Rocket Mass Heater Bells and Benches Discussion

This is a discussion of Bench and Bell construction and concepts for rocket mass heaters. Thanks to all of you for the great comments and questions.
Stove in video is my Tiny Masonry Cook Stove. This wood fired stove features a stove top, oven, water heating, and space heating all in a small footprint.
Easy to follow Rocket Stove and Cook Stove plans available here:
walkerstoves.com/
My Clay/Sand Mortar Mix Recipe: 3 parts dry sharp sand by volume, 1 part dry ground fire clay by volume. Add water to desired consistency.
Fire clay: amzn.to/2ykT2B0
Sharp sand: amzn.to/2LJKqa9
Commonly used parts: amzn.to/2XlBvko
Thanks for watching!

[@lemonbalm2604]

my new brain conditioner: "Don't be scared of masonry. Don't be scared of brick. It's really easy. . "

[@jaggervalentino3473]

I know Im asking randomly but does any of you know a trick to get back into an Instagram account?? I somehow forgot the account password. I appreciate any assistance you can offer me

[@justiceadriel8585]

@Jagger Valentino instablaster ;)

[@jaggervalentino3473]

@Justice Adriel thanks so much for your reply. I got to the site thru google and Im waiting for the hacking stuff atm. Takes a while so I will reply here later with my results.

[@jaggervalentino3473]

@Justice Adriel it worked and I finally got access to my account again. Im so happy! Thank you so much, you saved my ass !

[@justiceadriel8585]

@Jagger Valentino no problem :)

[@kevindouglas8768]

The one compound on earth that has the highest Specific Heat is WATER. This means water takes the most amount of energy to heat up, but also holds it the longest as it gives it back. ..making Water the PERFECT heat battery. . . .just don't let it drop below and freeze, or boil to build a head of steam. Saving you the study time as I hold a degree in Water Sciences. :-) Hope this helps

[@justinsane7128]

Only surpassed by candle wax.

[@kevindouglas8768]

@@justinsane7128 Especially if it's poured right on you. :)

[@scroogemcduck1462]

@@justinsane7128 Seen some people simply put a pot full of wax on top of their woodstove

[@B30pt87]

Ok, now I have to go find out if it's water or wax. Damn you, Internet!

[@mottthehoople693]

molten salt?

[@maukamarine939]

Thats great info! I had been thinking about building these in my octagon guest cabins where ea stove will sit next to a 22 degree bend inward wall using pipe. Now I know I have options without pipe. Exciting...Cant wait to get started. Will prob gather materials now and build in Feb.

[@MrLesiuxxx]

As You said right - You are not picking on flue pipe system - our ancestors in America Russia or elswere in the World they xid not have access to steel or metal pipes, but what they had was natural wisdom when they build theirs clay of simple bricks stoves channels and warm bell systems beds and benches. Thank You for reminding us this old many times forgotten wisdom, to those who wants to build their very own efficient rocket mass heater.

[@beebob1279]

Great statement

[@reuse_or_die]

Thanks this is great. I have a tiny cabin & building my first j tube RMH so will now go ahead with a bell bench rather than a tube one. I will share results asap x

[@kennetherickson2391]

Been looking at kang stove combos for a year or so (luckily I read & write a little Chinese). this small model fits the bill for a stove plus can run to a kang bench. brilliant. worried, at first, a little about the ceramic top, until I thought about it. They are designed for temp variation across the surface (my high-fire stoneware background had me worried but this is a different breed of cat). Brilliant use of light refractory material. I think having built a couple small wood kilns will make the build a breeze. Can't wait to start on this project this spring, man!

[@vsergeiev]

It's amazing indeed, I would say a game changer🏆🌠. Both tech peculiarities and design details of all your design esthetics look awesome in the interior. Two thumps up!!! I'd never seriously considered the classic rocket stock types for my house project for I am not quite into these tubes thing, but, after watching your videos about the stratification bell type, its concept blew my mind. 🌟🎉

[@philipsmart1453]

Another great video. Thanks for sharing.

[@salvothegermanshepherd4399]

Really appreciate your videos, thanks for sharing

[@jernejzore5604]

When I started watching, I was like...boring.. but your simple explain is great!!! I had to hit like button.. because it was very helpful

[@cocospops9351]

Very nice. I want to make basically this and was planning to have a "flue run" of sorts but really just a brick pathway for the gases to travel along, maybe with two levels. I'll ultimately be exhausting into an 8" inch masonry chimney located in centre of my house. It's got a great draught so I think I'll get away with fudging some of the calculations for these

[@reneebrown5598]

I'm with you on the fact that the only place wood has in a heating system is in the burn chamber.

[@Thomas-wn7cl]

Great explaination, thanks.

[@n-9249]

Beautiful design

[@tinbender998]

really awsome a heater bell, very cool i mean Hot , thanks steve

[@iisunny7218]

Love the bell

[@CandiceWA]

:D you looking so good in here! You remind me of my Holmberg boss Jeff White ! Thank you as always!

[@psargaco]

That was cool, thanks.

[@Audio_Book_Nut]

Great idea Matt. It seems so obvious yet no-one else on KZbin has described it. I know the principle is very old, but you have brought the idea into the modern age. I think I will built one later this year when my tiny house is delivering, putting the bench under the house, and enclosing the air space under the house so that the warm air from the bench is contained in another “bell” under the tiny house floor.

[@broaudio]

Thank you Chris. I love the idea of an underfloor bell, someday I too would like to build a system with that feature. Check out Korean Ondol design for some inspiration.

[@felicianocapicia]

Chris, could you explain this a bit more? Are you envisioning the rocket part in your tiny house, and then have the exhaust angling down through the floor to a bench underneath the tiny house? are you then going to have another bell in addition to the bench, all underneath the tiny house? since the bench is underneath the house, why call it a bench rather than bell #1? will you be able to go underneath and sit on it? what will you use to enclose the space? i'm fascinated by your idea. Thank you.

[@Audio_Book_Nut]

Hi Felecia, I hope I can answer your questions. Firstly the tiny house I an talking about is one on wheels, and is a type of caravan. It is not a small conventionally built house. The rocket stove would be structurally separate to the tiny house. The bell part of the heater (ie the part where combustion occurs) would be beside the house. The bench (where the hot exhaust gases heat thermal mass and heat is stored) would be under the house. It will need to be fairly low so that the axles of the tiny house have clearance. The flu would also be beside the house. It will designed so that I can build the rocket mass heater in situ and then move the tiny house over it. I would then enclosed the entire space under the tiny house perimeter with some sort of insulating board so that the warm air is not blown away. The space under the tiny house is very restricted. There is about 15" ground clearance ie the distance from the ground to the axles, so most of the bench will be below ground level and will have a maximum height of 12"above ground level. The bench could be built so that the bench extends outside of the tiny house perimeter, but then this part of the bench would loose most of the heat as it is the part exposed to the coldest air and to breezes. I can understand that having a warm bench to sit on on those chilly nights would be most desirable. there would be many nights when it is chilly outside but inside the tiny house it is nice and cozy even without any heating or minimal heating and the under-floor rocket stove would be over the top. I think that a second rocket mas heater for seating would probably be the way to go. Does this answer your questions? I hope so. I am no expert in this area, and this will be a first for me!

[@felicianocapicia]

Chris, thanks for your reply! i had some difficulty understanding your plans because of how you used the following terms: bell, bench and flue. As I understand it, no combustion takes place in a bell. I understand why you'd put the wood feed and core to the side - so you can access it, right? It sounds like you might be planning a 'double' system with a bell and a flue bench. Why not do one or two bells? are you still perhaps thinking of using the bench for warm sitting instead of building a second RMH? :) Why not put both bells under the tiny house? perhaps there is not room?

[@bretsprecher466]

on the bench,would it give you better heat distribution if at the far end you had an additional raised area so the hottest gases would then travel the length of the bench to get to the highest area,then work its way down ,seems like that would force better distribution and add additional mass for heat storage

[@lauray9793]

What wall thickness in the bell is most efficient in releasing latent heat in 24 hours? Would this thermal bell (taller version) work with a standard wood stove?

[@travelwithguyfromhimachal4706]

You are great sir

[@tyfitzpatrick3606]

Thanks for sharing the Bell design idea! Your stove is beautiful! Has anyone tried building a RocketMass heater with a firebrick top instead of a barrel? From what I have heard you need to replace the barrel every couple years.?.. I get that the metal surface is nice to cook on, but seems like if it was all masonry it could last a lifetime ...

[@gregtabor1342]

Matt, you mention that the flue temps are less with the bell bench versus the ducted cob style benches in a rocket mass heater. What are the flue temps you're experiencing in the bell system versus the RMH in your home? Second question, one of the advantages to the RMH Is the slow and extended release of heat that has been stored in the mass. Since you're only heating air and the primary heat release is through the surface of the bell, how does the bell design compare with the ducted cobb from a heat release perspective? Does it release more heat during buring and drop off faster once the fire goes out or what? Thanks for your insight.

[@broaudio]

Hi Greg, good questions! The flue temps are comparable in normal use, but the flue runs tend to climb more quickly and higher in longer burns. Average for both is around 200°F. High so far in the tiny stove is about 250°F, but I've pushed the flue run stove up to almost 350° on long burns. The two methods are roughly equivalent in terms of storage and release. Keep in mind that a cob bench with flue pipe is really mostly hollow as well, with lower density materials than the brick/stone of the bell used to surround the flues. My assumption is that my brick/stone bell has as much mass/thermal storage, if not more, than an equal sized cob/flue bench. Also, stone and brick are better thermal storage than cob. Due to these factors, the bells have just as long of a heat cycle, and no significant difference in performance. The thing about the cooler exhaust gasses is due to the fact that as the bell "fills up" with gasses, the warmer gasses simply become "deeper", heating more of the sides. The flue runs just get hotter and hotter on top. In this way I think the bells can store more in a similar volume, as they use their depth in a progressive manner, and more effectively. I think! Hope that helps, thanks for the great questions!

[@kandydilley8225]

with a bell system how do you deal with potential leakage of flue gas .

[@GTNBY2]

It's incased in media..

[@MiguelEstornino]

@@onebylandtwoifbysearunifby5475 well 1) you will do your masonry realy good, 2) there are CO detectors which you will use with this system. Smoke will be realy visible when they will have any crack to go through before any CO have any chance to enter room.

[@DRJMF1]

need to heat 40 gallon hot water tank (180deg F) efficiently with little wood fuel used. How ? eg would a 10 inch rocket stove with ceramic fibre board (and/or polished stainless 310 High heat resistance lining) make air flow better insider chamber and help to create a super hot flame (bunsen burner effect). That super hot flame would then be in close proximity (eg 1 inch) to under surface of a stainless steel water tank. Do you think enough BTUs to heat the water to 180deg F, or maybe use two or 4 rocket stoves together heating the same water tank which would be insulated ? Thanks

[@BrianB1963]

What about the draw comparison? does the flue through the bench seat draw stronger than the Bell design? And also if I use the flue to connect two Bell's together won't one bench seat be warmer than the other?

[@jamesevans1973]

water heat up in part ;;;;exust it100 yrds into green house floor man you are a great teacher it opened my mind GOD BLESS i gott 1000s feet gal pipe wow

[@peterburris4665]

James Evans not sure if your aware, but use caution with galvanized pipe with Temps over 500 deg F. you can get toxic gases from the galvanized coating.

[@jernejzore5604]

One question... Would break glass be good for terminal storage?

[@sweenie58]

If you run the gas out the bottom of the bench beside the hot barrel it will heat up the air inside the exhaust pipe making it draw better and you have less chance of downdrafts. Water can store more heat than almost anything.

[@billlyell8322]

Incorrect, water temperature tops out just over 200 degrees, over that temperature it starts flashing to steam. However, contrary to what he said, a sand thermal battery top out between 1000 to 2000 degrees. That's 5-10 times the ability of water. But both a water and sand battery MUST be much larger than a little bench will allow. It's more like the size of a medium to large above ground pool. Smaller, and they just don't store the heat long enough to justify the cost. Additionally, you're going to go through a lot of fuel to reach these much higher temperatures. You can do that with solar thermal cells, but unless you're going to run a blast furnace, you'll have a hard time doing it with wood. Plus, BOTH water and sand batteries require a great deal of insulation from the environment to stay warm. This is true even if they bury them. I do agree with a small storage option like a bench that either cob or stone is a better storage medium. The bell system is better if you're just looking to have a large radiation surface. But having an empty cavity will obviously sacrifice thermal storage it can have.

[@X02switchblades]

how can I rigidize a ceramic blanket for the riser?

[@ColumbcilleDougherty]

so this seems like the principals of Masonry heater, but without the reintroduction of heat to the gasses... what would happen if we used a few smaller bells to burn up the gasses before allowing it to the bench? would this be more efficient and cleaner?

[@bbruuse]

Beautiful, very nice, compared to having the 45 gal drum look. The "ONLY" concerns I have is how to regulate the amount of heat without literally cooking yourself out. I know you can always open windows, etc. to allow cold air into your space to cool the room out, but I live in a climate which the weather changes literally in a few hrs. I home built a outdoor fresh air wood furnace, with thermostatic controlled fan to suck warm air into the building I was heating, which by far was my favorite set-up , until -10 weather when it became impossible to heat. ( Trying to keep the place warm took truck loads of firewood requiring constant refilling ) Next yr. I am planning a beautiful system, like your's. The cushions I am planning to use to soften the top of the mass storage, will be made from " Old School Bus Seats pre 1960's" these are made with a "Memory Type Foam" salvaged from school bus seats, which cost next to nothing and I have yet to find anything as comfortable, ( Except Memory Foam ) However this foam is extremely "TOXIC" if it catches fire, so extreme care "MUST" be taken. to guard against combustion, Thanks for sharing your video, I was searching all video's looking for design I found pleasing. Which your's I consider, the most appealing.

[@kyleburdick8771]

I think I would try to avoid plastic-type cushions on the hot stone surfaces, they'd be more likely to catch fire as you've mentioned, and they might get stinky for a while before they caught fire. I would tend more towards something made of wool, it's less likely to catch fire, and when it gets warm it doesn't stink like burning plastic.

[@stacking4retirement222]

Would a reclaimed granite countertop be an acceptable top to the bench?

[@beebob1279]

If you have your bricks on the wood floor, how is it supported?

[@adamkremer1131]

beautiful stove!!! If you were to build this again what would you change? I was thinking about doing a similar build except with a longer, narrow bench with the exit on the end of the bench opposite of the stove. Same sq. ft. of your bench but different dimensions. Do you think this would work as well as yours? Thanks

[@broaudio]

Thank you Adam! I think your design will work great. I don't think one is superior, this is a flexible stove and there are many possible good configurations. I think you should build it!

[@MrFlyTWA]

Has anyone come up with a good refractory casting mixture that is affordable/DYI for the burn chamber? I have seen and researched many but most use portland cement. My 86yr old father was a concrete worker his whole life and said this would not be a good idea. I was thinking of incorporating the best of the recipes but lack of chemistry knowledge not sure how all would work..? parts 1-fireclay,3-Black Aluminum Oxide 70 Grit Abrasive Media,1-portland cement,1-hydrated lime,1/4-liquid glass,1-perlite and 1 to 2 parts water. Not sure if mixing with a foaming agent like aircrete would be ok for burn chamber or just for riser part. Any help?

[@moejaime2654]

Yep and how about ashes collecting in that bell ? Seem like it would collect a lot more residue ? !

[@chip582002]

So with the bell system what type of top, stone, concrete, etc will radiate the heat most efficiently?

[@sandrarobinson7661]

Hello...I need some help to get started with size of batchbox, exhaust etc. I'm in information overload and need to know if there are any books that show how to build a batch box system

[@BrianB1963]

Matt, I'm just about ready to do my rocket mass heater and I can't seem to find your recipe for refractory mortar for the batch box combustion chamber to hold the fire bricks in place, I looked up a recipe online that calls for Portland cement, silica sand, lime and fireclay. I can't find the fire clay anywhere locally. some people say that there is clay in the creek beds here in New Mexico do you think I could use that? or what's your recipe?

[@broaudio]

Your clay should be fine Brian. I use store bought Lincoln 60 Fire Clay and sharp mason's sand at 1:3 by dry volume.

[@bigdaddytreevo4120]

Thanks for the great video. I have never seen the bell option but it makes very good sense. I have one question. Have you ever tried it with two bells at 90 degrees for a corner application? I realize that you would have to go with a different stove design but, can it be done with any degree of efficiency?

[@broaudio]

You can certainly do it that way, or just one big bell with a 90 in it. Two bells work well, and the corner represents no obstacle to flow for a single bell either. A lot of variables in deciding which is best for an application, but either would work well. Hope that helps, thanks for the comment.

[@JimiJekyl]

Hi, I must say your videos are very informative and interesting. I'm only new to rocket stoves etc. You do put a good case for a bell bench as opposed to a flued cob bench. I've a few questions though: Do any of the materials for a bell bench have to be fire-rated or anything like that? How can you be sure that there will be no cracks or shrinkage affecting the gas tightness of the bell chamber? Do those slabs need to be supported in the middle? Thanks

[@broaudio]

Thank you Jim. Fire rating and material choice depends a lot on the system. In this system, I would not hesitate to use hardibacker and tile, but I wouldn't use wood on the bench/bell. I recommend inflammable masonry, but it doesn't need to be firebrick or refractory. The clay/sand mortar is the most flexible and reliable I've found. It creates a good seal and moves with the masonry. There may be some cracks on first warm up, but I always save the finish work and go over the joints when they are expanded. Aside from that, there is no reason to fear a little crack here or there. There is no such thing as an airtight masonry heater. The thermal mass means that after a fire, there is always draft. As such, any leaks will always be drawing in, so there is no risk of leaking exhaust into the space through small cracks. I have not found any reason to be concerned over air tightness, or cracks. In use, they don't manifest as a problem. In this build, the slabs are over a small span of 10" or so. They do not need support. A wider bench or bell could have a center support of brick piers, or steel angle across the span. Hope that helps, thanks for the good questions!

[@carlosdeleon8486]

finally, i found the light :) thanks for a great job saludos desde montevideo s33w55

[@MIhere2]

Thanks for your video. I agree with you about not using a loose pebble or gravel as a heat storage mass. This is my second winter using a partial run of that in my flue mass storage; and the other part is of a solid packed material. The solid packed material is much better. I am building a new house now and am planning to build a masonry heater, with some rocket stove aspects to it. The books I have on masonry heaters caution about having dead end pockets where unburned flue gasses can accumulate, for fear of a possible explosion. (must have happened in some). Do you think there is any concern with having a bench bell system like you have, where the the entrance and exit exhaust are on the same end? I think I may do something similar, but put the exit on the opposite end so there is no dead end pocket.

[@broaudio]

Hey CE, great questions. There are concerns with capturing unburned gasses in some systems, and in traditional masonry heaters this was a common caution. My experience is that while you can create some smoke in these burners with poor burning practice, it is very difficult to burn them poorly enough, often enough, to create chimney fire conditions. Due to that, it's very difficult to ignite downstream gasses, there just isn't any spark traveling that far, and once you are creating any significant heat in the front end, you will have draft clearing things out. And that's the real key. As long as the body of the stove is slightly warm, there will be draft and things can't build up. When you don't have draft, there's an obvious problem, and the stove is difficult to start. In this case the bypass is there for tough starts, so the smoke should really never happen. My belief is that this was much more of a concern in large European masonry designs that would be loaded with 60lbs+ of fuel, door closed, and walk away. They could smolder for a long time, often did, and often had significant creosote buildup and much more restricted flue flow paths. The dead end, or "blind bench" is, I believe, very safe when coupled to a modern combustion design.

[@felicianocapicia]

what would you consider to be poor burning practice?

[@dorkavenger42]

So here’s my question. Let’s say I have a wood stove that can cook with & heat my space but it is summertime. With something like this, and the heated seat be bypassed?

[@colemyst]

Maybe someone can help answer a couple questions. What size house will a RMH reliably heat? Is 1000sf-1500sf too big? I think a house with a basement would need a separate heat source for that level, but I could be wrong on about it. I'm hoping to build a small but NOT tiny house and have trying to find Eco-alternatives to mainstream building, heating, and energy. Most people around me say I'm crazy.

[@andrewjackson9853]

Condensing moisture and draining it as the exhaust gases cool in the bell, moisture from the wood? I'm a long time HVAC installer/business owner/designer. I love this idea for our cabin just wanna pick your brain a bit. Have seen some demonstrations where exhaust was as low as 110° F using extraction from the bench mass via Pex/hydronics as well for multiple "heating" uses. Gravity inversion hydronic closed and open loop systems.

[@aliciamcnamar8203]

Matt I understand how the RMH works(in theory) in heating an open floor plan but how does it work in a 3-4 bedroom single story home. (hubby asks and I have no clue). How does the heat move through out the house into the bedrooms and such? thank you for your help in explaining this to the Hubby.

[@broaudio]

Alicia, they are great heaters, but like all heaters there needs to be a way for heat to move through the space. It will heat the rooms as well as a space heater in the main room would. If you all leave the doors open and have good airflow it may work, but generally they heat the room they are in and don't do a great job of heating separate rooms. Thanks for the question.

[@thebackforty939]

nice video and RMHB stove.... is there an ideal length of the bell bench or max length where its no longer working correctly?

[@broaudio]

Thank you. Yes, but it will be different for all systems and vary by conditions; warm mass/cool mass, fuel type, outside conditions, etc. A longer bell may be used if the exhaust is coupled higher in the bell, heating mostly the top and less of the sides. A shorter bench can use a deeper exhaust exit to engage more of the bell as heat exchange. In this way varying lengths may be used. The best way to figure this out is to build the system and play with exit heights to find the best performance. This can be as simple as locating the flue pipe higher or lower in the bell, or blocking off the upper parts of a bell exit to adjust exit height. Great question!

[@vidard9863]

running a center brick wall in the middle of the bell bench might be interesting if one needed extra support for the bench top, or to force the air to flow around some corners. perhaps more interestingly if you had the center wall forcing air to move in a certain direction placing a wall perpendicularly to the flow of air with a gap at the bottom you could force the bell to work with a higher exhaust port so long as you could with your bell reach a point lower than both the inlet and exhaust ports... would only be useful in extremely limited applications though.

[@chefboyrdanbh]

How far is the flagstone above the bell? Is cob in between or just air?

[@haveyouconsideredtherapy]

Am I correct in thinking that the warm gases will force the colder gases out and up the flue? Someone I know is suggesting that having the exit flue too low will mean they don't have enough energy to escape at all.

[@MrFlyTWA]

Could the bell consist of two 50 gal steel drum welded together then placed on their sides and in cased in concrete?

[@broaudio]

Yes, that would work just fine Mr Fly!

[@thewoodensailor8722]

Very nice! Now you have me thinking about building another RMH lol. Any condensation build up in or around the bench?

[@broaudio]

Thanks Jon! Condensation is not an issue, the masonry can handle a little moisture on cold starts and once warm it stays that way and avoids condensation for the majority of the heating season.

[@LampWaters]

Thx

[@bobgunner3086]

Could this system be scaled down to be used to heat a chicken waterer? I am thinking having a stove of some sort outside of the chicken run, then run something into the run that I can set a metal waterer on to keep it from freezing. Right now I have candles in a cinder block and a metal waterer on top of that. It works decent when it works but overall its a PIA.Jokes about scrabled eggs aside, how would I scale something like that that could a.) burn some kind of fuel all day and b.) not be so hot as to be dangerous to the chickens? (an no jokes about scrambled eggs from the peanut gallery ;-)

[@jamesglenn520]

In the video you have a thermometer on the exit flue but didn't talk about the Temps that you get while running. I'm really interested in knowing what they are! Thanks 😊

[@reklawmd]

Hey Matt- Thanks for all the videos! how important is the volume of the bell? what would be the downsides of having the volume to large? or is there a target volume based on the stove size? Cheers!

[@broaudio]

Volume is important, but not an exact science as things change dramatically depending on weather, heater state (warm/cold), fuel, materials, etc. Too much volume means that exhaust temps are too low and there isn't enough draft to run the combustion core to burn efficiently. The solution is a start up bypass, to preheat the chimney and create draft in poor conditions. Using a start up bypass you can overcome a cold mass, or large mass, and allows for more thermal storage since you can start up small and then switch over to a larger volume. There are calculations for Internal Surface Area(ISA) of a heater in relation to how much fuel it holds(Power output, usually in Kw). I usually don't bother, as I haven't found the calcs to be a very useful guide, and I have a pretty good handle on what constitutes a heater that will run well. That said, many builders find comfort in pre build maths. Here's a link if you would like to learn more: donkey32.proboards.com/thread/1822/sizes-single-bell-systems Hope that helps!

[@broaudio]

Yes Hugh, the core is sized for a 6" chimney.

[@MiguelEstornino]

@@broaudio hey, first thanks for video, second I'm tinkering with my own bell and I have idea for making sort of moveable flap between bell and chimney so I could set different ISA. Like start rocket with almost none so chimney can preheat and than make bell bigger and bigger. For my 200mm chimney it could rise to something like 16 m2 of ISA I was said. But Peter suggests something like 9,5 so I wonder if there was some another attempt to have scalable bell. Do you know?

[@roach1405]

what is minimum flue gas temperature that you need to make it flow out the stack and have no moisture in the pipe

[@broaudio]

Good question, there is no one right answer as it depends on the dew point that day, moisture in fuel and system, and other variables. If your stack is insulated all the way to the top it can be as low as 200°F or even lower most days. An uninsulated stack is going to change this a lot, you will need to drive it with more heat to stay above the dew point all the way to the top. Hope that helps.

[@jimriddle7827]

You may have answered this before, or possibly have a build video somewhere? How do you ensure the exhaust gases do not start leaking out your bell? or bench area?

[@broaudio]

Jim, the stoves are well sealed, but even if it's not air tight it is always at negative pressure when there is a chance of gasses present. The warm masonry ensures consistent draft even through the end stages of a burn, ensuring the entire stove from firebox air intake and on is drawing room air inwards, rather than leaking out. If the stove were to leak, it would leak out the firebox air inlet as much as any leak in a bell or bench.

[@frankpoli873]

Great video just a question how detailed are the plans especially where the bench meets fire box

[@broaudio]

Thank you Frank. The plans are detailed, and show the flue runs from stove to bench. The bench itself is not included in the plans since every install typically has a different layout. Should you purchase plans and need details to help you complete your bench, I'm happy to help and can provide drawings or consultation to give you direction. Hope that helps.

[@frankpoli873]

Thanks

[@andreaspeper383]

Bell to extract more is confirmed through measuring yet?

[@dezertXer]

Your design looks really classy. I love the bell design. I wat to build one like this inside a greenhouse that is 44ft long. I would like to put a rocket stove and a long bell chamber at the end of the GH to keep it warm all night. How do I get a hold of you for help with it?

[@broaudio]

Thanks fellow enduro ripper. You can find me and my work here: walkerstoves.com/index.html

[@rnupnorthbrrrsm6123]

Hey, did you build one in your green house ?? I would love to see it :))

[@MrKDogg]

Both the mass and the bell systems are interesting. My question is, can they be used together? For example running flue pipe through a mass first then having it exhaust into a bell. Would there be enough heat for the stratification to take place and then exhaust trough a exit pipe. Also would this be any more efficient (store more heat) than either system on its own? I find this whole concept fascinating.

[@broaudio]

They can! kzbin.info/www/bejne/h4O4pHpoZbSeqaMm19s There really isn't a better or worse system, just different features for different functions. In the case in the link above, I had a long run that was too big for a bell so used flue pipe, but the end of the bench was slow to heat and cool. I use a bell there to allow the gasses to dwell and heat that space well, as well as allow a thinner top coat for faster heat up. It all depends on your goals and design, the different methods are simply tools you can use to achieve those ends.

[@MrKDogg]

Cool, thanks!

[@BrianB1963]

hello Matt, excellent video, now that I've seen this this is the design that I want to do. a couple of questions. 1) does the Bell only consist of the brick and flagstone? Nothing inside? if so, that's wonderful!: 2) is it okay to put the intake and exhaust on the same side? 3) I would like my bench SEATS to run both directions from the burn chamber each one about 8 ft. for the intake in the exhaust Can I use two 6 inch pipes going from the 55 gallon heat Riser Barrel on both sides going into the Bells and have two 6in pipes on each Bell going into One 8 inch pipe for the exhaust?. I was surprised that you talked about the inefficiency of Pebbles with wood aesthetic because that's what Paul Wheaton did and he said he only used 6 tenths of a quart of wood with a house in Montana for the whole year! that doesn't really sound that bad, I'm in Santa Fe New Mexico in the average overnight temperature in January is 20 degrees of course yesterday it felt like it was 60. during the day. I think I will use red brick on the front because it does look so good. thank you Matt, I appreciate your response. Brian

[@broaudio]

Thank you Brian. Paul and I agree about much, but not everything. Don't expect the results he reports, they are not the norm in my experience. Your plan is flawed in the splitting of the flow, that never works satisfactorily in my experience. Try to find a way to make the benches sequential and it will work great. Bell is hollow, exhaust and intake can be anywhere as long as elevation relationships are respected. Hope that helps!

[@darrelljacobjr2120]

Theres a way to build a valve-box between the two mass benches, with two valves, so that when the valves are open, the hot gasses go straight back from the drum to the chimney, bypassing the mass benches (useful during cold startup). When the valves are closed, the first valve guides the hot gas into the right bench, it flows around through the duct, comes back to the valve-box, the second valve guides the gas diagonally across into the left-side mass bench, it flows around through that ducting, and comes back into the rear of the valve-box to go up the chimney. I drew a diagram in MSPaint but can't post it in the comments. Pebble fill works great if you have mud/clay to connect all the pebbles together into one solid mass.

[@davewood7771]

Hi. I am very interested in Rocket Mass Heaters but being in the UK it is not the done thing. Do you have any UK self builders as customers? Regards Dave

[@navrbrda4647]

How big can be the bell to work or heat properly? I'm thinking to build one bigger, something like half wall. Is this possible? Thanks. Great video

[@MiguelEstornino]

Batchrocket eu have your answers, they tested out bell sizes with different cores and they came up with some charts for sizing batch box.

[@navrbrda4647]

@@MiguelEstornino thanks

[@BrianB1963]

that does help me, how about I take the exhaust from one side of one bench and connect it into the other bench so I only have one exit from one bench?

[@broaudio]

That'll work Brian!

[@broaudio]

Brian, you can also mix flue pipe and bells to achieve your goals. Check out this build of mine: kzbin.info/www/bejne/h4O4pHpoZbSeqaMm20s

[@SherryEllesson]

Matt, do you have anything like either an actual halved bell we could see, or a drawing? Also, a question: is it the expansion of the hotter gasses in the top of the bell that force the cooler gas to exit near the bottom? I can imagine the cooler gas leaving the bell, provided there's literally a drop of some kind, but having trouble visualizing those cooler gasses then rising out an exhaust. ??? Thanks for any light you can shed.

[@broaudio]

Hi Sherry. I don't have a drawing as it's just too simple. Turn a bowl upside down. That's a bell. As for the flow, think of a stand pipe in a pool of water. If we fill the pool, the water will not drain out until it reaches the level of the standpipe to flow into and down and out. The bell works the same way, but upside down. The bell fills from the top down with warm gasses, when it fills down to the exhaust exit it can then flow out. Hope that helps!

[@anomikak1062]

If the stove pipe needs cleaning out, then wouldn't a bell just have the same creosote spread over a larger area? You have had these stoves for years. I'm assuming you have opened one of them up. Are the bricks inside the bell not covered in creosote and does that not pose a threat?

[@BrantCasteel]

Is it correct to say that the top of your bell should be a level, flat surface to spread out the hot gases as much as possible for more efficient transfer before they sink to the bottom??

[@broaudio]

Great question Brant. No, that isn't necessary. Much like a pool of water, the deep spots will fill first, with the *coolest (hottest in this case) water(gasses). That said, every bit of elevation change will be a temperature change. More heat into the highest spots, with the low spots being cooler. In other words, a Bell with a tall section and a low bench section that are all one open chamber inside will heat the tall section then "spill" into the lower section. The tall section would be much hotter overall. In this way you might keep seating near the firebox cool and have a raised area out farther away that gets warmer. These features are neither good nor bad, simply tools that may be used to help design a system to reach your goals. Hope that helps.

[@tracywarren7332]

I have a question on the exhaust. The exhaust leaves the bell at the bottom but do you have a vertical flue outside of the building? We get lake effect snow and if there is a persistent band we could easily receive 12-18" in one snowfall potentially covering a low vent. Interesting topic.

[@broaudio]

Tracy, yes, there is a vertical chimney going above the roof line like any standard wood burning device. Thanks for the question.

[@tracywarren7332]

So my next question, instead of using some 30' of pipe for the flue to get 3' + above the roof line can I use an exhaust fan to create the draft while I have a fire going? We're not worried about smoke and creosote as the output would be just CO2 and steam. Perhaps a vedio on exhaust part of the system would be useful to more people than just me. Thanks. Tracy.

[@peterchan1741]

This is regarding the double sequential bell scenario posted by Green Energy Concepts (KZbin wouldn't allow me to reply to that comment). It does seem funny when I first think about it (the entrance and exit in the second bell being at the same height), but if I am understanding Matt's answers correctly, it shouldn't matter..... ? or should the exit still be lower than the entrance in the second bell, even though the entrance is positioned 'low' ?

[@broaudio]

In a double bell build where one is trying to maximize heat extraction ideally both entry and exit would be as low as possible. Exit does not need to be lower than the entry if both are low and there is room above to stratify. The real key is to have the exhaust low in the bell, so gasses must cool and descend from the upper level in order to find the exhaust. Ideally the entry is low as well, to encourage stratification, but in practice it will work either way. Exit does not need to be lower than the entrance, but it should be low in the bell. Check it out, he built it! kzbin.info/www/bejne/e33XXmdngsadqc0

[@BrianB1963]

Good morning Matt, Happy New Year, so does a batch box with design with a 8 inch flue and a 55 gallon barrel combination sound okay to use with this bench seat idea? and if I decide to do two benches would I take the lower Outlet from the one bench and put it into the higher side of the other bench? and then take the Lower outlet of the second bench to exit outside the house? and how is the draw (suction) compare with this idea from the duct pipe through the bench seat with mass idea? Thank you so much!

[@broaudio]

Happy New Year Brian. Yes, the normal RMH configuration, 6" or 8", into a barrel and then to a bench is a great design. Quick heat from the barrel, long cycle from the mass. Sequential bells are possible and increase heat extraction over a single bell of similar dimensions, so a two bench system is a good compromise of smaller footprint and better extraction. The coupling should be done as you describe, but it's not necessary to enter the bell low. It will change the performance a bit depending on the system, but exit height is most important. A high entrance into a bell means it may function as one bell with the proceeding bell. Draw is roughly similar, although velocity is reduced in the larger volume of the bell compared to flue run. At the chimney, it's all the same. Hope that helps!

[@BrianB1963]

awesome, glad to hear it, thank you so much for your input and response. I'm excited, I will post a video when I am completely done.

[@felicianocapicia]

Matt, did you mean that "it's not necessary to enter the second bell *high*?

[@broaudio]

No, I meant it how I wrote it in this specific case. His situation may extract too much heat if both bells are designed with max heat extraction so it may be beneficial to couple them high so more heat is transferred to the second bell.

[@felicianocapicia]

thanks!

[@davehoover8214]

Are you able to get home insurance with this system? And, sorry for the dumb question but, this is the first I have heard of the bell system, what we are looking at here is basically a horizontal chimney run inside the house with some strategically placed exhaust ports?

[@broaudio]

Yes Dave, this is a horizontal flue run used to capture and store the heat from the fire. Insurance possibilities are going to vary state to state, among other variables. In most states yes, in some, no. Hope that helps, thanks for the comment!

[@MartialGandhi]

Great Design, Matt! I have a quirky theoretical question. Let's say you wanted to make your bench as long as possible so as to maximize the amount of heat absorbed from the combustion process. How long do you think you could make that bench before you started seeing any diminishing returns? Thanks!

[@cordellscott]

Many factors to consider. Amount of heat at source ie size of fire. Width, height, and mass of bench per linear foot etc

[@peterrose5373]

seems to me that you're pretty much describing a hypocaust, here.

[@TheDave570]

Where the exaust is makes a difference. You want the hot gases to travel the longest distance possible to maximize the heat transfer !!

[@broaudio]

What you are missing is that regardless of where the exhaust is placed horizontally, it is the vertical location of the exhaust relative to the entrance that dictates how far the gas travels. You are correct in that we want maximum dwell and contact, but we get the same results regardless of which end of the bell the exhaust is on. A pool with a stand pipe must fill completely before draining, the water can not shortcut to the drain until it fill to that level. Same situation here. Hope that makes sense, thanks for the comment.

[@davinciconcepts]

Hi. you are forgetting the mixing that you show in the top half of your drawing. This allows hotter exhaust gasses than you think you are getting. It also explains the bench getting cooler on the end... if it was truly stratified the temperature gradient would be vertical and not left to right.

[@julier1080]

It would be interesting if he had a probe in the left side as well, to see the temp difference. It could be that the left side is closer to the burn chamber and is picking up heat from conduction, or, as you say it may be short cycling of the exhaust gasses. Might be worthwhile to have a horizontal plate in there to force stratification, so the hot gasses come in the top and have to travel the length to drop down and return to exhaust.

[@julier1080]

Some follow up- In discussing this with someone who knows far more than I do about physics, it was pointed out that the ‘pool and standpipe’ analogy is incorrect, since gasses do NOT behave the same as liquids. Liquids are very correlated to gravity, while gasses swirl and mix, and are more correlated to heat and air pressure. That is not to say the rmh model is wrong, since obviously it works very well. In general, the height difference between intake and exhaust creates the correct flow, but it could very well ‘short circuit’ at times.

[@dalevoigt8612]

His temp measure of the bell next to the stove was higher than the end of it. If he ran a pipe so that the exhaust exited towards the end of the bell I think that it would make the temp more even.

[@aliciamcnamar8203]

So with a bell(i'm new to all this with no knowledge of how to build either) how do you close / build your bench with it being hollow? is there Rock in there and the exhaust heat heats them up and that is your structural foundation for the stability for your bench? This is for a cob bench.

[@broaudio]

Hi Alicia, the bell bench is just a hollow brick box. No need for anything inside. Hope that helps, thanks for the comment.

[@aliciamcnamar8203]

So is that a wood box sealed in by bricks then? Sorry I am so elementary and don't know anything about working with bricks and such. thanks for your patients.

[@broaudio]

No Alicia, it's just the brick you see making a box.

[@alantarbet9349]

How do you decide how long you burn your fire to fully charge your mass

[@jamesglenn520]

The system in the video is not a mass heater. It's just a bell system with a bench to sit on.

[@chipseal9403]

As to efficiency, isn't drawing inside air counter-productive? Doesn't that mean that outside air is flowing into your living space? Why not have your burn chamber outside and the barrel inside? I am imagining one with the burn chamber in a mud porch outside integrated with the low part of the wall.

[@chipseal9403]

@frosty Or it could be on a porch or mud-room. But my comment was posted before I understood that mass heaters are meant to heat butts not heat air. I was still thinking heated air rather than heated people.

[@catherinec197]

I'm wondering what climate zone/area you are in and how to adapt this to my Ian.

[@catherinec197]

Hahahaha......own! Own climate, who is Ian!?

[@benveenema]

Does the stone bench top need to be sealed against the rest of the bench to prevent gasses from leaking out?

[@broaudio]

It does Benjamin. I just set the top sections in the same clay/sand mortar used for the brick work, and fill any gaps with the same. It's easy to disassemble to inspect, and seals well. Great question, thanks for the comment.

[@Grizzydan]

This was definitely my number one concern. I just wouldn't be able to get it out of my head someone accidentally breaking the seal.. I guess that is why the typical rmh's tape the pipe joints and have 3-6 inches of cob and clay sealing the entire shell. I love this idea and could see the benefit of making it even deeper. I guess I could run a half barrel shell covered with clay before adding the bench top to appease my paranoia LOL Thank you for the great ideas.

[@MrDavePed]

GREAT idea to use an open chamber and stratification instead of a snaking pipe system !! Your exhaust should be far away from the initial entry point of the hot gasses. There's going to be turbulence at the entry point. You want the gasses to stratify before the cold is expelled. I'm wondering if the next evolution to this system is to scale down the size and increase the duration of combustion to the point of more or less continuous combustion over the entire heating season. A automatic fuel feed of a very small amount of fuel continuously would offer some advantages. You wouldn't have to feed the fire at all. The size of the stove would be much smaller. The fuel could be pellets poured into an automatically feeding hopper. You could set it up to add a few pellets when the current burning pellets are reduced to a certain weight. When the burning pellets get lightweight some more pellets are automatically poured into the fire via and angled tube. Automatic feeding continuous burning rocket furnace. Maybe top off the reservoir with more pellets once a week at your convenience. There would be no temperature swings from too hot to too cold. Very even temperature output and with your cobb CHAMBER heat battery you would enjoy a very constant temperature. You could adjust the rate of burn by adjusting the weight at which additional pellets are fed or.. you could adjust the rate of burn by adjusting the air intake. This could be a much smaller scaled apparatus less intrusive to the indoor space. The cobb chamber should encircle the entire main floor of your indoor space. It could be narrow enough to allow a standard wood floor to support it. Perhaps an eight inch wide cobb chamber one foot high wrapping around all of your exterior wall at the floor. Only your exit doors would obstruct an entire circuit. ..

[@kyleburdick8771]

I suspect that the minimum viable fire (that efficiently and cleanly burns the gases) would be way too much heat to burn continuously. Paul Wheaton seems to think that a couple hours a day of burn is the maximum necessary burn, even in the coldest part of winter.

[@randalmoroski4423]

So one pipe in at the top and one at the bottom to exhaust...?

[@salmonhunter7414]

Do you need put a stop or shut off when you start the fire/ stove.

[@broaudio]

This stove has a bypass from cook top to chimney, shortcutting the bench. That does make start up easier, but it's only needed when the stove is cold and it's warm outside, shoulder season and such. Once the stove body is warm, it stays warm for 24 hours or so, enough to ensure draft and easy starts. Great question, thank you for the comment.

[@jeremiahshine]

Salmon Hunter I did maintenance on a build recently. I put a double wick candle in the fuel chamber for 20 minutes while I worked. Even cold wet cob couldn't stop the draw.

[@cheesemouse7774]

Considering build of a rocket mass heater. I live in the thumb area of Michigan. My house is two story colonial with a concrete block basement wall. Block wall is constructed of 12", 3-core blocks. House is wood frame. 4" side walls with blown in insulation and is 100% cedar siding. Trying to determine best location for the heater and the size required to heat the house. House is approximately 2,544 sq. feet, plus the basement, which is an additional 1,368 sq. feet. My main heating unit now is geothermal, which is 6 ton. House is 100% electric. No natural gas available in my immediate area.

[@broaudio]

Probably too much heating needs for a RMH, they just aren't that big and that's a lot of space. You could build one to heat a room you spend a lot of time in.

[@shanengivone3973]

Are you still available to help advise buyers of the tiny stove with bench? I love the idea of the bell and cooking on the mass heater that's warming the house... I live alone out in the country and I'm intimidated about doing this myself.

[@broaudio]

Yes, absolutely!

[@shanengivone3973]

@@broaudio may I have your email and or phone number or the link of where to find it, please? I'll try to contact you later this weekend or early next week after I get some measurements. Thank you and blessings to you!

[@broaudio]

Walkerstoves.com

[@thesundaycarpenter2514]

I notice that your rocket mass heater is built on a wood floor. I’m interested in changing from a pellet stove that is totally useless at warming my home to a rocket mass heater. But, I have a wood floor. Is it possible to build safely on a wood floor?

[@Thalanox]

Definitely not directly on the floor. Of the other videos of rocket mass heaters I've seen, they've made a foundation on the floor of bricks, sand, clay, dirt, or cob first.

[@broaudio]

Exactly as the Thalanox says, you want to build up on an air gap to insulate the heater from the floor and allow inspection. You will also likely need to shore up the floor from below to carry the weight. If in doubt consult a professional engineer is my recommendation.

[@jurok6920]

what is the fan on the stove?

[@jamesglenn520]

They run solely off the heat of the stove no electricity required and no noise at all.

[@HomesteadOC]

Id like to build a RMH cob bench/wall for outdoor use. Are there designs that will minimize heat lost through the riser/barrel and maximize it in the heat battery aka cob bench/wall? I want to have an outdoor winter area in SoCal for people and plant tropical fruit trees behind it, against a wall/backrest, espalier style. The bench/wall will be south facing and hopefully will store heat through the night and keep the trees above 50-60F. A cooking platform like you have here would be nice, but the main purpose is to heat trees, 2nd purpose to heat people, and 3rd to cook, slow smoke, or make jerky. I want the wall to be 6’ tall, circular and as long as possible. Ive read thar the flu pipe can be a max of 50’, can the bell/chamber be longer? Can I go longer if I have a bigger feed tube, riser etc diameter?

[@broaudio]

It sounds like an ambitious project! There are certainly some large RMHs out there, so I suppose it's possible. Good luck, please share when you build it!

[@HomesteadOC]

broaudio what is the downside on completely burying the riser inside a mass? Im guessing maintenance would be an issue, but maybe using a large thick piece of flagstone in place of your stovetop to send more heat into the mass downstream? Is there a limit to how long the bell or flu pipe can be when there are no bends?

[@broaudio]

No downside to having mass over the riser, that will allow longer flue runs. Length is going to vary depending on a lot of factors. In my designs I recommend no longer than 12'.

[@HomesteadOC]

broaudio is the 12’ rule of thumb because of bends? Im sure it becomes harder to create a draw the longer it goes. Do bell type RMH light easier than a flue type with a proper gradient?

[@billlyell8322]

Interesting video, I don't agree with all your premises. But I have a question, if you're looking for storage mass the why use a little bench at all? Why not turn the entire floor into your thermal battery? This was proven to work in Asia and by Roman's well over a thousand years ago. Run multiple flu branches off of the heat source and collect them at the exhaust pipe. For example for a 16x24 cabin you could have 3 runs that total roughly 50 feet each. That's a total of 150 of run. Additionally your radiation surface is 384 square feet. Obviously you need to insulate the bottom and sides but the top itself will heat the structure. This this layout is a simple straight run you minimize the back pressure as each total length is only 50 feet. And there is no snaking runs back and forth to get total coverage.

[@mysimplelifewithapril2428]

HI :) I am building a rocket mass heater and I am using the bell bench and my question to you is ... can I use cob for the top? if so how thick should i make the cob? will two inches retain the heat? or should I shoot for 4? I did not want to use the pipes because i am using an old futon mattress to sit/sleep on and I did not want to have to make up that much cob. I am in a very old house with not so great joists under the house and I am afraid all that mass would mean that I would have to shore up the floor before I could even start. Anyway ... I have cob, I do not have flagstone. Also one more question.. if I send you a picture of my bench so far could you critique it?? and let me know if I am on the right track please and thank you. I love your videos. If this goes well I am wanting to make a cookstove per your instruction. oh and .. does the pipe going out of the house ... the outlet.. need to be 8 inches as well? or can it be smaller in diameter.Thank you for your time. :)

[@broaudio]

Four inches would be ideal, but it can vary depending on the system and your goals. Chimney should be the same size as the rest of the system, or larger. Hope that helps.

[@mysimplelifewithapril2428]

broaudio Thank you :) I hooked up with permies.com and got some great information as well. Also I happened to remember that my mom has a flagstone walkway I put in years ago and now she wants brick sooo.. :) I dug up part of her walkway and put it on the bench and cobbed them in place. They are anywhere from 2 to 3 inches thick on top of 2" of cob. I think this will do nicely. Thank you for the pipe sizing. I was about to put in a six inch because the 8 just looks so big going up the wall. It will be fine though I'm sure :) I appreciate your advice. I love your channel.

[@videonatura8728]

I wonder how big is the room you are in the video, because the bench is no too big. Thank you.

[@stan.rarick8556]

By creating such a large chamber, aren't you reducing the mass which actually retains (and then releases) the heat? You need to maximixe the contact surface area to maximize the heat transfer and maximize the mass volume to maximize heat retention.

[@broaudio]

A bell maximizes contact area and the brick are far denser than cob, so this bench is higher mass and better heat transfer than a similar sized flue run bench. That would change with size, so it all depends on your goals and design. Neither construction is superior, they have different qualities that can be used to meet the builder's goals. Hope that helps.

[@bloodbought3029]

When you say bell do you mean hollow? Inoticed a wood floor runs under your box Did you cover the floor ?

[@broaudio]

Yes, it's hollow. The floor is covered with masonry and insulation.

[@wobdeehomestead1464]

Anyone know an insurance Co that would sign off on one of these?

[@jeremiahshine]

Enter at 1/3rd height exit at the bottom of the bell.

[@Pilot333]

That moment someone gets ready to write on the window

[@tonynewhouse1013]

How can you be sure no gases are leaking through or out cracks.

[@broaudio]

A functioning system will always be at lower pressure than the outside air, or it won't draw and won't burn. If it's leaking and you have no draft, you have bigger problems and will know immediately by the smoke coming out of the firebox. Hope that helps.

[@wadepatton2433]

Even wood stoves/stovepipes with gaps don't leak so long as you have a draft. I have observed this over 20 years of heating with wood. I see folks post this question over and over and it tells me they've not had a good flue or no experience with wood heat. If my stove leaks through a crack-there is a blockage. Same for RMH.