I am using full size fire brick on my first attempt to build a RMH. I am not sure if my J-tube measurements need to be adjusted. My measurements are: 8x6 feed tube; 6x6 burn tunnel that is 10 inches in length; 8x8 heat riser; 10" rise at feed tube and 32" rise in heat riser (from bottom to top on both). I don't seem to be getting enough draw and I am getting some smoke back out of the feed tube. This is without the barrel over the heat riser. I have tried adding eight more inches of rise on the heat riser but that does not seem to help. As far as I have read, these measurements should be close. Any advise would be greatly appreciated. Thanks, Mike

Mike: Is your core a dry stack, test build or have you mortared it together ? A dry stack will suck air anyplace it can and cause smoke back. If you have mortared it, then still being wet can have an effect on draw. Is your riser just a stovepipe sitting on top, or is it also mortared in place? Your measurements are good , I would go with an 8x6 burn tunnel but 6x6 is ok to test. We need more info and pictures to help you get up and running.

The entire J tube is mortared at this point.All the material is dry.The riser is fire brick on edge (long edge is horizontal) I will try to post a picture soon.

Picture was taken before mortar and is now squared up on corners of riser and the feed tube was made larger than is shown.It was 6 x 6 and is now 6 x 8. The lid is just sitting on top for my own reference.

Mike ; Your core looks good... should work fine... Try putting a brick on the feed tube to restrict the air , possibly the size difference is causing your smoke back.

Thomas,Thanks for your replies.I have tried closing off the feed tube in small increments and adding to the height without much luck.Could the size of the burn tunnel (6 x 6) compared to the riser (8 x 8 ) be an issue? If my wood isn't VERY dry could that be an issue?I wouldn't say my wood is damp,just not seasoned dry. In my reading I understand that the burn tunnel should be the tightest spot in the system and as short as possible.Would it be worth trying to close either the tunnel or riser down to a smaller size?

Hey there

I am on my first build so please take that into consideration when reading my comments.
When I fired up mine for the first time I kind of cheated. I read that when the RMH is cold it can be hard to get a good draft.
I started a fire at the bottom of the heat riser to get things started.
I found that once the "heat riser" fire died, the feed tube fire was good and the heat riser was warmed up the draft was fine.
Obviously you can't do that in normal operations but it will at least show that your set up is good.

Being that my burn tunnel is short,(only 10 inches) its easy to get the draft started and going,but for some reason I still get some smoke coming back through the feed tube.Almost like the draft could be stronger???

Mike; My understanding is the feed ,burn & riser should be the same dia., but if anyplace is tight it should be the burn tunnel . So what you have... is a feed tube & a riser sized APX. for an 8" system and a burn tunnel sized for APX. a 6" What size system were you wanting to build ? Unseasoned wood is not helping you any but I doubt that it is your problem. But let me make sure I understand the status of your build, other than the feed size & being mortared now, the picture you sent is what you're having trouble burning ? No barrell or mass , just the core Normally an open riser core with no barrel burns like crazy , the problems come when the barrel goes on ... then you look at your transition area and how wet your mass is for smoke back or poor draw problems. We need more info & pictures to figure this out . I should ask ,do you have ianto evans RMH book ? If not you should invest in a copy.

I didn't catch that. In an 8" system the feed tube, burn tunnel and heat riser should be 7" x 7".
This is Erika and Ernie's recommendation
Thats what I did except the heat riser (inner steel liner 5/8 thick) is 8" in diameter.

For 6" system you need 5" x 5" (double check that figure).

Thomas,Yes I have the Ianto Evans book and according to what I read on page 32-33 A (feed tube) could be 6 x 8 B(height of feed tube) should be kept short,mine is 10 inches C(burn tunnel) should be the tightest part but no dimension is given,so I went with 6 x 6 and the riser (F) should be 7 x 7 and I am at 8 x 8 The riser can be anywhere from 25 - 50 inches tall ,I am at 32 inches.So as far as I can tell,my only off measurement would be the riser at 8 x 8

When I have attempted to put the barrel on (my top of riser to barrel is 3inches or less) I really get smoke back out of the feed and the burn slows considerably in the burn tunnel.

Apparently the dimensions of Ianto and Ernie differ

Here are some numbers I learned from E&E on the DVD with paul wheaton.
Don't rent from Vimeo... had a lot of problems viewing the videos on line, get the DVD's.

8" system is 50 square inches meaning 7" x 7" or 6" x 8", etc..
6" sytem is 30 square inches meaning 5.5" x 5.5 " or 4" x 7.5", etc...
(I am sure there is a limit, 1/4" by 200" for example)

burn tunnel length no more than half of heat riser
(that is from the front of the feed tube to the back of the heat riser)
(from the bottom of the burn tunnel to the top of the heat riser and not the barrel that covers it)

heat riser at least 3 times longer than the feed tube

the space between the heat riser and the barrel should be 1/4 of your system size.
ie: 8" system should be 2" and 6" system should be 1.5"

the feed tube, the burn tunnel and the heat riser should have the same cross sectional area
(meaning they should all have the same square inch area)

6" system can't have more than 40 feet of linear duct
8" system can't have more than 50 feet of linear duct
(from the barrel to the exit of the house)
And every elbow you lose in potential length because of drag (5' or 10' per elbow...I forget)

proportions suggested by E&E 1 : 1.5 : 3 (feed tube : burn tunnel : heat riser)
I was given another proportion by a permies user of 1:2:4 but if you compare them the only difference is the feed tube is a little longer if the heat riser is the same length

Hope this is helpful

Ramzez,Thanks for the great replies! I just got back in from reducing the riser size to 6.5 x 8 and also reducing the feed tube to match the burn tunnel size of 6 x 6. I used some concrete block soaps (1.5 x 7.5 x 15.5) in the riser and a fire brick into the feed tube.Only to adjust my measurements for the test.I got a decent fire going and the sound is good and rocket like and seems to be getting more draw.BUT ! The fire never does get very hot.....so I am thinking now that my fuel is holding things back??? At least the smoke back from the feed tube has been stopped.My goal for Saturday is to find some really dry oak for another test run and see if that makes a difference.I will let you know,Thanks again

Mike

I think the problem may be start up technique and not dimensions.

I am wondering how hot your heat riser is getting. When I was doing my first J-tube I was a little impatient for the results. If I waited long enough the heat riser would heat up and give me a strong draw.

My second thought is on the size of the wood you are feeding to start warming the J-tube. Start small. I like to start mine with twigs and place the twigs deep in the combustion chamber. As the J-tube heats up and my draw gets stronger I bring the wood forward and put in bigger pieces. Twigs = size of pinky finger or smaller

My third thought is getting a 'cheater' (fossil fuel start up heat source) to help start the draw. I use a propane torch with a push button trigger to help start the draw and warm the J-tube. Usually after 1 to 2 minutes I have enough draw to take away the torch my twigs are now starting to become coals and they are hot enough to start some larger sticks on fire.

Hope this helps

Brett,My start up sounds like yours.I use a propane torch with a long flex hose so it can get down inside.I had it going for about an hour this evening.Starting with some paper and small cedar kindling and slowly progress to larger pieces up to the 1 inch square range.Even after that hour the wood still seems to have to be played with a lot to keep it going even though the draw seems sufficient.So I am hoping with some other fuel that will get a hotter fire going that doesn't need so much attention.Thanks
Mike

Mike; If you have access to plate metal and a way to cut it, build yourself a (P) channel to help air flow, adding one to my rmh had a very noticeable increase in velocity & temperature. If you don't know what they are look for my post (experimental P channel) and you will see pictures showing mine. Edit- added a pic for you

Thomas,Thanks for the info! I have some flat barrel steel that should do for a trial run. I can try to bend the edges in that 1/4 " or use nuts/bolts also.I will get on this today!

Mike; Peter Berg suggested increasing the height 2" up & 2" wider (both left & right) outside of the feed tube, separating the two air flows even more. My 1/8" galvi plate I used is now starting to warp in at the bottom. Amazing that from the top I can see how this changes the fire flow. Turned it around so the warp was to the feed tube side instead of the burn tunnel side and again from the top I could see a difference in fire flow however it was not a good difference! Took it back to the shop and hammered it flat again , works as good as new ! When I replace it I will try peters suggestion of increasing the height.

Thomas,I looked up your original post and read all of it.I did see the part about the 2" extra and have got one made up.I'm still working on getting some dryer wood and then will test it out.Thanks

Thomas: Can you show a photo of what it looks like without the wood or fire?

And I don't see why you did that when you can simply cover the feed tube and get same effect.
Or partially... adjust as needed.
Your plate is permanent
E&E do that.

Ramzez: The purpose of the P channel is to change the flow of air mixing with the fire. The plate extends 1/4 " past the roof of the burn tunnel & sits 1/4" out from the wall, it creates a vortex just inside of the burn tunnel from the air rushing down and rolling that burns hotter than a normal feed tube. Really makes my dragon ROAR ! The plate is just sitting there and could be removed at any time. I also use 2 bricks to restrict the feed tube , but their purpose is air restriction, they do nothing beneficial for air mixing . unfortunately I don't have any pictures of the plate or without the fire going at this time, but can post some here in a few days. The plate is simply 1/4" longer than my feed tube wall and the four bolts purpose is simply to space it out 1/4" from the wall. The two short wings hold it from dropping down. This is the spot that peter suggested making 2" taller and extending 2" left & right to separate the air flows . I suspect that E & E haven't had opportunity yet to experiment with these P channels , but when they do i'm sure they will approve.

Thomas/Ramzez,Well I am failing miserably here.Thomas I tried the P channel but didn't really notice any audible or visual difference.That being said,it may just be my design.I will ask now more about the burn tunnel and the distance the flames travel in your systems.My burn tunnel has a 10" long ceiling,with a total of 18" to the back of the riser.When I fired it up yesterday and got a good roar going,the flames are just getting visible at the riser.I'm wondering if this is now my issue?Should the flames be coming high into the riser or at least part way up?I believe my riser is high enough because even when i add height to it the draw stays the same as far as I can see and hear.I believe we have cured the small amount of smoke back in the feed tube by decreasing the measurement there and tightening up the riser As soon as I place the lid over the barrel now,it's almost an immediate shut down.As far as I can tell ,my measurements (2-3" from riser to lid) around the riser (more than 2" all the way around) and 50 square inches at the outlet should be good???

Sounds like there is a restriction somewhere, probably between the manifold and the cap on your chimney. In a cold system, no fire going obviously, there should be detectable draft pulling room air into the feed when everything in the RMH build is correct. In some conditions, when draft is not present, priming the chimney will be necessary to get the draft going before lighting up the RMH.

The riser to barrel top gap can be very accurately checked by placing, i.e. a ball of soft clay, on top of the riser. Firmly place the barrel lid in place so that it is fully seated and compresses the clay ball. Remove the lid and measure the height of the compressed clay ball to find the amount of gap present.

Updated pictures

These are my current measurements Feed tube is 6 x 6,burn tunnel is 6 x 6 with a 7.5" ceiling length,riser 6.5 x 8 x 32 top to bottom,2.5" gap from riser to cap. I have the barrel offset and the sheet metal covering my insulation is 4" or more for a length of 19" from the barrel.From the floor to base of barrel is 7" x 16".The 19" and 16" measurements are straight across not with the arch of the barrel.Both of which should be good when compared to the 6"x 6" burn tunnel.The only 2 things that I can see that could be wrong are the riser dimension is a bit big and the fact that it is not hooked into a chimney,possibly limiting the draw that it might get from that.Thoughts?

There's your main problem, for the present anyway, that being "no chimney". A proper chimney is a must have, an important part to a well functioning rocket mass heater. There is a saying going around Permies, when the barrel goes on, the chimney must go up.

You riser could stand to be resized to the same 6x6 as the feed and burn tunnel, but will work in its current configuration.

The only other thing I notice is that the barrel seems small, leaving little room around the riser. I like to have at least 2" of clearance along the sides between barrel and riser. The barrel offset will compensate for the most part, as long as the offset is on the side where your manifold will eventually be built etc.

It is 55 gal.

OK then, it's the full size firebrick filling up the space. Just looking at the latest photos it appears to have enough riser/barrel space.

I have read that it is possible for these systems to not even need a classic style chimney? Just a vent to the outside.I do understand though that a chimney does create draw too.I guess that being said I need to add in a test chimney and try again.

It is possible under just the right conditions to have a functioning RMH without a chimney, but that depends on constant wind direction and a short house that doesn't generate any significant stack effect, and probably other things. Any RMH will work better with a good chimney, and the majority need it for good operation.

Thanks Glenn.