Rocket seems to be running cool

Hi all,

My wife and I have built a test RMH in a shed. Didn't have a lot of space so we used 6 inch exhaust pipe about 21 feet long with 2 180 degree turns and one 90 degree turn to exit through a wall. The two 180s were negotiated with one T and one 90 degree elbow each. Used a 55 gallon drum so maybe a mismatch there. We built the feed tube, burn tunnel, and heat riser with brick. Here are the dimensions and description.

Feed tube opening: 6" x 4.75"
burn tunnel cross section: 6" x 4.75"
burn tunnel length (D): 14"
heat riser cross section: 6" x 6"
heat riser height (E): 41"

The insulation around the heat riser is contained with 1/4" hardware cloth and stuffed with vermiculite and slip mix. It has a flat top at 41". The distance from the heat riser top the the bottom of the barrel top is 2.25". The space between the heat rise sides and the barrel sides varies (didn't get the cloth exactly circular) between 1" and 2". When we fire the stove there is pretty even heat around all the barrel sides.

We've had about 5 or six test fires so far. The first couple were with minimal cob. Just enough mortar and cob to seal the barrel and the connection from the barrel to the exhaust pipe. Even then without all the pipe turns in place it seemed to run cool. I could touch my hand near the top of the barrel on the sides momentarily. It was hot to touch but didn't burn.

Since then we laid out all the pipe in the final configuration as described above and covered all with cob so we are sweating out a lot of moisture from the system.

Yesterday I got a bi-metal stove thermometer and dropped it on the top of the barrel and fired it up again. After preheating the primer port and a few smokey minutes the fire draws nicely. The first fire I built was with scrap wood and very small windfall branches (not the best density or heat content). I focused on using small pieces to get the fastest time to highest heat. After about 40 minutes or so the temperature registered at 420 F. I kept messing with it pushing more wood into the feed tube and subsequently blocking the burn tunnel with hot embers so the draw stalled out and filled the shed with smoke. I pulled all the wood I could grab out and the stove started to draw again. I let it burn down for quite a while. I took a break.

The 2nd fire: I cleaned out all the ash and embers with a small scoop and some bent sheet metal to reach into the tunnel. This time I used sticks split from our regular supply of cord wood - very dense hedge (Osage orange) - split very small. Again in about 45 minutes I had a good hot fire going and the temperature registered about 450 F. This time I was careful not to push anything into the burn tunnel so it remained relatively clear compared to the first test.

So that's the story - to me - compared to what I have read and seen in videos this is not as hot as I expected. I know that the distance from the top of the heat riser to the top of the barrel is more than ideal for cooking but I didn't think it would be a problem for generating a hot fire. On the other hand this is first time I have ever seen a rocket stove in person.

This test stove is going to be used to heat a small shop in the winter (if it works properly) and as a learning experience for a possible retrofit into our existing home.

Is this reasonable performance for a rocket stove and if it isn't what tweaks, adjustments, redesigns are needed to get it running hotter?

I am using the Ianto Evans/Lesli Jackson's book plus the 6" rocket stove plans from Ernie and Erica.

Paul Brook : I expect the 'fix' when found will turn out to be simple, though nothing jumps out at me ! Could you add a description of how you handled the 90 degree turn from the bottom of the barrel
through the Rockets brick base and out at the horizontal run of stove pipe, this is the 1st spot that seems to slowly fill with wood ash, - you did not mention a clean out there (?).
I'm asking about insulation around your Rocket stoves brick base just because your start-up time seems a little long, perhaps these answers will give someone else a clue to help you !
PYRO-magicly yours Allen L.

allen lumley wrote:Could you add a description of how you handled the 90 degree turn from the bottom of the barrel through the Rockets brick base and out at the horizontal run of stove pipe, this is the 1st spot that seems to slowly fill with wood ash, - you did not mention a clean out there (?).

Where to begin. We used brick to support the insulation and the barrel on three sides. On the side where the barrel vents to the exhaust system I created two walls parallel with the direction of the exhaust pipe about 12-14 inches apart (also supporting the barrel) and bridged it across the top and down the front with a several layers of chicken wire (it's what I had). I cut a hole in the front of the chicken wire mesh to accommodate an arm of a 6" T. The leg of the T is the primer/clean-out the other arm connects to the rest of the exhaust pipe. I used cob to build the roof of the manifold/exhaust collector and once that was set I poked the T arm into the hole but just enough to make the connection. Then I sealed the T to the mesh with cob. I can reach into this area when I uncap the T to clean it out. So far there doesn't seem to be much build up of ash. This is also where I light newspaper to warm up the pipe. The inside of this little cave isn't particularly smooth but relative to the barrel opening and the 6" pipe it's pretty large.

allen lumley wrote:I'm asking about insulation around your Rocket stoves brick base just because your start-up time seems a little long, perhaps these answers will give someone else a clue to help you.

The shed is built on a concrete slab. I laid down one layer of brick directly on top of that (no vermiculite or perlite). From there I built the feed tube, burn tunnel, and heat riser. There are several inches of cob around the brick foundation below the barrel and around the feed tube and burn tunnel.

There are definitely some defects in the workmanship. The barrel we got already had one end removed and the remaining end has two small caps so I could inspect the top of the heat riser after placing it. So I'm pretty sure about the distance from the heat riser to the barrel top. After a couple of fires when I measured that distance the insulation cap made from slip cracked and fell in about an inch or so. The insulation/slip may have shrunk when drying out - or we may have not compacted it enough. In any case the top of the heat riser/insulation layer isn't smooth and may be collecting ash or maybe cause more turbulence than expected. Some of the feed tube mortar joints are breaking down - maybe from getting bumped but if the same thing is happening in the heat riser that could also cause some problems.

We fired up the stove this morning and got the barrel top heated to about 400 F. We didn't need to preheat the pipe because it still was warm from last night's burn. I mostly used hardwood after it got going by the time it hit 400 the burn tunnel and feed tube was filling up with hot coals. I needed to adjust the brick on the feed tube to constrict the air flow or the feed tube would smoke.

Thanks for taking a look at this.

Paul Brook : This direction will only make sense if you have a Vertically loading 'J-Tube' wood feed. Its a small thing but its counter-intuitive,
try loading your wood so that it is in front of your coals, instead of pushing coals further in !

Mortar, what did you use for a final ratio, ? clay and sand, none of the mortar joints should ever exceed 1/4 '' .
there are two techniques of sealing up the Heat Riser, Soaking the bricks a short time to keep the bricks from drawing all of the water out of the mortar
causing it to dry before it can set .

The second technique needs careful explaining, the bricks are dipped in a thin-ish pancake batter like clay slip / slurry and then carefully stacked
with the interior of the Heat Riser being carefully wiped down, after the bricks have had a chance to nearly set-up, the exterior is spritzed with water to
partly re-hydrate the clay slip so that the mortar that is then applied to the outside has something to lock onto ! This is not some carefully guarded secret
it falls under the ' but i thought every one knew that one !

Really wet bricks exposed to a hot fire will chip, or spall ,or just go bang - so your weather will have to co-operate,
to allow for careful drying, not the time to build a large fire to dry out your cob!

These tips come with a large disclaimer - Y.M.M.V. PYRO - magicly yours , - Allen L.

allen lumley wrote:
Really wet bricks exposed to a hot fire will chip, or spall ,or just go bang - so your weather will have to co-operate,
to allow for careful drying, not the time to build a large fire to dry out your cob!

Yep it's cold here so I have only built a few small fires since I last logged in.

First off I reread the "book" and the plans from E&E and came away with a few new understandings. 1) the 55 gallon barrel is too big for the cross section area we targeted (6") so... there's too much surface area on the bell to really get it hot. 2) it's a game of inches in that the heat riser is about 3/4" too far below the top of the barrel it should be 1.5" and it is 2.25", 3) the burn tunnel is too long for the heat riser.

This last point I was able to do something about without removing the barrel. I rebuilt the burn tunnel and removed two of the bricks that make up the top or bridge portion of the tunnel so the feed tube is bang up against the edge of the barrel. Other adjustments will have wait until spring. This gave me a reduced bridge length of 9" or a total length of tunnel (from outside edge of feed tube to farthest end of burn/heat riser) of 20". So this helps with starting the fires and keeping them going smoke free but it still burns cool and if I'm not careful it can back draft and blow smoke out of the feed tube.

In the Spring I want to pull the barrel and rebuild the burn tunnel with more insulation around it. Since I can't make the tunnel shorter due to the diameter of the barrel I am going to add a few more courses of brick to the riser to go from 41" to 49".

I have a question about this: The barrel is about 35" tall and sits on top of the burn tunnel but now I am going to raise the heat riser about 8+ inches and I'll need to add almost that much height to the bell. I plan to do that with brick wall and seal that up on the inside with cob. Will that work or are there other ways to extend the height of the bell? I don't know how to weld.