6" vs 8" rocket mass heater

Hey guys, just discovered rocket mass heaters and was wondering what the difference was between a 6 inch and 8 inch heater? Is it just a matter of size of wood that can be burned, or does it effect the amount of area that can be heated. Think this demands some more looking into.......... thanks from a newbie!

A 8" system can have twice the gas flow a 6" system can. So an 8" system can eat wood twice as fast as an 6" and put out about twice the BTUs. In the same home with the same mass you would have to light a fire and burn roughly twice as long with an 6" system.

A 8" in a small house would be burned maybe not often enough, so the mass gets too cold and you get condensation. A 6" is a too big house would have to be run too often and could be a pain in the ass. In some medium sized/insulated houses either system could work.

Lots of free 6" piping to be found, used 8' piping is harder to come by.

I almost wish I used a 6" system since I could have gotten more mass in my couch (limited space) that way.

Great info Daniel. Before reading this I would have thought the difference was only slightly marginal, e.g 25% greater.

What about difficulty of use? I am just putting my prototype together for an 8" system now, and the feed tube looks quite large (7" x 7.5") to me. When I did my first burn, as a noob, the thought crossed my mind that a smaller feed tube would be easier to manage and get started. i.e. after you got the initial burn going with some newspaper, you would need a smaller batch of kindling. Is there any truth to this?

Rob; Lighting a rmh that has been run every day, is the easiest thing you will ever do ! Nobody mentioned that you could literally restart with a match and some kindling ! No kidding , the residual heat makes restart a snap the horizontal draw pulls the fire right away. I did have some start up draw issues when my cob mass was wet, it would start & run a few minutes ,then want to back up the feed tube. I used a small electric fan to blow down the feed tube and within 20 minutes the first day and 10 the second day by day three my mass had dried enough to not need and extra push to get started. Now that it has been running for months i don't know how i went without one !!!

This is great to hear Thomas. That makes sense come to think of it that the warmed up RMH is going to continue with the draft. So this means even that initial burning of the kindling (on a warmed up RMH) is going to suck most of the smoke out the chimney?

This says that a 8 incher J tube can output about 14kw per hour

http://donkey32.proboards.com/thread/965/approximet-output-20cm-tube-batch

Satamax Antone wrote:This says that a 8 incher J tube can output about 14kw per hour

In that post is it saying that there was almost twice as much wood burnt in the hour? and twice as much Kw/h in the 6" compared to 8"? Isn't that backwards? Perhaps I misunderstand, but wouldn't that imply (in his system) he needs to run his 8" for twice as long as the 6" to produce the same energy release?

Rob Irish wrote:

Satamax Antone wrote:This says that a 8 incher J tube can output about 14kw per hour

In that post is it saying that there was almost twice as much wood burnt in the hour? and twice as much Kw/h in the 6" compared to 8"? Isn't that backwards? Perhaps I misunderstand, but wouldn't that imply (in his system) he needs to run his 8" for twice as long as the 6" to produce the same energy release?

Yep, you read across the post.

He's comparing a 6 incher batch rocket with a 8 incher J tube.

Oh I missed that important word 'batch'.

I haven't heard of it until now. Initial impression is that a j tube seems slightly self loading as the wood burns and falls downward, but a batch version burns more faster, however it is loaded horizontally and pushed in as it burns i.e. requires more attention. You might also get a bit of a visual fireplace from a batch rocket.

Have I understood correctly?

Is it just a matter of size of wood that can be burned, or does it effect the amount of area that can be heated.

In an 8" j-tube, you can burn larger pieces of cordwood. In addition, it can create a lot more heat by burning quite a bit more wood at one time. The idea with rocket heaters is to keep the feed tube full of wood all the time so that the burn is the most efficient. Consequently, if the feed tube is so much bigger, it is going to hold more wood.

but a batch version burns more faster, however it is loaded horizontally and pushed in as it burns i.e. requires more attention. You might also get a bit of a visual fireplace from a batch rocket.

The batch box requires a door. It is loaded and then the door is shut until all the wood is consumed. The fire starts at the back and then works its way forward toward the door. Casting the batch box correctly is tricky and the whole unit is more finicky than a well-designed j-tube such as a Dragon Heater. The same guy, Peter van den Berg, designed both.

Rob Irish wrote:Oh I missed that important word 'batch'.

I haven't heard of it until now. Initial impression is that a j tube seems slightly self loading as the wood burns and falls downward, but a batch version burns more faster, however it is loaded horizontally and pushed in as it burns i.e. requires more attention. You might also get a bit of a visual fireplace from a batch rocket.

Have I understood correctly?

Well, to me, a batch rocket requires less attention, because the load is far bigger than a J tube. And fuel falls down to the shape at the bottom, becoming embers.

Cindy Mathieu wrote:

but a batch version burns more faster, however it is loaded horizontally and pushed in as it burns i.e. requires more attention. You might also get a bit of a visual fireplace from a batch rocket.

The batch box requires a door. It is loaded and then the door is shut until all the wood is consumed. The fire starts at the back and then works its way forward toward the door. Casting the batch box correctly is tricky and the whole unit is more finicky than a well-designed j-tube such as a Dragon Heater. The same guy, Peter van den Berg, designed both.

Well, Cindy. A lot of people use a door, but that doesn't mean you necessarily need one.

Satamax Antone wrote:Well, Cindy. A lot of people use a door, but that doesn't mean you necessarily need one.

Max,
Allow me to refine that statement.
The batch rocket is able to run without a door, yes. The excess air will remain comparetively high however, so a lot of air which isn't contributing to the combustion is heated up and sent out the chimney. Also, there is a fair chance of smokeback now and then, the better the chimney stack the less smokeback.

Using a door on the other hand, the excess air can be minimized to about a quarter of the riser cross section area while a well-built stove will remain burning clean. The net effect will be lower excess air, a hotter afterburner, much higher combustion efficiency and in spite of that last fact, higher exit temperatures. The stack effect of the chimney will be stronger because of that, not bad for a stove which relies on high gas velocity. In my opinion it's much more desirable to loose, say, 1000 cu ft of air at 300 F per hour as compared to a tenfold of that volume per hour at a temperature of 175 F.

The goal has been a combustion system where the shape inside the stove induced turbulence, lots of it without imposing much drag. Minimizing the excess air at the same time as far as a condensating boiler burning natural gas will go, something like 4% oxygen in the exhaust gases.

Door or no door, both methods have their merits, the sound of this stove when burned open is impressive, to say the least.