newly build rocket mass heater with smokeback

Heya,

Our intention: We want a rocket mass heater that can keep our small house warm during winter, preferably without lots of wood.

Our design:
We used 2 barrels of the same diameter (38cm). The bottom one we halved and cut a square hole where the fire chamber will be. We build a fire chamber of fire bricks. the inner chamber is 2 bricks (40cm) long and 1/2 brick width (10cm). As a roof for the firechamber inside the barrel we used a vermiculite board, where we cut in a circle for the heat riser. The heatriser's inner pipe has a diameter of 10cm. It has an outer pipe of 16cm diameter. The space in between is filled with clay mixed with a lot of sand (we didn't have much clay left). There is approximately 4cm between the heatriser top and the barrel top. On the other side of the bottom-half barrel we made a smaller hole for a 130mm T-shaped steel pipe. One end we put in an loosely fitting steel lid and taped it over with aluminium tape to make it air-tight. The other end we put almost 10 meters of aliminium bendable pipe, which we put in several S shapes to give as much heat as possible to the bench. We put the whole thing on a layer of stone, with underneath old carpet and the original wooden floor. The exhaust comes out low in the wall and goes inside, where it goes 2 meters up to exhaust above the roof. The wood feed starts where the barrel ends (there's no fire brick 'bridge'). The feed itself is therefore pretty long (16cm), 10cm width and 12cm deep.

The Problem:
It's only burned for 2 days. But even now I think it's not getting hot enough. There's a lot of residue ash, both in the fire chamber and inside the barrel. When I've burned it a few hours the ash and hot coals start taking up a lot of space and eventually I start getting a lot of smokeback. I think it's not rockety enough. There's just too much charcoal left too quickly. Even if it seems to work the exhaust is defnitely also real smoke.

How can I change my rocket mass heater so it will function better?

I thought the wood feed might be wrong and had several attempts with adding 1-4 bricks to make the wood feed a bit deeper and less long. Doesn't seem to help a lot.
At this point I have the most success with no smokeback when I make the fire deep inside the fire chamber (inside the barrel).
Is the heat riser too small?


thanks,
Rein

Ok so I did a few things:

- I checked the barrel top to the heat riser. It was only 2 cm, so I changed that to the intended 4 cm.
- I made the wood feed a bit higher and smaller

It seems to function only marginally better than before. After some hours of moderate success the fire is pretty hot and I put more sticks in vertically. Somehow the fire doesn't burn enough sideways. The sticks start catching fire too high and eventually the fire comes back out of the wood feed, killing itself quickly afterwards with lots of smoke. From this point onward I tried everything, with only little succes. Many times the flames go in the wrong direction. Sometimes only partly - but with lots of smoke as a result.

My next idea what's wrong is the heat riser. It's inner pipe is only 10cm, while on the design it is 6 inch (15 cm). That's 1 1/2 times my core. Can this be the problem? It's seems a pain to start again with a new heat riser, but if you guys think this helps I will.

Rein Baarsma wrote:

My next idea what's wrong is the heat riser. It's inner pipe is only 10cm, while on the design it is 6 inch (15 cm). That's 1 1/2 times my core. Can this be the problem? It's seems a pain to start again with a new heat riser, but if you guys think this helps I will.

It's best to keep the cross sectional area the same throughout the system. Your riser is less than half of the feed tube in this respect. Not a single part should be smaller than the burn tunnel, so yes, I'd think your stove is not built to the right proportions. The book of Ianto Evans and Leslie Jackson is quite clear about this. Stick to the rules and provide something of a vertical chimney stack, the stove won't run very well without in most circumstances.

And another thing: the burn tunnel and riser should be insulated. A mix of clay and sand isn't insulating at all, it's functioning as mass and as such it's extracting heat. Use vermiculite or perlite, wood ash will work as wel but will compact over time. And last but not least: when the stove is finally working like it should the metal pipes will burn out in no time.

One more test I got from another thread:

I removed the barrel when there were only some hot coals burning, almost dying down.
Instantly the coals lit up and a draft was obviously going through much better. I added some wood and the whole thing was LOTS more rockety.

I tried putting the barrel back on and holding it at different heights. Putting it down decreases the rockety effect.




(I wrote this before your reply Peter. I'm going to look into what you say)

Hi Peter.
Thanks for answering. I'm Dutch too, and unfortunately have a bit of trouble with your wording, so I'm just going to see if I understand you correctly.
I'll write in English so this post can maybe help others or get more feedback.

Peter Berg wrote:Not a single part should be smaller than the burn tunnel

Okay. The burn tunnel is about 13x12 square. That means a heat riser with 6 inches diameter would be bigger and therefore be better.

Peter Berg wrote:Stick to the rules and provide something of a vertical chimney stack

Not sure what a vertical chimney stack is exactly. But my exhaust pipe is going out at ground level, but afterwards going up pretty vertically about 2meters. Isn't that a vertical chimney?

Peter Berg wrote:the burn tunnel and riser should be insulated

Okay. For the new heat riser I'll use vermiculite (or perlite).
You say the burn tunnel needs to be insulated too. Also inside the barrel? For me the concept of insulating the whole burn tunnel is new. We did put extra stones around it, but I guess that's not insulation. It also begs the question if the vermiculite board that we used as a roof on the burn tunnel is allright? Or is it better to put fire bricks as a roof and insulate the whole thing with vermiculite?

Peter Berg wrote:when the stove is finally working like it should the metal pipes will burn out in no time.

I'll have to find a new solution for the heat riser anyway. I have the feeling fire bricks won't fit in the 55gallon barrel. What should I use as the inner core for the heat riser?

Thanks a lot for helping.

Rein Baarsma wrote:The burn tunnel is about 13x12 square. That means a heat riser with 6 inches diameter would be bigger and therefore be better.

Correct. That way, the tunnel is slightly smaller than the riser. Mind you, the feed tube should not be larger than the riser though.

Rein Baarsma wrote:Not sure what a vertical chimney stack is exactly. But my exhaust pipe is going out at ground level, but afterwards going up pretty vertically about 2meters. Isn't that a vertical chimney?

First, a proper chimney will reach above the highest point of the roof. Those 2 meters bare stove pipe outside doesn't count hardly as such. Much better to have the chimney inside the house and insulated as well.

Rein Baarsma wrote:For me the concept of insulating the whole burn tunnel is new. We did put extra stones around it, but I guess that's not insulation. It also begs the question if the vermiculite board that we used as a roof on the burn tunnel is allright? Or is it better to put fire bricks as a roof and insulate the whole thing with vermiculite?

Stones doesn't count as insulation. The idea is to achieve the highest temperature as possible inside where it counts. The vermiculite board is OK as insulation but again, when the stove is rocking properly this will wear out rather quickly. Fire bricks for the tunnel ceiling is OK and insulating the whole thing with garden grade 3 vermiculite will work. May I suggest using a p-channel to protect the first brick?

Rein Baarsma wrote:I have the feeling fire bricks won't fit in the 55gallon barrel. What should I use as the inner core for the heat riser?

I've got the feeling you are picturing the wrong barrel. The diameter of a 55 gallon drum is short of two foot or 58 cm. The smaller drum of, say, 15 gallon or 60 liter is just short of 38 cm diameter. And that would be too small, yes. You could try to use an inner riser pipe of stainless steel and a mild steel outer pipe of 25 to 30 cm to hold the insulation. That would give you a nice side gap, no problem.

Rein Baarsma wrote: There's a lot of residue ash, both in the fire chamber and inside the barrel. When I've burned it a few hours the ash and hot coals start taking up a lot of space and eventually I start getting a lot of smokeback. I think it's not rockety enough.

How can I change my rocket mass heater so it will function better?

I thought the wood feed might be wrong and had several attempts with adding 1-4 bricks to make the wood feed a bit deeper and less long. Doesn't seem to help a lot.
At this point I have the most success with no smokeback when I make the fire deep inside the fire chamber (inside the barrel).
Is the heat riser too small?


thanks,
Rein

Do you have any pictures you can show ? I know with my build, I had the same problem, and I almost tore apart my riser and put in a new barrel, - I measured the diameter of the barrel, and minused the inner pipe, and found that the riser and the barrel fit together ( 6 inch diamter all the way thru the system) so that wasn't the problem - until I read a post on here about the base of the barrel and the manifold being the most difficult part of the stove to get right - the size of the bottom - barrel exit has too be alot bigger than you think... at first, I used 1 brick to hold the barrel up (first pic) after reading the post, I raised the barrel 1 brick , so its 2 bricks high - making the opening bigger for more hot gasess to "rocket" thru . . (second pic.)

Also, as it has turned colder, and the mass is being heated and cooled repeatedly, it is really drying out . . and is rocketing better and better as it dries.

hope this helps. . .

I just re-read your post, when you say "bendable pipe" - you are not using that corrugated stuff are ya ?