My rmh mod - high heat safety questions

I have a rmh i have been using for a year or so. It seems to kick out quite a bit of heat, but where I am at when it gets really cold the heater struggles to keep up just a bit. Basically I need to bolster the heat efficiency for the house a bit so that I get more instant heat into the air to counter the fact that I have very weak insulation. Currently I just have a masonry riser chamber that is insulated very well. It goes straight into the rest of the system from there.. Which is two clay flue tile bells that capture. So the system takes a bit to really start kicking out heat i have noticed. Basically about an hour or so from a cold start. Which seems good becuase it means heat storage is happening. The thing i notice is that after a while i have taken readings that my exhaust is about 400° F (usually a little less) after a six hour burn. So i take that as the point where max heat holding capacity for the whole system is being reached. As i find it doesn't seem to be grabbing the heat from the burn fast enough before it dissapates outward. Otherwise i would think the temp of the bell would rise continually. Basically concluding that the mass indeed is holding and relessing too fast as its storage capicity is getting close to maxing out. So obviously I do plan to add more mass eventually (like a bunch of bricks, or other materials on top the "big" bells, and possibly at some point face the bells with some rock or tile of sorts too). But for now my plan was to get more heat out now and store lots more later. So i plan to slap a metal drum of sorts top the heat riser area. That is currently capped with a refractory cement brick that I casted for your information. The top of that cap is only a little over 17 inches in diameter squared so a 55 gallon drum and even 30gal one often used are too big for my configuration. Plus I want to reserve more energy to the mass at the end of the day. So a smaller solution came to mind.

My solution has become going to the junk yard and finding an old tank to use. Forgive me for not knowing much about tanks, but this one seems to be about 30 gallon or less? I have a pic attached that represents a similar tank. So what I did was basically where the valve would go(removed) I measured from there down about 2.5' or so, drew a line, and cut off the bottom of the tank at that point. So what I'm left with was the two and a half foot barrel rounded at the top with a little tiny hole in it(I plan to plug with fiberglass insulation), and completely open at the bottom.

At this point I plan to cut a hole in the refractory cap attop the riser and slap that piece on there. Oh and I should mention that I plan to burn it out heavy strip all the paint off the outside beforehand that's left(not much). So to my question...


Is this going to be safe at high temps? Has anyone ever used this particular type of peace to construct a metal "bell" for a rmh? Any chance of this particular steel type off gassing something poisonous or strange? I'm not sure on the exact thickness, but I believe it will hold up for at least a season or two. Usually as what temps I have gathered I don't expect this metal to get above 900 degrees often or ever. But the possibility is definitely there for up to 1200 I'm sure if i guide the burn meticulously. But the plan would be to never have to run this thing more than 4-8 hours a day on the super cold days. Any thoughts, ideas, or concerns please. I have a family in the house so I plan to at least put my feelers out there befor I make a final decision. Anything would be appreciated. Also btw the dia. of the barrel peice is about 14".

Also here is my heater below. The lower leftmost "bell" is where i intend to put the tank piece. Thoughts?

No thoughts from anybody on this? I basically would like to know at what temp plain ol steel off gasses. Anybody knows where to get that information please send ity way. Many thanks in advanced.

"Plain old steel" is not going to offgas at all until maybe when it melts. You don't want the tank to get significantly past 1000 degrees F or it will begin to soften... but the setup you have is not at all likely to do that. The tank added to the top of the first bell will radiate enough heat that it will stay safe, and start to make you comfy immediately.

Thanks a ton for the feedback. I actually just ran accross some stuff about this, and simlars facts were noted. I really do appreciate the confirmation. I really dont think it will go far above 600-800° F on average. Its what i was thinking anyway, but this things seems to be able to burn quite hot at times so ibwill see for sure after test fire. I actally plan to start the mod tonight. I will try to rmember to add a photo, and some details on temps soon after completion. Thanks again!

So I "finished" the mod by adding the metal bell and a new cleanout door that was much needed. One more cleanout door to go i think too will be needed. Also i use the quotes because i will eventually face this whole thing with rock or tiles of sorts, add a few hundred pound of mass to the caps of the bells, and the whole wall behind this thing is coming out too to expose the room behind it better. The facing would add a nice chunk of mass and to drastically improve the appearance. Although i specifically think it looks pretty darn cool. I will mention that my wife also would greatly appreciate a cosmetic overhaul, lol I must say though she has been trooper since the beginning when i started this project by cutting a huge hole in the kitchen floor over a year ago, heh heh.

Now to the brass tax... It seems i achieve about 500° from the middle of the barrel after about a 2+ hour burn which is grand. Pretty much exactly what i was looking for. I get this old, drafty, and poorly insulated house of 1150 sq. ft. from 60° F or less to 70° F in a matter of an hour or so when its snowing, 26° F, windy, and starting this thing dead cold. Also the metal bell seems to peak out its heat holding capacity and radiation ability at this point. Seems the brick starts to absorb the heat well at this point. It was my worry that the unit would pour out too much heat immediately and not store it like i would rather it do overall. Turns out my fear was not a reality. There is a nice balance happening now.

It used to take under similar circumstances about 5 hours to make than kind of jump in temp in the house. Not saying it didn't already heat the home well, but it seems to be much more efficient at it now. It also seems to take less wood over time now that heat levels have increased. Basically less feeding times due to comfy temps being met sooner.

The brick only holds heat for about 12 hours at most even on the really cold days. So it usually requires to be run bare minimum once a day for almost 3ish hours to keep the house above 60°(usually more like 68° F as of lately), and two spaced out runs of similar length for the 70°+ temps if desired. Usually before the mod it seemed to need to be run about 8 hours minimum to keep the house warm all day and night, but now it seems a 4 hour burn does as much and more. I think at this point adding more mass is where its at for maxing out the total heat efficiency of the beast. The 8 hour burn will be needed still I'm sure, but now can be reserved for the days of the month and half every winter we often stay in the negatives(about -15°-20° F easy and colder). Also i will remark that last year running this thing all day(12 hours) on the coldest days would leave the house at around 70° max and not all night after( by moring it would dip below 60° at extremely cold nights(-30° F with wind chill making it effectively -60° F!). I'm hoping to have better results in a third of the wood burning time. Extra mass at this point seems to be my next move for sure max this baby out. What do you all think? Check out my photo!

you may get a bit more out of your system with more insulation. looks like you have some kind of unique custom RMH/batch box going on. i like it. you may want to check out my RMH for some ideas if you like First Rocket stove heats water with pics i have a new concept idea on heat risers and RMH cores.

So i really dont think there is much need for insulation, but do correct me if i am wrong. I have a full 1.25 inch casted core and riser. Fully surrounded by perlite and ceramic insulation to seal the perlite in place. The base of the unit outside the feed chamber barely reaches 80 degess F after a really long burn. I appreciate your approval, but i really think I need more mass. Not that more insulation would hurt at all, but I do feel I have achieved optimal burn temps with my current rig. Also not really a batch box going on... This is a J-tube design somewhat designed from Peter Burg's notes. You may want to check out me photos closer. The cleanout doors may have thrown you off. I will throw a few pics down to clear this up for you. They are from the early days before ever burning this thing.

With that kind of insulation I would fully agree that more mass would be the thing to work for. Your first bell looks large enough in cross section that it might work to stack bricks inside lining the walls, possibly in an open pattern so the gases can contact the flue tiles as well as the bricks. Dry stacking will allow you to experiment with different quantities and arrangements. If you have space outside the bells, adding thickness there would of course be good.

The bulk of my mass is for sure in the first bell, and a good internal surface area/airspace to boot. Acting well as the first holding chamber to capture and store heat. Also if you look, you can see there is an air gap between the inside and outside flue tiles so that if expansion of the flue tile does occur due to heat internally.. there is space to do so. I don't think i really want to sacrifice any internal surface area because i feel it is my best method of heat extraction from the gasses other than my recently added metal bell attachment. I do think the main idea is to slap on the sides and top somehow with some stones, brick, cob, plaster, tiles, or whatever i decide in the end or have easily and cheaply available. Thanks for bringing up the point of dry stacking. I really like the idea of dry stacking... and was planning on basically doing so on top the two bells with bricks and stones at least to start originally in my thoughts. Possibly filling it in with cob or some sort of plaster/mortar mix or blend eventually for sure. But this unit is my first prototype and i plan to move in a couple years, so ultra permanent is not my main goal at the moment. Basically i plan take it down and move it to my next home. So maybe a light mortar job, or carefully sealed joints at few key places, but just enough to hold things together. It seems i'd like to get a tile or brick array to stick to the sides of the bells, and also maintain the ability to be able to be moved easily with a light tap with a hammer and taken apart somewhat gracefully. I maybe could wire everything in, and cutting the wire and harness would be the method of disassembly. A few ideas i have had also anyway... Others are welcome

Talk of mass brings me to questions for anyone with the knowledge.. Any thoughts on either concrete, clay, or other type of brick that would work best for heat storage? In theory mass is mass i know, but there maybe something more idealic. So that is why i was wondering if anyone would have a good reference for best materials to be used for heat storage. Keeping in mind durability, safety, cheap, and of course the ability to be taken apart.

Matthew, if piled on the outside, anything will do prety much. Mind you, having done the mistake of leaving an air gap between the bell's skin and the mass; lately. I would advocate sticking the mass to your flue elements with clay. The air gap delays the heating of the mass big time.

One thing i've thought about a lot, for mass, is pavers. They usualy come cheap here. When they've goten all mossy and uggly.

I also advocate to use the mass around the first bell, where it's hotter. And make the last in line to be a radiator. That works far better imho. You have a lower temp radiator ( usualy the barrel) So you have to run longer hours to heat up your place. But don't fear overheating nor burning flesh. And by burning longer than you would do usualy, you charge teh mass more, and don't have the temperature dip occuring between the fast heat from the radiator, and slow heat from the mass.

I wonder if it wouldn't be intresting to fille the gap between the two flue ellements with sand. I haven't started experimenting with sand yet, as a mass. I know it doesn't work, as a bench mass; But talking with Pinhead, it still can be usefull.

Any thoughts on either concrete, clay, or other type of brick that would work best for heat storage?

Yes. For your purposes, they're equal. There are different characteristics you can measure with respect to how useful a mass is for storing heat.

Concrete, clay/cob, fired brick, they're fairly close. Water is much better, but has several dealbreaker downsides (corrosive, doesn't stay where you put it, expands violently at 212f).

Soapstone, should you be blessed with access to some, is also vastly better. It's like God invented soapstone just for the purpose of masonry heaters. I'll see if I can find the chart showing the relative numbers for the materials. It's startling how much better soapstone is.

In any case, no, don't worry. An inert, solid mass is what you need. One kind is good as another.

Noted on the air gap concerns. I can see why... As with heatsink mechanics theory it makes sense to have a cuductive layer to attach the mass. So that the mass can draw enough to begin with. As far as filling in the gap between the flue tiles in the "first" big bell with sand I would be weary. Although i have had this exact thought already. So for two reasons.. sand is not the greatest storage device. Seems on its own it makes a crappy insulator at best, but can work im sure. Usualy seems the idea is to mix with mortar/clay. The second concern i have would be that little air gaps purpose is to allow a tad bit of expansion room interally for the clay flue since that bell is the hottest at the center. It is in fact closest the opening of the heat riser exit point. Not sure if there was a good material that would be a little forgiving that could be crammed in there or not. Other concepts welcome.

Also thanks for the materials advice. I will see if i can come across the prized soapstone if possible for cheap. Otherwise concrete or clay brick seems very accessible and relatively cheap. Any other ideas or recommendations still welcome.

I wanted to add it was 18° F outside and i ran a 6 hours burn. My house was 79° F and stayed above 60° F through out the whole next day. Didnt start another burn till 8pm the next day. So i.meight be getting a bit better sorage than before mentioned. The thing is seemingly running better than ever as far as i can tell. Thanks to many for that on permies. Spent a good amount of time talking on the forum last year. Been a great time thus far. some great experience here folks.

Ok, here's the material information I promised:



This is the chart from Ken Matesz's book Masonry Heaters: Designing, Building, and Living With a Piece of the Sun, which I heartily recommend. (That's an Amazon affiliate link.)

In columns 2, 3, 4, 5, and 6, the higher the number, the better. See how soapstone just excels? It's nuts.

Mike Cantrell : here is some more related information some just restating the information another way which often helps understanding !

Actually I started this but some of the clarity got lost in a kerfluffle so the definitive part starts with Erica Wisners Dec 20th 2013 post!

see link below :

https://permies.com/t/30551/rocket-stoves/Fake-fire-brick

For the good of the craft ! Big AL