Low heat out put.

It is winter now in the high deserts of Az my RMH bench is dry, I get a rocket noise when the wood is in just the right way and it does draft, but the temperatures are very low (magnet temperature gauge) it barley gets to 300 deg. 90% of the time it stays at 250F or lower or about 100C this is on the side of barrel at the very top. Could my ash pit be too big and causing a problem with the burn? I have attached 2 pictures (I hope) that show the ash pit section any comment would be very helpful. TIA

Just in case pictures do not turn out.

Ash pit measurements 12+H at the barrel 24" across at barrel tapers down to 8" pipe about 22" long.

Hi David; What you are calling your ash pit is commonly called the horizontal transition area. It is not to big , I don't believe that it can be . Your problem must be elsewhere. What are your J tube dimensions ? Riser ? barrel top & side gap ? 300-250 F is low for a barrel side temp , something is not sized correctly or possibly you're not letting everything get heated up long enough. We need more information to help.

Well, first of all, i see no insulation around the feed tube and burn tunnel.

After, i would give the same comments as Thomas.

The whole system is 7"x7" my heat riser is 44" the gap at the top is 3" the gap along the side is aprox. 2.5" I don"t know what to do now.

Side gap ok!

"plenum" fluer transition (what you call ash pit) OK!

Top gap?

Lengh of pipe?

Elbows? Dry mass, wet mass? No mass?

How high is the chimney? Do you have a chimney cowl of some kind? How high is it above the apex of the roof?

Feed tube/burn tunel/heat riser of a constant section?

The gap at the top where barrel and heat riser is 3" the heat riser was finished flat on top ? The bench has 4 elbows 1 45 with about 12'+ of 8" piping which makes it look like a big P going to the chimney which goes up through the roof and 2' above that with a wood stove spark arrester cap {it is clean}. the burn channel is 10" long the heat riser is 44" top to the bottom of burn channel the cob around the piping is dry. A mixture of cob,river rocks and 1/2 to 3/4 crushed granite. David

David; everything sounds close enough that it should do better. Is your chimney 50 square inches or more ? Could it be slowing you up ? Can you easily remove the spark arrestor cap , it could be slowing you up. Your riser gap could be 2" instead of 3" How long have you kept it burning ?

Yep, everything seems good!

Thomas, leave that 3 inch top gap alone!

But you said it, the spark arestor might be a problem. If it's a grid with little holes, the friction given by the small gaps slows the gases passing through.

You need 49sqin of open way for the gases to go through. I'd say, if the gaps are smaller than 1/2 you need at least three times that, and if smaller than 1/4, may be 5 times that. As velocity of the gases is the key for a rocket.

David, you're absolutely sure you haven't forgoten a rag in one of the pipes? Crushed it with a stone?

How long ago have you made your mass?

How long after lighting up do you take your temperature measurements?

everything is 8" all the way through the roof I took the spark arrester off last night and did see some improvement but not enough to say that was causing the problem. I took pictures of all the pipes in the bench and everything looks good no rags everything is round. The bench is dry it has been a month or so drying I am putting off the final coat till I get this thing running correctly. David

Count more on 6 or 7 months for drying a whole mass of cob naturaly.

Use a fan, and burn, burn and keep burning. Till it's all dried out!

David; Max is correct in that a new built cob bench takes months to completely dry out and heat up. It also takes time to heat up a firebrick core & riser I would like to see a picture showing more of your piping and your chimney. Is your chimney metal all the way or is it a block chimney ? Is it mostly indoors with only the last two feet exposed outdoors ? Leave off the spark arrester, you won't be getting anything but steam out the top anyway ! Put a "coolie cap " on the chimney to keep rain out. How much (long) are you burning it ? How dry is your wood ? A new build brick J tube & riser and then a cob mass needs to be burnt at least 8 hours at a time, when you can , to get it dry and to get it started heating up for the season. If necessary use a hand held fan at the mouth of your feed tube to Turbo Effect your rmh. I recommend a cheap (10-20$) handheld laser temp gun , so that you can monitor the temps at the barrel and at different locations along the mass. KEEP BURNING it will get better !

Hello David,

Your build sounds a lot like my RMH build. I typically get 250 to 350 F, but I designed mine that way intentionally. My heat riser is 45 inches tall and I have a 4 inch gap between barrel and riser. I wanted my barrel temperatures to be colder because my RMH is next to a cement mortared wall (thermal decomposition of cement mortar ~ 500 F). The cross sectional area of my feed tube is smaller than a typically RMH build to so I cannot put a lot of wood in all at once. I can get higher temps (450 F) if I have a hot coal bed and then dump in a lot of small twigs. I also get higher temps on windy days because of a Venturi effect on my chimney.

To get a hotter barrel temperature I would suggest one or all of the following:
*check for any competing convective air flow sources: air flow out upstairs windows, aif flow out central HVAC exhaust, a leaky HVAC duct that leaks into the ceiling space that is then vented outside, bathroom fan vent
*decreasing the heat riser height and also lowering the barrel too, keeping the 3 inch riser-barrel gap
*put a ducting fan on the chimney outside your house
*decrease your ducting run
*burn a hotter faster burning wood (I get hotter temperatures with pine from pallets and cedar)

Satamax Antone wrote:Well, first of all, i see no insulation around the feed tube and burn tunnel.

After, i would give the same comments as Thomas.

I would have to agree with this one. With my rocket, I installed 6 inches of perelite and clay ALL the way around the fire chamber, the more heat you can hold in, the hotter the burn - more efficient. I can melt copper pipe inside my burn chamber since there is nowhere for the heat too go . . .

Also, I noticed that your burn chamber is 7 x7 ? My brother built his rocket at the same time as me, and his system is a 10 inch system (10 inch pipe) - interesting thing is, that his burn chamber is 7x7 ? When he has his rocket burning, you know it's a rocket stove ! whoooosh !

One of the things I did was too first calculate the space in the pipe you are using . . 8 inch is equal to "A"
Then make sure that your burn tunnel size ( 7x7) is the same as " A " all the way thru to the top of the heat riser.
Then (and this was the hard part for me) figure out how much space your barrel has inside which would be "B"
Then figure out how much space the outer pipe of the heat riser takes up and minus it from "B" - I figure whats left should be more than the "A" - since the hot air in there is expanding and contracting, and doing all kinds of funky things. Just as long as it's not less than "A" and making a bottleneck in the system . . .

Your manifold looks good, as well as the space at the bottom of the barrel for the hot gasses to exit (lots of space).

I had a few burn temps that would not get over 500 F at the top of the barrel, and I know how frustrating it can be. . . I was even turning on the video camera on my smart phone, and sticking it into the pipes with the camera light turned on - just too see what was in there (if anything) . . You probably know all this already, but if it ain't burning hot, you may want too double check - everything - one more time.

Just a suggestion... oh, and post more pics as you go !

Another thing, i hope you haven't made your burn tunnel low and wide. That has been proved to be a problem too.

Satamax,

Could you give an example of problematic low and wide? This is the first I've heard of it.

If low and wide is a problem what aspect ratio should the burn tunnel be?

Brett Andrzejewski wrote:Satamax,

Could you give an example of problematic low and wide? This is the first I've heard of it.

If low and wide is a problem what aspect ratio should the burn tunnel be?

Well we've discussed that at Donkey's with Peter. But just as a statement, that a wide and shallow rocket wans't working well. Peter tested it, i tested it. But i can't remember having a discution about the why behind.

Mind you, it's just speculation, but a very wide and shallow rocket has more surface to exchange heat at the top, with the bricks or else, thus cooling the burn. Same goes with the mixing with fresh air. Too much fresh air with too much mixing, because usualy, the sticks lean towards the front of the burner. The boundary layer might also be suspect, where the hottest gases are, slowing theses down.

HTH.

Max.

I remember the rocket stove videos that I have here, and there is some mention of (if possible) making the burn tunnel deeper - rather than going wider. - ash collection ?

Do not remember the reasoning for this either, but I did notice that with my rocket stove last night, I loaded it up to get it nice and hot, and after about the third full load of wood went into the hole, it started to back draft - came to find out that the embers were piling up and blocking the burn tunnel (no air flow) - I gave it a push further into the tunnel with a piece of wood and it helped, but it would only be a temporary solution too letting it burn down a bit . . . that is with a square 5 x5 tunnel.