8" batch rocket mass heater in bale/cob house

Here is my 8" batch rocket mass heater completely build to Peter's specs. The riser is square but with the octagon pieces placed on the first half of the riser as built by Peter in his Mallorca build. I used a cast iron griddle to span the top of the fire box and have used ductwork channels rather than a bell bench design. I have ran it twice now with promising results. No priming needed and the fire began drafting through the port immediately. There was some leaking at the cleanouts as I did not seal them properly beforehand and was only doing a test run which may have led to the resulting low draft. I felt it could definitely be running harder but I didn't load it fully with good dry fuel as the lid for the barrel was not properly secured but only pressed on and I didn't want a mishap.

Possible reasons why I wasn't getting a nice hard draft going through the system?
1) leaky pipe cleanouts
2) Didn't load the stove up with enough fuel/enough time to get it roaring
3)the cob around the firebox is still wet--the rest of the bench is dry
4) the house isn't completely sealed yet. No doors and not front windows which is an entire wall
Any other ideas???

Also does anyone with something similar get smoking out of the door issues when they go to reload the stove? I had to load quickly after the initial start to keep the smoke to the minimum and keep the smoke from coming out of the P-channel as well.

and a video of the first burn. It wouldn't let me attach an MP4 so here is a link

https://drive.google.com/open?id=1S2HLaDyEHfhkNd0p7Gb22dfLE0orJlq2

Hi Daniel;   REALLY Nice looking build you have there!
First & best reason for less than optimal draft is the wet cob ... my bench sat 4-5 months while building, I thought for sure it was dry... 5 weeks later after daily burning it finally was dry !!!  Use a temp gun and monitor the heat on your bench you will see it warming more as the cob dries.
Your leaking clean outs didn't help, and not having a full load of wood didn't help. Your house being unsealed should not have anything to do with it.
You haven't told us what your chimney is like, that could have a bearing on it as well.

Thanks thomas,

I imagine the cob could still be wet, it has been sitting since may so i suppose it could still be wet near center. The chimney is single wall stove pipe running vertically next to the barrel-- about 8 feet to the ceiling and then it switches to double wall insulated chimney pipe a foot to get through the ceiling and is 4 feet above the roofline. I'm getting white smoke coming out of the chimney, possibly just steam but it is pretty cool. I need to get a better thermometer for this but it is just hitting the air and dropping. I put my hand entirely in the chimney cap and it was warm, I'm guessing  around 120F. I'm tried to shoot for the ideal 200-250F range. I think that is what Peter said the ideal temp was for clean burning, I'm hoping my bench isnt too long. It is about 40ft of pipe including the elbows at 5ft each.

I've sealed up the cleanouts now and will find some courage to load this thing up and let it go.

White smoke is steam, it would be blueish if it was a poor burn.  When your cob is dry AND your wood is dry there will be nothing but shimmers coming from your cap.  Temp at my chimney cap is apx  F 130  , the stove pipe where it leaves the mass is 130 . So I think your exhaust temp is good. I'm not sure about the 200-250 range that Peter suggested as optimum, my J tube sure hasn't gotten there.
Sounds like your chimney is perfect as well.
Its a little warm this afternoon but tonight or early morning load her up and stand back I think you just need to dry it out.
Ebay sells digital temp guns under $20 worth every penny!

Thanks again Thomas,

As this is my first batch build and only second RMH build it is nice to hear encouraging replies from others. I might stop at my local hardware store on the way home and grab a digital reader. I'll load it up, take some photos and post my results.

Daniel, good looking build.

Two details tho.

I'm worried you won't have enough primary air.  All those little holes , with lot's of brick or vast iron surface create a lot of drag.

And i hope you have thought about a way to bypass some of the bench, for the nasty days when the stove refuses to start.

Almost all rocket mass heaters are unwilling to run ferociously at first. Clean burning at the same time is relatively common, that white steam indicates there's a lot of water involved, either in the heater or in the fuel. Besides that, smoke coming out the p-channel and door is an indication of weak draft, probably caused by the core and bech not really dry yet.

On a more serious note, I spotted a couple of errors but I don't know if those are real problems.

The p-channel looks quite small for such a large system. Is the (internal) csa as large as 5% or more of system size?

You are using the Mallorca build layout, that one is tested in combination with a floor channel only. I honestly don't know whether or not this would lead to malfunction in combination with a p-channel. The drawing of the Mallorca build as well as the diagram in the Core Designs chapter clearly shows a floor channel, though.

Batch rockets aren't tested when combined with a piped bench. The 8" version at Paul Wheaton's is a notably exeption, although the coupled piped bench in that case is a pebble-style one, unlike yours.

Satamax-- The little holes I measure along with the slider on the cast iron door and it came out to around 10 square inches which is just slightly smaller than the recommended 10 1/16 ich area. I didn't think about the drag though. I could fill these holes and create a 3 3/16 x 3 3/16 opening. I thought that the holes would be a slick way to prevent sparks popping out from the box, but performance of the draft is definitely my top priority.   I haven't put a bypass in, but put a cleanout at the base of the chimney so that I could prime it from inside. This is how I have started up my J style for the last three winters, it will be nice having this feature inside the house this time.


Peter-- The P-channel is exactly the recommended size found on your diagram for an 8" system- 2 7/8 x 7/8 with a 7/8" overhang which is 5.8% of the CSA.

The only part of the Mallorca build I used was the riser layout with 45 degree triangles at the back to get the double rams horns, everything else is to the layout specified in your diagram. I realize you didn't put the backsweep in that build either due to the floor channel and based on you double shoebox design. I could still install a sweep if you thought that would perform better. Do you think also that the stove would draw better if I used a single 3 3/16 x 3 3/16 opening for the primary versus the brick slats? Totally doable.

We will call this an official test of the piped bench batch rocket since the bell style is much more common. After lighting it the first time in semi warm weather with leaking cleanouts and still having it draw all the way out the chimney I'm very pleased.

Thanks for the comments and suggestions. I'll post my results as they come.

New photo of the almost completed bench and a new video link.

Reloaded it with the new air intake made--filled up the brick holes and cut a single opening.The batch smoked badly at first, but I'm pretty sure it was user error as I put the wood too far forward toward the port and once I reached in--burning my hand-- and used a stick to drag the flames backward from the port it leveled out and had a really nice sound. Found another small leak where barrel meets cob, but I put some sandy clay slip and it went away immediately. Ordered a temp gun on amazon so I should get some temperature reading soon, maybe by the weekend. Sorely tempted to purchase a CO reader just for fun and see what PPM I'm getting.

https://drive.google.com/open?id=1wqDOxUcqlJCecI04AgoO87NSCiOyhAe0

Daniel, i don't know if it's the video, but it seems on the verge of pulsating. Nothing extremely bad for a "wet" heater.

Well, for lighting up, here's how i do it.



Handful of twigs, or split sticks, in front of the port. Then, i top up, until i have a good fire going. And fill the box then.

A good trick that Kirk Mobert uses. An incense stick in front of the port, or air intake tells you how your draft behaves.

Thanks for the video satamax. I'm going to give the incense stick a shot and just see how the draft is. I saw a video of someone loading theirs up all the way and then lighting from the bottom. I'm guessing it may work better to follow your video tutorial instead. I will get a video shot of how it is performing and post it. I suppose I could always add another few feet to the chimney to increase the draft if it continues to be an issue.

Daniel Ray wrote:I saw a video of someone loading theirs up all the way and then lighting from the bottom.

Been there, done that.

The most idiotic way to do it, i think. The rocket, in order to work well, needs the heat of the flames at the port, their velocity too, dragging air in. So, if you light opposite of the port, and at the bottom, where all wood might catch fire there. against the door. And have no flame reaching the port, to light the afterburner.

I have done loading the full box too, and lighting on top, right in front of the port. Works a bit better.

But in both cases, if your system stalls by reaching dew point, you get a nasty smoke back, filling the whole house.

So lighting a small fire in front of the port. And ramping up your fire, is the best bet in my opinion.

satamax, could you explain the dew point issue. I don't remember reading about that in "the book" or on the proboards. What is it about this that causes smokeback? This is definitely what happened when I tried the full load from the back. Or is there another thread where you've explained this?

Well, a quick explanation.

There is moisture in, the air, and in the wood.

That moisture takes up energy to be transformed into steam. And if the steam condenses into a cloud in the flue, the gases get heavier, and tend to come back down into the chimney.

http://donkey32.proboards.com/thread/1103/dew-point-flue-gas

https://en.wikipedia.org/wiki/Dew_point

https://books.google.fr/books?id=fayqDAAAQBAJ&pg=PA146&lpg=PA146&dq=dew+point+chimney+stall&source=bl&ots=Pn8RUJ6PlE&sig=rHR8_nqKZeTceiZIKXVSQeXWeS8&hl=fr&sa=X&ved=2ahUKEwi85_-foqfdAhUC0BoKHY7-DmwQ6AEwAHoECAAQAQ#v=onepage&q=dew%20point%20chimney%20stall&f=false

very cool stuff. Thanks for the links. I'm still new to the combustion game of RMH so I'll keep learning and hopefully pass the knowledge on.

Quick update. I'm still lacking the greatest of drafts. From your video it seemed that the initial ignite begins pulling hard through the port, however mine is not pulling quite that hard which leads to some smoke drifting out from the front when the door is open and eventually a bit out of the P-channel.

I tested the draft prelight with an incense stick and the draft was definitely pulling everything the correct direction.

Once lit, the small kindling ignites, but once more is added it is somewhat smothered which leads to smokeback issues. I also had some pulsing today, but once everything is caught and it has warmed up a bit it ceases to smoke back but still doesn't have the same sound that is particular to batch stoves. On the plus side the chimney only had a small trickle of steam today, but I would sure like to fix this issue.

We have success! The stove is burning nicely now, still had a bit of smoke drift out of the door when wide open during mid burn when everything was ablaze. Got a nice roar going with only a half sized fire. Barrel at top got to the 500F range, bottom of barrel stayed around 400F. Single wall chimney pipe at the ceiling was just over 100F so I'm guessing the exhaust temps were pretty good as well. What are people using to gauge the exhaust temperatures in the center of the stream? Turkey thermometer?

Possible reasons this burn went better...
1) temperature differential--was 33F outside and 60F inside, this is the coldest it has been outside so far.
2)Bench has dried out more from previous burns
3)Added 3' of chimney on the roof to get it up to 16' total. I was compliant with the 3 feet above the roof 2 feet above anything within 10 feet, but still wasn't taller than the highest point of my roof--now the chimney is even with the highest point of the roof.
4) Closed the upstairs window that was open.


I've linked another video to the burn for anyone interested. Thanks for your comments, I'm excited to keep my house warm this winter.

https://drive.google.com/open?id=1At4QROF09NTTFJ6fW1KOk9ywRR4hpJqs

Looking great Daniel ! Video looks good as well. Your going to very happy this winter.

Update on this stove. I couldn't be happier with its performance. I let it sit for over a week and dI'd a cold start with no problems whatsoever. After three consecutive burns over 3 days my house is 75f without sealed doors and 30f weather. The bench was still 68f after leaving overnight.

Barrel top is hovering mid burn about 600f with the top of the exhaust pipe at 120f. Not sure what the exhaust midstream is as I don't have an accurate way to read it. Full burns last about an hour with lingering coals for another half hour.

Thanks again to Peter for such an excellent design and all of those here that gave their advice along the way. Just awesome work!!!

Daniel Ray wrote:Thanks again to Peter for such an excellent design and all of those here that gave their advice along the way. Just awesome work!!!

Glad to be of help, always nice to see people half a globe away having build it and are happy with it.

On another note: this might be an opportunity to place it as an example on the batchrocket site. Since it's the first full batch box with barrels and a piped bench that I know of. Could you write a comprehensive article about your build, proportions, where you aquired the door and fixed it to the firebox, pictures, that last video?

The end temperatures could be measured with a turkey pin thermometer. I won't expect you to buy a Testo gas analiser but most of the builds in the applications chapter never saw one either.
Let's say another month or two and then evaluate and write the article?

Peter, I would love to write that up for the site. Where should I send it when I've put it all together with the videos and photos? Cheers.

See how it goes the coming couple of months, there might be some changes in behaviour while you learn to operate the heater.
Just send me a purple moosage (personal message) when it's ready and I'll send you the mail address to deliver it to.

Sounds good, I'll document well.

Hi, does it still work after all these years?
It would be interesting to get feedback after all this time

Hi Cedric, the stove still runs well after five years. It has had some modifications  and this is a good place to put the results from the original.

1. The original firebox was made shorter to keep from taking space in the room, a mistake that i've fixed this year by extending it. The main issue was that wood had to be cut short, under 14" to load. This ends up making hard work for more cutting, but impossible if you are ordering wood from someone as they don't cut it that short.

2. I switched out the cast iron door with a glass fronted door from firespeaking. Not a performance issue, just nice to see the fire.

3. We had a baby and I decided to remove the extremely hot cast iron top from the firebox and replace with a kiln shelf, cob and tiles. This required removing the cob from the sides of the firebox and wrapping new insulation blanket around the entire box continuously so no cracks would form in the cob.

4. I took advantage of removing the cast iron plate and removed the P-channel which had degraded less than an 1/8" and replaced it with a floor channel which can be changed out much easier.

5. The triangular bricks at the back of the riser came loose after three years of burning. As this is the hottest part of the stove I wasn't so surprised, but I took them out and didn't replace--didn't have a massive change so I haven't replaced them.

I cleaned out the manifold access for the first time last year which had about half an inch of fly ash in it and could have went probably a decade or two without needing being cleaned out. The only reason I did was because randomly a bird flew down the chimney and made it all the way through the piped bench to the manifold. Crazy!

That is gorgeous, Daniel. It deserves a place in the list of Beautiful Rocket Mass Heaters.