Advice Needed ASAP

I've started a rocket mass heater in my basement and I'm having a few problems. I'm not sure if i made the system too large or what. I have a bed of crushed stone that is about 5'x10' that I'm trying to heat. The stove is not the typical design with a barrel outside and chimney inside. I welded steel tubing together to make up the stove and buried that in the stone bed up to the top of the fire box. It drafts ok, not as rockety as I'd like, but the stone is wet from being outside for a while. The bed has about 35' of pipe in it and then runs up a chimney. I've gotten the stove up to 300 F. Any idea's about how I might improve my results. I do have pictures but will take me a little while before I can post them. Thanks!

My first thought would be that crushed stone has air pockets between the pieces, air pockets = insulation, therefore the "mass" may not be transferring heat efficiently.
I'm still in learning mode so ignore me if you wish....

I did think of this in my design stage. I used B2 unmodified (Crusher Run) stone. It has a lot of clay, stone chips, and dirt so it packs in very well. As far as the mass, I'm thinking that will work just fine once I get it dry. I'm just having trouble getting it to that point. The stove doesn't have a very strong draft. It will draft, but with some smoke back and the fire doesn't get very hot.

The first pic is the stove in the shop after welding. The others show the stove pipe being installed.

Adam,
This won't work very well in this form, I'd say. The riser is coupled with a downdraft channel I will refer to as the downer if you don't mind. The drive of the rocket is the temperature difference between riser and downer. Since these are coupled back to back it won't reach enough difference. My advise: separate those two and insulate the riser, burn tunnel and feed tube but leave the downer bare. The exhaust ducts through the bench, are these the same or larger as the riser cross sectional area? When they aren't, you have to buy larger ones I'm afraid.

It's possible to produce a rocket heater this way, having the gas stream through a duct of the same size and without a barrel.

I have two things to point out about your overall design--

1) With all of the bends and convolution in your exhaust, the air is having to do an awful lot of work to get out the other end. You could probably dispense with one of those big secondary clean-outs and replace it with a regular elbow...would help a little. You're actually going to catch most of your ash in the first ash-pit since it's a vertical drop.

2) The other thing is that your modified design has gone away from a couple of key principles that are going to give you, not only better draft, but higher temps as well.

First of all, your burn chamber is not insulated. By only containing the point of combustion in steel that is exposed to the surrounding air, you are losing a lot of thermal energy that needs to be retained in order to get the really high temps that promote a clean, high-velocity burn-- BUT, just surrounding your stove in cob or masonry would not solve the issue and give you better draft because there is a larger issue...

You don't really have a heat riser and a heat exchange in your stove. While your design is elegant in its symmetry... by welding two steel channels together, all you are doing is forcing the exhaust through a large radiator before it has to make a long slog through the mass with the fire as the only draft engine. The point to having an insulated firebox and heat riser with an exposed radiant heat exchange is to create, in essence, a difference engine that is powerful enough to maintain a high-temp/high-velocity burn and force the exhaust through a large mass. It is the difference in temperature between the hot gases rising in the insulated chimney and then being cooled rapidly in the exchange barrel that gets the system moving. Having the rise and the drop share the same uninsulated steel wall doesn't give you much difference in temperature between the two and hence, not a very powerful difference engine. Now, if you had just welded up the feed tube, burn chamber and riser tube... encased that assembly in masonry/cob... ensured that the riser tube had at least 2" of insulation (bentonite and perlite mix is good)... and then put a 55 gal barrel on as an exchange... you'd have a much more powerful draft.

To compound the issue though, it looks as if your CSA is the same throughout your stove (8x8?) and then your exhaust is 6" diameter. Not only is your exhaust CSA smaller than your input, but your burn chamber should be a bit smaller than all other CSAs. The riser/exchange create the draft and a choke point at the burn chamber create the higher temps.

Without a complete redesign of the stove end, I don't think removing a clean out will do much good.

Thank you for all the advice. What if I cut firebricks and stack them in the fire pot, burn tunnel and up the heat riser? that way I could cut down the inside dimensions of the stove from 8x8 and then it would be even with the output of the 6x6. I could secure the firebrick to the steel using furnace cement. Any idea?

If you can manage to cement it all together inside the unit, then I think you may have a feasible work-around. If you could line the riser side of the drop channel as well, then you will prevent the steel from conducting the heat from the riser lining and radiating it into drop channel. Overall, it beats a full cuss/chop/weld rebuild . The only drawback to this solution is that you are still relying on the heavy gauge 8x8 tube stock to be your radiant transfer, which is much less efficient than the thinner steel and larger surface area of the drums you see used in so many other designs-- but I think your lining idea might give you an adequate enough performance boost to merit implementing it and see what happens.

well, that gives me hope. The whole top of the stove is removable so I can clean it if need be. I should just have to pull the top off, line with firebrick, and re-bolt the top on. I was also thinking about firebrick on the outside of the heat riser as well as the brick on the inside. Not sure if that would be pointless or if the idea has merit. Either way, I'm going to try and fix the inside with the brick tomorrow. I will let you know how I make out.

Thanks again!

I've been able to make some improvements on the draft. I lined the inside front part or heat riser, burn tunnel, and fire pot with firebrick and for good measure stacked firebrick on three sides of the heat riser on the outside. The draft is working very well but I'm still not getting good temp on the stove or the far end of the stove pipe. I may have made the mass to big or this may be that the mass is wet. I'm thinking about taking the rock out and switching to sand. any thoughts?

How long did you burn?
How much did you burn?
What did you burn?
How was the fuel prepped? (split/whole, long/short, etc.)

I think that, overall, you have to keep in mind that you are charging up quite a large mass. If you suspect that your mass still contains moisture, you may just have to burn longer and keep drying it out. Then, you might have to experiment a bit and see how long it takes to charge up your mass once it's 'cured'. You will still have to go through the 'curing' process if you took the effort of switching the rock out for sand, but on the other hand-- sand provides a more even dissipation of heat (both in and out), plus, you would probably have a few hundred pounds less mass to charge.

as long as your riser is sharing a surface with the downer, this will not draft the way you want, or should. My original steel core version of RMH would glow red. Thats about 1600 degrees. 300F is like bathwater. The feed box, the combustion area and the riser all need to be insulated. There needs to be a surface after the riser to dump heat, cooling the gases and continuing to power the drive through the downer. If you can not insulate the riser on 4 sides - because the 4th side is still shared with the downer - this will never be very efficient.

firebricks are used because they deal with the thermal stress without breaking. Lining the burn tunnel with a brick or half brick does not count as insulating. Not to mention it changes the cross sectional area. Which you dont want to do. Bury the feed, burn tunnel and riser in like 4 inches of vermiculite and clay slip. THAT is insulation.

Yes, I'm seeing this was a very big mistake and I've listened to the advice of some people in the design stage that I should not have listened to. Not sure what to do with this thing now, other than a boat anchor. I really should have followed the traditional model. I have a lot of time and money invested in this thing, so if anyone has any more ideas how to make it work I'm all ears.

Your best bet is to cut off your version of the downcomer and take what's left insulate it and put a big barrel over it, and connect it into your mass pipe and see if it works...

Your issues are why Ernie doesn't like to comment on threads like these...

Adam : I sent you a P.M. suggesting that you separate the pipes and you will have the start to two rocket stoves, don't give up, try something new and keep us posted here ! Allen L.

I had a thought so please tell me if it's stupid or worth trying. The stove I have has a removable top with a "rope gasket." If I take the top off, insulate the stove, and put a barrel over it? Just to clarify I would still leave the section of downer intact just remove the top. Do you think the barrel would cool down the gasses for the draft and still vent out the existing stove?

YES ! give it a try !, I would fill the 'downer w vermiculite or what ever, and leave any insulation you have around your heat riser! Best spacing between top of heat riser and the underside of your barrel should be in the 1 1/2'', 2'', but there are a few cases reported with the gap measured at 3'' , go ahead and give it a try, I expect the side with the 'downer' to be the coolest side.- what do you have to measure temps ? let us all know how you made out, who knows something good could come out of this ! Pyro-maticly yours Allen L.