can add later if need be. For now I don't see the need.

The basic design is a hybrid between this design I saw the builder placed on his fireplace hearth made from a discarded water heater tank with an all steel J-tube and the traditional model using fire brick. For my application I think this merges the best of both into a compact design that not only works well but is fully serviceable and sized correctly for my 240 sq-ft insulated stick frame workshop.

Pretty much everything is explained in my 3 videos covering the construction but here are the critical specs:

Tank is a salvaged 17gal compressor tank with 1/8" thick walls, 36" long by 12" I.D.

Vertical flue stack is 3 groups of 9" long fire brick, 3.5"x3.5"x27" (12.25 sq-in across, about the same as a 4" round pipe).

Vertical stack surrounded by 8" x 24" galvanized pipe with insulation stuffed in the open spaces.

Gap at the top of the internal flue is 2" from the center of the tank (A little less at the edges because of the convex surface).

Gap between the 8" pipe and I.D. of the tank is 2" around.

The horizontal burn tunnel is 5.5" long (measured at the top edges), 3.5"W x 3.25" H (11.375 sq-in across).

Vertical feed tube is 3.5"W x 3.25"D x 7.75"H (measured from the top edge to bottom inside).

Virtually all of the critical dimensions needed I found in this one diagram posted at Ernie and Erica's website. The only dimension I could not find in that diagram was the capital letter D which I deduced is the burn tunnel length by process of elimination. Can't thank those two enough for sharing their knowledge and expertise. I am calling this first build of mine a complete success. I plan to post my plans including the brick cut sizes at my web site alt-nrg.org. Hope to inspire many others to build one too! Now to install it. Here are the links to my 3 construction videos:

Video 1
Video 2
Video 3

Enjoy!

Z

Thanks for sharing! I look forward to seeing how it performs to heat your shop. Can you speak to the legal/permitting side of your situation? Don't ask/don't tell?

Looks good! I recently completed a 6" stove that looks very similar to yours. I used a 20" dia. truck propane tank for the body. I have the exhaust feeding into a 10' loop of 6" pipe which is inside a 5'x2'x18" steel box (discarded fuel tank) full of rocks and clayey gravel. Both the stove unit and bench are mounted on 6" legs, so I was able to install the whole thing on my wood floor without any modifications. I've been heating my 16x16 cabin with it for about a month now and am totally happy with it. I burn it for about 4hrs a day, and the bench "tank" stays warm for 5-8 hrs after the fire goes out.

John Danks wrote:Thanks for sharing! I look forward to seeing how it performs to heat your shop. Can you speak to the legal/permitting side of your situation? Don't ask/don't tell?

Actually, wood stoves are quite common in my area. I will be looking into my local building requirements for insurance coverage purposes but I WON'T be inviting the local inspector, IF you know what I mean.

Likewise,
kudos on the latest photos and video: very clear, good shots.
However, (!) please be aware, and share with newbies some of the following materials concerns:
galvanized pipe or other steel is to be avoided because of fume poisoning potential at high temperatures,
and I likewise doubt that what seems to be household grade fiberglass insulation will stand the temperatures
you should be able to acheive in such a unit. I'm using refractories good for 2300F +.
On the other hand, you've sparked another approach I'm going to model in CAD for a replaceable
input (feed) nosepiece, thanks
Andor

Andor Horvath wrote:Likewise,
kudos on the latest photos and video: very clear, good shots.
However, (!) please be aware, and share with newbies some of the following materials concerns:
galvanized pipe or other steel is to be avoided because of fume poisoning potential at high temperatures,
and I likewise doubt that what seems to be household grade fiberglass insulation will stand the temperatures
you should be able to acheive in such a unit. I'm using refractories good for 2300F +.
On the other hand, you've sparked another approach I'm going to model in CAD for a replaceable
input (feed) nosepiece, thanks
Andor

Yes the fiberglass is household insulation. It's use is purely experimental to see if it will hold up. Any fumes that might come off the galvanized pipe will vent outside, although I feel the outside jacket will not get hot enough to off-gas. I'm going to be adding a wire frame to the input to allow much longer sticks and longer burn times between refuelings. Think tomato plant frame.

Hi {:^}>
I've been getting the permies newsletter for a while now but only just registered at the forum. I'll be watching this post eagerly for further developments as I've been thinking about a similar project myself. I'm particularly interested to see the test results on the fibreglass and galvanized tubing... Seems I joined at a very exciting time !

Tony Hedgewolf wrote:Hi {:^}>
I've been getting the permies newsletter for a while now but only just registered at the forum. I'll be watching this post eagerly for further developments as I've been thinking about a similar project myself. I'm particularly interested to see the test results on the fibreglass and galvanized tubing... Seems I joined at a very exciting time !

I should have some hard data as soon as early next week. All that's left is to strip and repaint the outer tank flat black and punch the hole in the wall for the flue thimble. The insulation definitely is a question mark but even if the galvanized pipe needed to be changed every few years, that would absolutely be no big deal at all. The design is completely serviceable. In this photo you see where the stove will make its home. The brick hearth is just laid on the plywood floor and will be held together with moulding strips tacked down. The cement board is mounted to the walls with 3/8" thick wood slats leaving a 1.25" gap at the bottom so that radiant heat absorbed by the board can be released into the room through convection on both sides. Since the structure is raised off the ground, when I get around to playing with a directed fresh air inlet, I will just pop a hole for it in the floor.

This here is probably one of the last videos I will produce on the rocket stove. Thing performs REALLY well. Data is presented in the video. I'm adding this last photo that was not included in the video showing the simple wire frame that guides long pieces of wood for burn times between refuellings >2hrs. It's just coat hangers tack welded to a 1/2" flat bar, screwed to one side of the removable firebox door. Enjoy!

Z

Neat project. I'm looking forward to seeing the data (can't get to it through the company firewall).

Fiberglas insulation won't burn but the glue that holds the little glass fibers will melt at high temps. If it's an issue for you, get some vermiculite from a garden-type store.

should work just fine. the breakdown in 4 inch systems is in exhaust length. so a short exhaust works well. the harvesting of heat is the problem in that short of span. I am thrilled by the clean burn not so thrilled about the waste of heat.

This is looking increasingly interesting to me, as I currently live in a place which isn't my 'forever home' so anything I try out here needs to be low-impact in terms of the future sale value of the house. Ernie has commented about the short exhaust run being good for such a small burner, on which I have a couple of questions;
1) Do you feel that the vertical exhaust is significant to the success of the burn i.e. does it provide chimney draw?
2) If a horizontal run is possible, what do you think would be the maximum length with this system?

Thanks to all of you who make it possible for such lively, honest and productive discussions to take place !

in my testing of 4 inch systems yes the vertical exhaust is important. you get a much better and cleaner burn. As i said my problem with 4 inch systems is the waste of heat.

Ernie Wisner wrote:should work just fine. the breakdown in 4 inch systems is in exhaust length. so a short exhaust works well. the harvesting of heat is the problem in that short of span. I am thrilled by the clean burn not so thrilled about the waste of heat.

Hi Ernie. As I stated at the start, the primary goal for this build is heat on demand. Had it really stoked up last night. Tank top was 667F, flue just before it exits the workshop only 157F. I'm pretty happy with those numbers. No, correction, I am VERY happy with those numbers. Thanks for all that you have shared to help me make this a success the first time.

BTW, I made an addition to the wood hopper. Dumped the wire frame idea. Instead, two sections of 6" vent pipe (1st section 6" tall fixed, 2nd 18" tall removable) so I can use irregular sticks without getting hung up. 3" or 4" air inlet will enter the lower section from the floor for fresh air intake. Pictures coming.

I've been wondering about the vent pipe temp. Jeff Crutchfield built a similar sized one with a 100 gal propane tank and he has around 100deg f vent temp. see link.... http://www.youtube.com/feed/UCoxXpjp4ROpPf1aW3R8HuGw/u . I am wondering if it has to due with better insulation on yours and the smaller volume around the riser on yours.... hotter burn plus less time in the tank and less tank surface area to transfer heat from the exhaust?

Kevin

I'm sure you are correct. More time hot gases spend swirling around in the outer chamber = more heat given off into the room. I suppose if I really needed it I could always add some vertical radiator fins around the top of my tank to increase the surface area and raise the rate of heat given off. I just think it would be ugly and I don't really need it.

I tried a few tests with my new wood hopper that vented from the outside last night. When I added the tall vertical extension and closed it up with some longer pieces of wood inside, the radiant heat given off by the firebox was enough to cause the wood to off-gas and start burning above the firebrick. Not good. So, I'm leaving the 6" tall by 6" dia vented collar open at the top and combining it with the wire frame to guide longer pieces into the firebox. Even with the top open like that, the negative pressure being created by the stove is still being relieved by fresh air taking the path of least resistance, through the 3" fresh air pipe from the floor and down into the firebox.

Don't want you to think that I'm being negative, its just that this the type that I need as I'm putting heat in the shop as well. So I think I'll use your feed tube, burn chamber and riser with a bigger tank. I had a compressor tank just like yours and was going to build insulated steel internals, but I like the internals of yours better.
Thanks for posting this stuff.

Your rocket heater is impressive, and your descriptions very thorough.

I intend to copy your design soon. and need to decide on a 4" or 6" chimney.

Could you list the size of the garage it heats and the R values of the walls and ceiling.

At what outside temperature will it keep the garage at 65F?

Thanks,

Don

All good questions. The structure is a newly renovated shed, 12'x20', open ceiling, about 2150 cu-ft airspace. It is built above ground on 6 posts, 2x12 floor joists on 16" centers, two layers of 3/4" 5-ply with a layer of heavy roofing underlayment in between. Walls are 2x4, R13 with sheetrock. Roof is 2x6, R19, ventilated, also sheetrocked.

Last night it was about 32F outside. Brought the interior from 50F to 65F in 60 minutes. Another hour later it was 75F, not 80F, because I cut back on the wood I was feeding it. I was told by someone in a chat room that equated to about 35000 BTU/hr net gain. I have a simple idea for some radiator fins on the tank I'm going to try as well. Just some angled aluminum roof shingle flashing cut into sections and strapped to the outside of the tank. We'll see how well that works too.

Z

Z,

Thanks a million!! Your numbers are a big help.

Don

don ho wrote:Z,

Thanks a million!! Your numbers are a big help.

Don

No problem. I'll send you a bill!

Tonight's latest mods. Dropped the flue temps 40 degrees across the board. The fin material is just aluminum drip edge cut to 11" lengths, sprayed with high temp flat black and clamped to the tank. Works so well I might expand the concept further.

This thing is looking great! I have got to build me one ASAP. It was -4 here this morning. Thanks to all of you for your insights!

Just a thought; could you improve the heat transfer to the fins by creating turbulence in the downflow, using a couple of simple baffles inside the chamber?

First question (probably stupid). What does "4 inch" RMH mean ? I see no dimension listed which is 4".

Second question. I'm not a welder or a DIY person in metal at all. Are small fireboxes available readymade - in the UK or France ? Indeed, does anyone supply thewhole unit as an easy-to-assemble kit in the UK or France ?