just wondering...

Hi
I've been reading Ernie and Erica's book and watching Paul's DVDs  and I'm getting ready to build my own RMH. But I have a few questions: 1- I have a cement base of 3ft by 7ft to put the heater on, no basement underneath. On the other side of the 7ft wall is a bedroom. Is there enough room to put a RMH and have enough mass. my house is approx 700 sq ft. fairly open concept.

2- We have a lot of sandstone on our property and I would like to make the heater look somewhat like a masonry heater, I understand the idea behind not covering the barrel with bricks and stone because of the air flow but I was wondering if a person built a stone wall in front of the barrel to hide the barrel and left a space between the barrel and the wall, and even left a few holes in the stone wall, especially at the bottom for air flow, would that change anything?

Just wondering...

Is that galvanized steel roofing lining the wall behind the stove?

Hi Helene;  Welcome to Permies!
We really need some more info on the layout of your house  700 sq ft should be easily doable.  
What kind of chimney do you have or plan to have?
Where are you located ? How cold is it going to get ?
Are you familiar with the brick bell style of RMH?  One of them could fit in the size you have avalable.

Thanks guys for the replies. Yes Aaron, that is galvanized roofing on the walls that was the best I could find ;o)

Thomas, the house is in New Brunswick Canada where the weather can dip below -25C in winters, not sure where that would be in Fahrenheit, We're in the middle of a snowstorm as I'm writing this. As far as the layout of the house, the room where the stove is is about 10 x 10, so is the bedroom behind the stove wall. Across from the stove on the right of the picture is the open kitchen-living room and front entrance roughly 20 x 20. The chimney is double insulated typical chimney installed by a professional renovator. We had our first snow storm 6 weeks early in November and ran out of power for 2 days, the house wasn't finished yet so I borrowed this ancient stove from my brother's barn. it's not legal and not very efficient but better than the high electricity costs.

By "Brick bell style of RMH" are you talking about the heat riser made out of fire brick or are you saying that the bell would be built out of brick instead of an old steel drum? Is there a picture of one in Erica and Ernie's book? Or where would I find a plan for that? I was kinda hoping to use an old drum as I have a bunch of them lying around in the yard but if there's not enough room on my 3X7 cement pad....

Hi Helene;
What size is your chimney ?  6" or 8"  Block or metal pipe ? Is it all indoors or outside the wall ?
-25 C is -13 F .
A standard rmh as described in the builders guide, uses a barrel as a bell, a transition area where the exhaust starts moving horizontally thru the pipes surrounded by cob and buried under large rocks. This bench is commonly used as a couch or day bed. Some folks use cob on the whole bench , some like me surrounded the cob mass with brick or other hard material to avoid repairing chipped cob off the bench, and as additional mass.
Your concrete slab is too small for this style of mass.
There are options here though.  To start, we know you have no basement. What is your floor ?  Wood ?  Is it or can it be supported to hold the weight of a solid mass ?  Simply adding a few blocks under the floor joists is sufficient.  Your RMH does not "HAVE" to be on your slab...  People routinely use bricks laid horizontally on the floor with air gaps between , then place concrete board on top of the brick. You build your entire rmh suspended 2" off the floor.  You protect the floor from excessive heat, you gain another side of the mass to radiate warmth into your home. And in your case you might find that you have room to make a traditional bench! That should give you some options to concider.


Now lets talk about brick bells or another name is a stratifacation chamber. I looked and this is not in the builders guide that I could find. Plenty of info here at permies about bells.
Here is a link to a post descibing a ladys build of a small brick bell using a batch box design rather than a J tube design. She had limmited space (much smaller than your slab) and she built a superb little rmh to heat her home...  You will also no matter what style or type of RMH you proceed with. (permies.com/t/43809/Masonry-stove-diy-build-feasible)  its a long post but worth reading.
A brick bell RMH has a core unit the same as a bench style , risers are the same. The difference is that there is no pipes buried in a mass. This allows your RMH to go vertical rather than horizontal. Saving space on the footprint .The hot air from the riser enters your brick bell and rises to the top. This causes the cooler air to sink towards the floor. Your chimney attatchment is located at floor level. Only the coolest air leaves . The entire brick box heats up and radiates warmth thru your home. This is similar to a Russian masonry stove but not built by a certifed Mason. Unfortunetly the insurance man likes Masonry stoves because they are universally accepted as a safe form of wood burning in a home. A RMH is also a safe form of wood burning... but your insurance man will not understand this...

This is enough food for thought for now.  Come back and let us know what you are thinking.  I've included 3 photos showing you my traditional rmh with bench and my brick bell rmh (that uses a barrel for instant heat) Lots of options including the choice of bulding materials for your core.

I'm not claiming expertise in this area but I have read some things about galvanized metals releasing unhealthy fumes when heated, they might have to get pretty hot before being dangerous, but you might consider using a different material in the long run. I did the same thing a while back in a little tiny cabin of mine and someone mentioned it to me, best of luck in your project! -Aaron

Thanks for the info Aaron, i didn't know that,  actually that heat shield was meant as a temporary solution until i get my RMH.

Thomas, my chimney is metal, 6 in diam. The double insulated part is about 6 ft long from the ceiling straight through the attic to the roof.

The floors aren't done yet so it's just plywood sitting on 2x6s.

Thanks a lot for all that info, lots to chew on. Looking at at glance it seems like my best option might be something like Galabriel Freden did. I guess i have a lot of reseach to do.

I want to make sure that whatever i chose will be the most efficient for my house and that i can use sticks, not chopped wood as I'm not interested in cutting trees and chopping wood.
Thanks again for all your help.
I'm sure I'll have more questions in the future though 🙂

Hi there

I started this tread a few months ago and had to stop to finish building my house and do some more research and thinking about what I really wanted to do as far as a RMH.

Here's my conclusion: I don't have the experience, manpower or resources to build the heat riser/burn tunnel assembly and frankly it kind of scares me. And since I'm no spring chicken I don't have the desire either so I settled on purchasing the Liberator Rocket Stove https://www.woodcookstoves.ca/rocket-heater.html   instead and just build the mass for it.

Here are a few rough sketches of my house plan which is about 700 sq ft. And sketches of how I envision the mass to look like. According the instructions on the Liberator website I can use no more than 4 elbows of 90 degrees in the mass for best efficiency. I'm planning on using field stones and leftover cement that we used to build the pad that the RMH will be sitting on.

Since you guys are the experts on RMHs I would like to ask you a few questions and would really appreciate your advice.

1- Is it OK to put the stone mass directly on the drywalled wall without a heat shield?
2- I have a few bags of cement left over from building the cement pad, is it OK to use that with field stones to build the mass? If so, should I mix it with anything (soil, sand ...)?
3- Do I have to wash the field stones before making the mass walls to help the cement stick to them?
4- The stove pipe that runs from the entry point to the T-joint is on a bit of an incline, is that OK?
5- What do you think of the whole idea? Does it make sense?
6- any advice?

Let's try downloading these pics again

Hi Helene;  Welcome back!
Let me try to answer your questions. I am not familiar with the Liberator stove and how it compares with an evans design core.
#1) My own , I used cement board for long term safety. I would "guess" Sheetrock would be OK with the lower mass temps.
#2)Yes, as long as your away from the core.
#3)No not at all, use lots of stone and as little (binder ) cob as you can. The rock hold heat better.
#4)Yes , that should be OK, but why not have it level ?
#5) It seems to be a very short run thru the mass ?  What are the measurements inside the mass ? Do you have the dimensions of the liberator core?  
I'm sure that a RMH will heat your home just fine. The more mass (rocks) the longer it will shed heat.

Thanks Thomas.
Good idea about the cement board, I think I will do that for safety.
The reason the pipe between the entry point and T-joint is not level is because the T-joint is higher than the entry point and I can't lower it in order to have an elbow under it for my clean out pipe.
You're right about the short run through the mass, I wish it was longer but the space I have doesn't permit it and the makers of the Liberator advise not having more than four 90 degree elbows in the run
The measurements inside the mass will be approx 10 ft in total length plus three 90 degree elbows and a T-joint. The Liberator stove bell is 36” high and 16” wide and requires a 6” diameter chimney.

Thanks again for your help Thomas. Can't wait to get my stove and install it. I'll send pictures when all done

Hello again
Here's an update and a couple more questions )

Due to lack of time before winter sets in, resources and confidence, I decided to purchase the “Liberator” Rocket stove and build a mass around it. My RMH sits on a 7' X 3' cement pad. I wish it was bigger but that's all the space I have for it.

The stove and pipes are now assembled and I did a couple of test runs. Everything works fine, no smoke leaks etc. But there's one problem: the stove doesn't get very hot. I put a pot of water on top and after 2 hrs the water was steaming hot but still not boiling and the lid was on the pot! I don't have a stove thermometer but my room thermometer registered 27C/80F, it should have been cooking hot in that room. The company website shows a video where the Liberator heats up really hot.  https://www.rocketheater.com/  (look at the second video, not the first).

The instructions say not to use more than four 90* elbows, I only have 3. On the pictures you will see that the whole system is a bit slanted. The reason for that is that the pipe that comes out of the stove is 7” off the floor but the other end of the pipe at the “T” joint, the pipe sits at 11” from the ground because I had to put an elbow under it that leads to my clean out door so it makes the whole system a bit slanted. So this is what I'm wondering:

1- The bottom horizontal pipe leans upwards towards the chimney while the top horizontal pipe leans downwards a bit, could this affect the draft?

2- Maybe I'm putting too much wood, or not enough in the feed tunnel, that may also affect the draft?

3 does it matter if my assembly including stove is not perfectly level?

4- What else could result in low heat?

The stove measures 32” high, 16” wide and the feed tube is 25” high. I'm using 6” pipes, total length of pipe from stove to ceiling cap is 11' 4”, and clean out pipe is 27” long.

I sent an email to the stove company and waiting for an answer, but I really value you  guys expert opinion also.

1) I doubt that the short run of pipe going slightly downward is affecting your draft. It could potentially be a possible condensation trap though.
2) Generally, the more wood you put in (up to a point) the draft or rocket/whooshing sound should increase
3) Doubt it
4) Is your wood dry? Also, some woods burn hotter than others. I would experiment with this the most.....  

Hi Helene;
A Quick guess on my part, is the quality of the wood you are burning. Maybe not dry yet ?  Rounds rather than split wood?
Your pipe run looks fine. The slightly sloping pipes should not be an issue, nor if your liberator is slightly out of level.
In your pictures, it appears the cleanout pipe is not capped? That could have an effect on draw.
Are you planning on encasing the pipes with rock and cob ? It will help with draw.
It sounds like your not having any smoke back or hard starting issues... There is no cob to dry out , it must be your wood.

Hi guys , thanks a lot for all the comments and ideas. So here is what I did. I slanted the downward sloping pipe slightly upward. Yes I did have a problem starting it. It seemed like the wad of paper was burning out before I had time to throw the kindlings in, don't know if I wasn't putting enough paper in or maybe not lighting it thoroughly before pushing it in there. And yes the wood was mainly old dried up branches from my forest. Yes there is a cap at the end of the clean out pipe, it just looks like there isn't ;0) .

So today I borrowed my brother's propane torch and got a bunch of 5-10 year old cedar kindlings from his cedar mill (cedar doesn't rot). I figured that would make it burn faster.

Well the stove did light up faster and easier with the torch but about half an hour into it the fire was coming up the feed tube. I don't know if it was because of the cedar or because I was pushing the kindlings down into the bottom but it was creeping up and black smoke started coming out of the feed tube so I got scared and put the lid on. After a minute or so I opened the lid again and a cloud of black smoke came out so I shut it again.

So what did I do wrong this time?

My recollection is that the liberator is a mag style rocket stove. You stuff wood in the vertical feed, cap it, and air comes in via the bottom in the L portion of the rocket. So if the bottom air entry point is open when you open the feed tube, then this can lead to smoke and flame ascending the feed tube. If the bottom air port is closed and this still happens it means the draw is insufficient.

Which means you have to look at the chimney above the roof to see if the issue is there.

Hi Graham
The bottom trap door is for lighting the fire and clean out purposes, once the fire is lit, you close the bottom trap door and feed the fire through the top of the J tube. I sent the same post to the makers of the stove and they told me that the problem was that I don't have enough draft. they suggest putting a damper on the upper pipe. I'm not sure how that can help?

if the bottom door is closed, and the feed tube is also closed off, how does air get into the system?

If the air gets in via the J tube, and it's smoking/firing back, then the feed tube is acting as the riser because you have insufficient draw.

So, there's a chimney issue.  Do you have a picture of the chimney on the roof?  Are there trees around?  Is there an inversion layer?  Is there a cold air plug in the system?
Can you take a picture of the chimney when the fire starts?  Does the smoke rise vertically?

Can you try running it with the feed tube closed and the bottom door open as a L style rocket ?

Graham's questions and comments are all spot on. I would add, how tall is the chimney from floor level?

A different subject is the heat exchange area. Comments about it being too small/short are correct; no matter how well you get the core burning, there will not be time for most of the heat to be absorbed by mass before it goes to the chimney. I think you really need a bell instead of pipes here. A hollow stone box on your base, with the Liberator exit going straight in the side, and chimney flue pipe dropping straight down through the box to about 6" above the box floor, will let you collect and store all of the heat you generate. A 6" batch box core would want a total of 57 square feet of interior surface not counting the floor; a J-tube style like yours would want a smaller area, maybe something like 40 square feet. An 8" thick masonry box with inside dimensions about 3' wide x 4' high x 1 1/2' deep would give you 40.5 square feet of internal surface. This will decrease drag to the absolute minimum, as if you had no elbows at all. There are other relevant details if you decide to go this route; I or others will be happy to give further advice.

You do want a double wall for the box if you can, for safety in case there is a crack in the wall. A layer of bricks on edge, or a smooth layer of a few inches of cob that is separated from the stone & mortar layer by say a sheet of cardboard, would be ideal. At the least you need to not have any stones going all the way through the wall, as any crack in the bedding would leak. A good chimney will give enough draft that you will have negative pressure inside the bell except perhaps at startup, reducing risk further.

I neglected to mention that the Liberator shell would also count to the heat exchange surface, so you would want a correspondingly smaller masonry box/bell. It is always possible to effectively reduce the exchanger area of a bell with the chimney dropping from the ceiling, so you don't have to worry about making a mistake there.

Thank you so much Graham and Glenn for your prompt response and very instructive ideas. Now I'm starting to think I was crazy to start this project! I'm a senior (female) who has never done anything like this before and looking at Erica and Ernie's book and Paul's DVDs on RMHs looked simple enough but I didn't trust myself to build the RMH core, that's why I bought the Liberator, then all I would have to do is build a mass for it (or so I thought). Now trying to wrap my head around things like cold invisible air traps, and building a bell heat exchange box and all kinds of things I don't understand is really scaring me. Plus it was 4*C/38*F   last night and my mass isn't built yet. Looks like I really got myself into a pickle!

OK to answer your questions, Yes the feed tube is opened when I light the stove (lighting from the bottom). Yes I believe you're right about the insufficient draw.  I haven't seen the smoke coming out of the chimney after I light the stove. The total height of the pipes from the floor to the top of the chimney (if the chimney went straight up) is roughly 15-16' and 6” in diameter. The added pipes inside the mass only adds about 4 feet plus 3 elbows. As far as the inversion layer, if I understand it correctly, it has been fairly warm and damp outside when I light the stove in the evening. As far as your suggestion about lighting the stove like an L style vs a J style, That's how I'm supposed to light it. There's a trap door at the bottom of the J (feed tube) where I light it with paper, then close that trap door and start feeding the stove through the top of the feed tube. Maybe I'm supposed to fire it gently first for a while until the invisible cold air trap disappears before I try to get it hotter.   But then the other day I fired it for 2 hours and the stove never got hot enough to boil a pot of water.

The nearest trees are at about 50- 75 feet from the house. This bell box sounds like a good idea but I'm not sure if there would be enough space for it. The total space I have for the whole system including the Liberator is 7ft long by 3ft wide and my ceilings are at 7ft 6 in from the floor. I can't go any bigger than that as there is a door at each end of the space. You're also mentioning an 8” thickness on the box wall which would make it even bigger, right? What do you think?

If that's the best idea, how do I go about building that by myself?
Here's another question, if the Liberator works so well on the video without a mass why doesn't it work just as well in my house without all these bells and whistles?

I forgot to mention also that what they have suggested was to add a damper to the pipe. Would that add to the draw?

Looking at the last picture, seems like my 15-16ft height estimation of the chimney might be a little off. Looks more like maybe 12ft?

Helen, the liberator video is blocked from my current location but I suspect this is how it works

1. Put kindling in the feed tube.  

2. Close the feed tube cap

3. Open the bottom cap, and use a propane torch or whatever to start the fire

4. Leave it running like this.  Do not closes the bottom cap.

When you need more wood

5. Open the feed tube cap

6. Close the bottom cap

7. Insert some wood

8. Re-open the bottom cap.  Close the feed tube cap.

The air entry is via the bottom to allow the wood to burn.  If you have both open at the same time, it will lead to fire and smoke going up the feed tube.  If you have both closed at once, it will starve the fire of air.

If the system is not hot enough, then that's because you're not managing to burn enough wood.  And that might be because there's a lack of draft.  The draft is normally defined by the height of the chimney which pulls air into the fire.

You've posted  your message in the rocket mass heater group, but yours is a rocket stove without mass.  The mass bit would involve building a bench or a bell to capture the heat after it leaves the primary bell.

The bell size I suggested would not take up a lot of space. 3' x 1 1/2' x 4' high inside, plus 8" walls, would give an exterior of 4'-4" x 2'-10" x about 5' tall. The dimensions can be tweaked to fit your space, like narrower and taller, or whatever. If you can lay up stone as your photo shows, you should have no trouble building a brick (or stone) box.


12' from floor to chimney top is not a lot, but my system has a similar height and draws fine. The dryness of the wood is critical - it must be bone dry to work properly.

Graham, according to the instructions on the video, you only use the bottom of the feed tube to light up the  initial pieces of paper to light the stove, then you shut the bottom trap and do not use it again. You then throw the kindlings and another wad of lit paper down the tube while the air sucks the fire down through the kindlings and lights up the kindlings on it's way to the center of the stove. Then you start feeding the feed tube with wood.
My system Will be a RMH after I build the bench which will be as soon as this draft puzzle is solved

Glenn, Ok that system doesn't sound too complicated, do you have pictures, instructions or drawings of your bell so I can have something to guide me? Is there a link somewhere that I can check? As far as bone dry wood, I just went in the forest out back and grabbed dead wood that had either fell down or that I could pull down with my bare hands. Would that be considered bone dry? It hasn't been chopped though, I just got round branches. Is that good enough or is it better if I get chopped wood?

My home system build: Rocket mass heater with 8" J-tube and bell

Thomas Rubino built a fairly simple bell for his shop heater: Brick Bell Shop Heater

A damper in the chimney or exit flue would not increase draft in any conditions; it could moderate a too-strong draft if you had that issue.


Depending on your climate, standing dead wood might or might not be really dry. If you have had a long hot dry spell and the wood is out in the open, maybe, but if it is in a shaded forest it is not likely to be as dry as you want unless you have a very dry climate. (If you frequently have wildfire danger, your climate is likely dry enough.)
I would store wood in a sunny sheltered space for at least a week, longer for wood over an inch and half or so, and split anything over 2". Split wood dries faster than round wood. Most commercial firewood I have seen is not dry enough until it has months under shelter and off the ground.

Helen, since you're not using dry wood that's likely the reason why it's not working. The fire can't get hot enough to get the draft going.

Try running it my way. That will let the wood in the feed tube dry out before it burns in the burn chamber.

If you watch the pellet video that's how he runs it as well.

Hi again guys. I didn't bother lighting the stove yesterday it was too hot outside ( I remember reading somewhere that if it's hotter outside that inside it can create some kind of back draft or something?) but I took the chimney cap off just to eliminate the possibility that the draft problem was up there. They were calling for 5*C/40*F this morning so I could try it again. I also split some old dried cedar pieces into kindlings. I figured I should learn how to light the stove properly before tackling more drastic measures like creating a bell.

So I tried lighting the stove the way Graham explained but the paper/wood burns too fast and keeps wanting to fall out of the trap door. I can see that could become dangerous, that's probably why the instructions say to close the bottom trap door after the initial paper lighting. I also didn't feel that taking the chimney cap off made a big difference. I suspect my biggest culprits at this point are my lack of experience at lighting the stove and like you guys mentioned, the wood may not be dry enough.
So The next few days are supposed to get really warm, I will concentrate on getting good wood and stealing more dry cedar kindlings from my brother's cedar mill ;o).

Thanks a million guys for all your comments I really appreciate your help, I will keep you posted

Hi Helene! I've been following your post, as I am just about to purchase a Liberator also! Hoping I love it. Now - I'm a novice - just been following the rocket stove posts for a couple of years - and particularly the ondol from Korea  - so this is a question from a novice and I hope someone can jump in and help.
In my understanding, if you have the stove pipe winding through a bench - for heat - then that stove pipe should end up exiting the house at approximately bench level. This is the way ondol works.  In fact for a few years I have been trying to duplicate it - with a lower level kitchen and fire smoke/heat winding its way under the floorboards of the house. If anyone at Permies has done this I would love to find out about it. I only have my Korean friends feeding me info about their homes and especially their older country houses.

If we were to create this system, why would we even need additional stovepipe going up out the roof? seems that is dividing the circulation and confusing the system. I've also sent this question to the good Liberator folks and look forward to their answer. Their instruction manual specifically calls for installation of a standard stovepipe going up. At this time I am not planning to add a bench or mass but just use the Liberator by itself but am interested in the possibilities.

Hi Hazel,  Ianto Evans of Cob Cottage Co. who did a lot of the research and early development with rocket stoves often exited the chimney low on the cob buildings and worked mostly fine. However, his circumstances were uniquely different than most houses which require a chimney to be around 2' above the highest roof peak within 10'. This is not only code in some areas but without it, your stove will most likely not draft properly (or at all). So its a pretty necessary thing to have. Hope this helps and good luck with your purchase. Would love to hear how it works out for you too!

Hello Ianto thanks for your reply! I appreciate your insight into the need for the height of the chimney. it sounds good. I'm just so confused - watching videos and reading comments on the Liberator - with no mass to radiate heat long after the burn and  just a loud hissing 700 degree thing in my house. Do you have any comments on the Liberator from your wonderful expertise?

You need a draft to power any stove. A chimney provides a thermo siphon to pull air up and away as hot air rises.

But the corollary is that cold air falls. So your chimney exit can be below the hottest gases which displace the cold gases in the system which then fall out pulling fresh air into the fire. Just imagine a normal water siphon.

One reason to have the chimney exit at ground level is to hide your existence. But then you're also having noxious gases exit at ground level which isn't healthy.

Thank you Graham!

Helene, the wood burning fast is good. Not so good is it to fall out of the bottom feed opening but it should only fall on to the hearth. That shouldn't be too hazardous.

In your situation I'd experiment further. Maybe construct a chain fence to prevent burning fuel falling out but still letting air into the system. The thing is to get the draft established and then maybe you can switch to how it's intended to be used.

Hazel Brand wrote:Hello Ianto thanks for your reply! I appreciate your insight into the need for the height of the chimney. it sounds good. I'm just so confused - watching videos and reading comments on the Liberator - with no mass to radiate heat long after the burn and  just a loud hissing 700 degree thing in my house. Do you have any comments on the Liberator from your wonderful expertise?

Sorry, but I don't have any experience with the Liberator stove so I can't make any claims to its performance. Once thing I can say for sure though that my name is not Ianto but Gerry.

ha ha! I realized that! thanks for getting back to me!

FYI here's the original thread for this stove

https://permies.com/t/54288/Building-code-compliant-prefabricated-rocket

great thank you