Feed enlargement and other RMH optimizations?

I came across Rocket Mass Heaters in a Youtube video about seven years ago and I was immediately hooked. After several years of experimentation and a few prototypes in the middle of my living room (I really have no idea how my wife puts up with me), I eventually built one (with a lot of help from friends, family and the good folks here on Permies). This will be our fourth winter living with an RMH, but, the truth is that I am somewhat underwhelmed with the results and am looking for advice on how to improve. Don't get me wrong, I love my RMH, the kids love it, and the cob couch is our main living room couch. But, there is no denying that there are serious downsides:

1. I have a 6" system and the resulting feed is just too damn small. The wood I have available is often somewhat crooked, and is too thin to split easily and too big to use as-is. I spend way too much time splitting wood with a hand axe to get it to a size that is optimal for my stove.
2. Whether because I used dense firebrick, or because the burn tunnel is too long, or both, my system takes far too long to reach peak temperature. In the fall and spring this is tolerable, but in the dead of winter we usually need an additional source of heat to tide us over.
3. The peak temperature is just not as high as I expected it to be. I've never gotten anywhere near a visible red spot at the top of the barrel. I think the most I ever had it was around 300 degrees Celsius, and that happened only once and after running the heater for over three hours. At best it can keep a 30 sq.m living room warm, but it definitely doesn't reach the whole house (which isn't particularly large - 95 sq.m.)

Last year we just stopped using the RMH in the middle of winter because I was busy at work and I didn't have time to split wood and sit next to the stove maneuvering it through the feed. We are at the beginning of heating season and I need to improve the stove in a way that will require minimum effort and give maximum benefit. Here are the things I was thinking of, and I'd appreciate any additional ideas. Also, if my ideas suck I'd like to hear about it :-)

1. Rebuild the entire combustion area into a batch box style heater. This will supposedly solve all of my issues, but this is highly unlikely to happen right now. Winter is here and it will take me too long to design, gather supplies, and implement.
2. Remove the dense firebrick heat riser and insulation, and replace with ceramic fiber board. The ceramic fiberboard is fairly lightweight from what I saw in the specs, so I'm guessing it's insulative enough? This should actually be a fairly quick fix. Any reason not to do it? This would theoretically make the system burn hotter..
3. Enlarge the feed. I spoke to a local builder who suggested this is possible, although I couldn't find anything on it in the forums... He said I'd just have to make sure to not increase the amount of wood I put in, which is fine with me as long as I have more wiggle room for the same amount of fuel. Is this a good idea? Implementation would be fairly easy - I could just remove the first brick in the burn tunnel and the cob above it (see pic below). This would also have the additional benefit of decreasing burn tunnel length. If I could remove all of the bricks from the feed and just leave the cob, I'd have a very reasonably-sized feed.



I'd appreciate any feedback on my ideas above, and obviously any additional ideas are welcome too.

And now, for some grainy RMH porn:



Thanks guys!

Removing the bricks seems easiest, but it will alter  system size, which is supposed to be a big no-no.
However it's probably the most reversible.
Another approach,  recruit your minions into collecting branch wood.
This can be done with safe simple  tools,  loppers at most, no need for splitting with axes.


A riser upgraded seems likely to make the biggest difference.
The ceramic fiber board is kinda expensive,  and there is the 5 minute riser alternative.

Hi Daniel,  Some of your reasons for looking at improving the function of the J tube were also my reasons for switching over to a batch box. Certainly not a simple switch, especially since you'd be crunched for time but definitely worth looking at for next year in my opinion.
I notice that your barrel is pretty small. Pretty, and small :)
If you had a larger barrel, it would release more immediate heat into the room and would help your place warm up faster. Not a very big job and could be done in a day or so.
As far as enlarging the feed, I don't see what that will improve. Generally speaking, if there is one place where the system size is reduced, it should only be the feed tube, but not enlarged. The reason for this is that you'll be increasing the draft with a smaller opening and helping to prevent smoke-back - very similar to the function of the two bricks you use to adjust the size of the feed opening.
If your using a dense firebrick riser, switching over to a 5 minute riser or even a square cf board riser would help reduce the density stealing your heat as well. Making the whole core out of cf board would be better yet.
Keep us posted with your findings and include pictures...we like them too.  

I definitely agree with the five minute riser upgrade. Making it taller along with a taller barrel should help the speed of heating. What is your chimney like, and how good is your existing draft? If the draft is strong, you might be able to increase the system size slightly, with a larger diameter riser and corresponding feed opening. I have a 7 1/2" J-tube in a masonry/cob bell with a temporary (for 4-5 years now) 6" metal chimney, and it draws fine. The key is having good natural draft. The worst that could happen in my opinion with this modification is that you find the exhaust can't keep up and you slim down the feed again.

Thanks for the ideas!

Enlarging the barrel is, unfortunately, out of the question. Keeping the barrel as small as possible was pretty much the only condition my wife had for this project to go forward.

I do have fairly strong draft most of the time (depends on wind - the windier it is outside the stronger the draft), so I do think I will try to remove a brick or maybe even two. It isn't likely to affect the heat output very much (the burn path will be slightly shorter, so maybe), but it will make it easier to work with firewood that isn't the perfect size. If there is smoke-back I'll still be able to use bricks to constrict the airflow somewhat. Either way, it is easily reversible...

From what I could briefly find online, the five-minute riser is made with ceramic wool blanket? My current riser is insulated with a ceramic blanket, and I hate working with it. I think I might make one out of cf board. It's expensive but it seems like it's not very difficult to work with. Will I still need the ceramic blanket insulation if the riser itself is more insulative?

I'll post updates as soon as the work is complete.

Thanks again.

Hello Daniel.

Look at my barrel.





You might be able to sell that to your wife. A nice oven. Plus, you could brick the sides of the metal container. And even the face. So it looks less dodgy! My whole build is there

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

Daniel Granovsky wrote:From what I could briefly find online, the five-minute riser is made with ceramic wool blanket? My current riser is insulated with a ceramic blanket, and I hate working with it. I think I might make one out of cf board. It's expensive but it seems like it's not very difficult to work with. Will I still need the ceramic blanket insulation if the riser itself is more insulative?

The 5 minute riser is ceramic wool blanket that is encased in a metal tube. See here: Working-Morgan-Superwool-ceramic-blanket
If you go with cf board, then you would not need to wrap it with the blanket.

I have a couple thoughts.  First I'd stop trying to split your wood into smaller pieces with a hand axe.  Instead you should get or make a kindling cracker.  Here is a blog post I did about making mine.  I have a 6" system myself and have found this tool amazing for all the splitting I do, making it quicker and MUCH safer to do.

My system is also inefficient as far as RMH go because I simply didn't have the space to make heat exchange tube runs as long as would be optimal to draw out all the potential heat.  (It's also not as efficient because I went with a pebble style to reduce the weight on my mobile home floor!)  Early on I found I had a strong draft as well, making it super easy to start the fires, but like you my barrel top didn't seem to get as hot as other were reporting.  Part of this was due to me having a larger gap between the top of the heat riser and the top of the barrel.  If I remember correctly mine is around 4 inches.  Anyway, I think another factor was that my draft was so strong the fire/heat moved through too fast.  My exhaust temps were also higher than I'd like.  So I tried a trick that seems to have worked.  I still need to write another blog post about it sometime.  What I did was think, "How can I slow down the draft?"  I would need to make the tube runs longer to increase the drag, or make the tube walls more irregular to impede a smooth flow of air.  What would happen if I put a fire brick, or parts of them into the tubes at the clean out ports?  This would cause obstructions in the flow of the hot air and should slow it down, reducing the draft.  It seemed easy enough to try, and easy enough to remove if it didn't work so I tried it.  It seemed to work for me!  My exhaust temperatures at the chimney going out of the house dropped by roughly 40 degrees F.   The average temperatures at the hottest part of the barrel top also increased by at least 100 degrees F if not more!  It is a bit more challenging to start a fire because the draft isn't as strong, but as I understand it that is always the trade off, easier starting or more efficient.   I'd say if you've got an extra firebrick or brick parts around toss it in a clean out port and see how that changes things on your system.

If you are interested I also did a different blog post about the making of my RMH.

Clever idea David. Good on you.
I suppose you could remove the brick(s) when you first light up, then put them back in when its going good. Assuming your cleanout port is readily accessible and a glove on standby to grab the hot and dirty brick.

I've actually found there is no need to remove the bricks.  I just have to be a bit more careful in arranging the starting paper and twigs.  It still starts easily enough, well, other than the initial cold start for the season.  There I just have to use more paper shoved farther back into the feed tube to get the draft moving.