Can I build a RMH on this wooden floor? Can I install it in this position? Can I remove this wall?

Hello! I would like some advice about placement of a RMH in this property.

Continental climate, average Jan temp −2.3 C / 27.9 F average July temp 22.9 C / 73.2 F.

Walls all concrete block. All floors 1’’ thick floorboards with some kind of hardboard sheets on top, apart from concrete in the kitchen. All windows apart from kitchen double glazed / PVC. Gas boiler + 5 modern radiators. Rooms are large-ish, so it is a bit expensive to heat the house to a warm temperature. My ideal is very warm. Next fall I plan to build an earthbag / polycarbonate greenhouse along the southeast facing wall of the house. This will likely improve the thermal performance in winter, but may give problems in summer.  



I would like to install a RMH with an enlarged thermal mass, space enough for a bed, and some ledges for cats to sleep. See photo for concept I would like to follow (I understand flames will not be visible for a RMH).




I want to partially remove this concrete block wall, which seems thinner than the other walls in the house. I am not 100% sure if it is load bearing. See photo of the attic space. The roof is “stick built”, and I have indicated the location of the wall in question. I can’t yet identify the positions of the joists because on top of them is some kind of insulation material, and then a thick layer of mud / clay. The attic is cold. Can see daylight in many places. It is likely that the roof will be completely redone next year with factory-made trusses and metal sheeting with a rainwater catchment system.



If it is possible to partially remove the brick wall, the barrel of the RMH could take an almost central position (A) between these two rooms.  

Long term I would like to put in stained concrete floors which would be heated by a circuit of fluid-filled pipes which wrap around the heat exchange channels of the RMH. Potentially a second RMH in the kitchen or next to the gas boiler.

Any advice would be really appreciated, thanks!
1. I am concerned that the weight of a large RMH and thermal mass would be too much on these floorboards? What else can I do? Remove them and pour a concrete foundation for the RMH?
2. Again, is this possible to do a heated concrete floor on top of floorboards, or too much weight?
3. Can I partially remove this wall?
4. Is it essential to have the final chimney completely vertical, or can I put it laterally out through a wall?

I've been reading more of the textbook, and the answer seems to be "no, don't build a RMH on flammable materials".

So I guess I have to remove the floorboards where the RMH will stand. I expect there will just be earth there? So I have to pour a concrete foundation? Or can I make a foundation out of concrete blocks and cement? I have many concrete blocks here already.  

And in the textbook, it also says it should be insulated, otherwise lots of heat will be lost to the ground.  

I think you need to get a qualified builder to visit you house as it is too difficult to advise about knocking holes in walls from an armchair but i would say “yes” you can make a opening in virtually any wall if the corresponding support is catered for..ie a concrete lintel.
Mass heaters will always be better sited on a concrete base and generally speaking that base will need some form of insulation.
Have you checked your building regulations and insurance regarding rocket mass heaters?    

Hi M,
I agree that advising on the wall is difficult to do over the internet. However, if it is concrete block... why do you want to remove part of it? It would act like additional mass and soak up the heat and send it out the other side, without needing to remove any of the wall.

About the RMH, there are two pieces of the puzzle. One is whether the floor can hold the weight. This is possible, as Paul built a RMH on a wood floor, but had to add additional supports underneath. Does your house have a basement underneath? A crawlspace? Or build right on a slab?

The second piece is whether the floor/wall can handle the heat. I wouldn't be too worried about the floor, as all the RMH I have seen have quite a bit of rock/cob/firebrick on the bottom that should dissipate the heat. As for the wall, concrete blocks don't burn very well, so that helps :). There are sheets of heat shielding that you can buy for wood stoves to put on the wall and reflect the heat back and protect wood/sheetrock walls. I suspect something like that could help for any wall issues around heat.

.
Hi M;
I  will try to answer your questions.
#1)  Some RMHs and mass can weigh in around #4000.  If this load is spread over many floor joists it may be ok, but a qualified builder should be consulted.
It sounds like your home is one story with no crawl space.  If the ground is close by, then removing a wooden floor and pouring an insulated slab is a better safer alternative.
#2) No concrete on top of the wood floor, that is just asking for problems in the future.
#3)  Only someone qualified on-site can determine if you can remove a section of the wall.
As Fox mentioned, a new support must bridge the gap.
#4)  It is possible to exit a wall but not recommended.  An insulated metal chimney must be installed beyond the roof peak to maintain a good draft.
A vertical chimney through the house will stay warm and draw properly.

From your photos, it appears that you want a traditional J-Tube RMH with a piped mass.
Are you aware of the newest design RMH?  A hollow bell to hold your heat rather than a heavy piped mass?
And then there are Batchbox RMHs that burn longer and produce more heat than a J-Tube design.
There have been many improvements and changes in the last few years.

Check out some of the posts in the RMH forum and read all about the newest inovations.

Fox James wrote:I think you need to get a qualified builder to visit you house as it is too difficult to advise about knocking holes in walls from an armchair but i would say “yes” you can make a opening in virtually any wall if the corresponding support is catered for..ie a concrete lintel.
Mass heaters will always be better sited on a concrete base and generally speaking that base will need some form of insulation.
Have you checked your building regulations and insurance regarding rocket mass heaters?    

A little difficult since I speak the language only at about 50% fluency, and I fear that pretty much any "professional" I consult will charge me significantly more for their trouble. I'll have to try and get a recommendation for someone reputable. Silver lining is that building code is very relaxed here, as long as I don't want to build over 3 floors or within 3 meters of my neighbor's fence, basically you can do as you like. I was not planning on getting insurance.

Thanks for advice. I need to find a good source about pouring a foundation for a RMH and insulating it. Maybe I will only remove part of the wall? So half of the barrel sticks into the next room.

Matt McSpadden wrote:Hi M,
I agree that advising on the wall is difficult to do over the internet. However, if it is concrete block... why do you want to remove part of it? It would act like additional mass and soak up the heat and send it out the other side, without needing to remove any of the wall.

About the RMH, there are two pieces of the puzzle. One is whether the floor can hold the weight. This is possible, as Paul built a RMH on a wood floor, but had to add additional supports underneath. Does your house have a basement underneath? A crawlspace? Or build right on a slab?

The second piece is whether the floor/wall can handle the heat. I wouldn't be too worried about the floor, as all the RMH I have seen have quite a bit of rock/cob/firebrick on the bottom that should dissipate the heat. As for the wall, concrete blocks don't burn very well, so that helps :). There are sheets of heat shielding that you can buy for wood stoves to put on the wall and reflect the heat back and protect wood/sheetrock walls. I suspect something like that could help for any wall issues around heat.

I was thinking of removing the wall because I read that a RMH heats most effectively those areas which are in the line of sight. Also, I like large spaces. Unfortunately, they are harder to heat.

No basement. I don't think there is any crawlspace. I don't think this area suffers from frost heave. Seems to my very uneducated eyes, that the house just sits on a concrete foundation. Thank you for your insights about fire risk and heat shielding, interesting!

At the moment I am leaning more towards the "remove part of the wooden floor" option since long term I want to put down heated + stained concrete floors in most rooms. Seems like a good start)

thomas rubino wrote:.
Hi M;
I  will try to answer your questions.
#1)  Some RMHs and mass can weigh in around #4000.  If this load is spread over many floor joists it may be ok, but a qualified builder should be consulted.
It sounds like your home is one story with no crawl space.  If the ground is close by, then removing a wooden floor and pouring an insulated slab is a better safer alternative.
#2) No concrete on top of the wood floor, that is just asking for problems in the future.
#3)  Only someone qualified on-site can determine if you can remove a section of the wall.
As Fox mentioned, a new support must bridge the gap.
#4)  It is possible to exit a wall but not recommended.  An insulated metal chimney must be installed beyond the roof peak to maintain a good draft.
A vertical chimney through the house will stay warm and draw properly.

From your photos, it appears that you want a traditional J-Tube RMH with a piped mass.
Are you aware of the newest design RMH?  A hollow bell to hold your heat rather than a heavy piped mass?
And then there are Batchbox RMHs that burn longer and produce more heat than a J-Tube design.
There have been many improvements and changes in the last few years.

Check out some of the posts in the RMH forum and read all about the newest inovations.

Thank you for great answers!
1. Yes 1 story with no crawlspace. Pour concrete foundation, understood. I already have 2 huge builder's bags of cement here. Normally I hand mix it with a shovel, but I guess I will have to get a concrete mixer for this job. And build forms etc. So much to learn.

2. Understood. It is a longterm plan. Maybe I will only replace some of the floors with concrete to make the job more simple. How can I connect a RMH to a supply a heated concrete floor in the longer term?

3. Yeah, I'll need to get someone in. Maybe it could be done at the same time the roof is redone... I'd like to put a few more southfacing windows at the same time too.

4. Yes, I read this in the the textbook after I wrote the OP. I guess the more of the chimney that is inside the house, the warmer it is and the easier it is to get a good draft.

I wasn't aware of newer designs. I am working from the Wisner textbook (2016). I'll look up batchbox RMH, thank you!

thomas rubino wrote:.
Are you aware of the newest design RMH?  A hollow bell to hold your heat rather than a heavy piped mass?
And then there are Batchbox RMHs that burn longer and produce more heat than a J-Tube design.
There have been many improvements and changes in the last few years.

I found this website, linked on wikipedia. It seems like a pretty good source, but I don't know when it was written: https://batchrocket.eu/en/

M Ferguson wrote:I found this website, linked on wikipedia. It seems like a pretty good source, but I don't know when it was written: https://batchrocket.eu/en/

Most of the material on the batchrocket site has been written in 2016/2017/2018. There has been more recent updates, those are mentioned here and there on the site. Development has been going on through the years, latest additions are the double shoebox designs, the latest one (#3) isn't on the site yet. I am busy with development of a compact batchrocket core at the moment, results are looking good but no firm final design in sight as of now.

In general, the batchrocket.eu site has the most authoritative information on proven designs and methods. Posts from Peter himself addressing specific questions are also to be relied on.

Peter van den Berg wrote:
Most of the material on the batchrocket site has been written in 2016/2017/2018. There has been more recent updates, those are mentioned here and there on the site. Development has been going on through the years, latest additions are the double shoebox designs, the latest one (#3) isn't on the site yet. I am busy with development of a compact batchrocket core at the moment, results are looking good but no firm final design in sight as of now.

Hello! Is this your website? It is very useful and educational, thank you!

I read the whole website. Now I understand the advantages of Bell vs exhausting gas via ducts through a thermal mass. I also like the other advantages of Batch box mass heaters, such as greater capacity of wood, more efficient, and esthetically, I really like that the flames can be visible.

I have zero experience with casting, so I'll need to do a brick core. I read your notes about Double Shoebox Rocket, but space is not an issue for me, so the designs I am interested in would be "7: Another core layout" and also "Bell with dead-end benches" (except I would plan for only 1 extension of the bell to act as a bed). The "baffle board" is very ingenious. I've downloaded the 3D models. Thank you again!

If you have any other advice:

1. Would you place the RMH at point A in this house? (see original post for floorplan). These are the most used rooms, apart from kitchen and living room. I am thinking about putting another heater in the kitchen (already has a concrete floor), which will heat and also double as an oven.

2. What is the simplest way to make foundations for the RMH and to insulate them? I read on your site that someone used portland cement / vermiculite mix. Could I use perlite instead of vermiculite? I made a test cavity into this flooring, and found that there is no slab foundation. there is about 1 foot of crawlspace, if that is the correct term, and then just rubble and earth under that. I am thinking of using concrete blocks to build up to floor level, and then put a layer of some insulation similar to shown in "pool heater and pizza oven".

3. Long term I want to replace this wooden floor with an acid-stained concrete floor, which ideally would be heated. I don't know exactly how to do this safely. Could I use a copper pipe which would closely articulate with some hot part of the RMH and then connect with a looping tube system embedded in the concrete to circulate heat via convection? What is the easiest way of doing this? Thank you for any advice!

Hi M,
I have no actual experience with RMH's, but I do read a lot on this forum and watch a lot of youtube videos. I don't think copper pipe in cement would be a good mix.

I can think of two ways to do this. The most common way is to embed pex pipe in the cement floor and pump hot water through it. This can be achieved with a lot of different equipment, and should be possible using water heated with a RMH, but I think to do it safely you are going to need quite a bit of other equipment like pumps and relief valves and whatnot.

Another possibility is to dig down deeper, so that the chimney goes through the floor instead of a bench. Essentially just dropping the whole thing lower than normal. I have seen one guy experimenting with this in his garage. He seemed to think it worked ok.

Matt McSpadden wrote:Hi M,
I don't think copper pipe in cement would be a good mix.

I can think of two ways to do this. The most common way is to embed pex pipe in the cement floor and pump hot water through it. This can be achieved with a lot of different equipment, and should be possible using water heated with a RMH, but I think to do it safely you are going to need quite a bit of other equipment like pumps and relief valves and whatnot.

Another possibility is to dig down deeper, so that the chimney goes through the floor instead of a bench. Essentially just dropping the whole thing lower than normal. I have seen one guy experimenting with this in his garage. He seemed to think it worked ok.

Sorry, I was not being very clear in my words. Copper would be for the section of the system that wrapped around the flue of the RMH or otherwise was closely adapted to receive heat from the RMH. The system of tubing in the concrete would better be a different material, agree. As far as I can see, this is exactly how the current gas heating system works. Small pump to circulate the heated water to each radiator, and 2 relief valves, one for gas, one for fluid. I struggle to maintain this system at the "ideal" pressure, for some reason it is always too low or vents because I let in too much water. I can't see any leaks in the system... maybe it is just evaporating a bit the whole time the heater is running?  

To sink the RMH lower into the floor, it sounds like what you are suggesting would be similar to an "ondol" or hypocaust system? Yes it's an interesting idea

M Ferguson wrote:2. What is the simplest way to make foundations for the RMH and to insulate them?

Directly on the ground I would:

1. Put at least 5 cm of baserock and compact it.
2. Build wooden form, 20 cm high.
3. Put 12 mm rebars in a grid 20 cm on center, for bottom and top of the slab.
4. Pour concrete and vibrate it well.

This would make a proper foundation for building a quality heater. On this slab you can build canals from bricks laid on stretchers and fill them with perlite or crushed glass. On them you would lay bricks and that would be your base for building.

These specs would make a significantly structural foundation which would support a tall concentrated bell structure forever. For a bench style of layout, you could use a thinner slab just as well since the load is much more spread out, as long as the ground beneath is evenly compacted.

Cristobal Cristo wrote:
This would make a proper foundation for building a quality heater. On this slab you can build canals from bricks laid on stretchers and fill them with perlite or crushed glass. On them you would lay bricks and that would be your base for building.

Thanks for info! I'll follow this advice.

Would you advise to build a RMH (with bell) on an existing concrete floor, without any knowledge of how it was poured, only that there is no basement beneath?

The spaced-out bricks to hold the mass up off the slab are a good plan, but you do not   want to totally fill the spaces with insulation (an inch or so would be fine.) Insulation does not stop heat flow, it slows it down, and without freely moving air, heat will build up over time. This  is unlikely to actually damage concrete in a residential-scale application, but a wood floor could be overheated and eventually char and catch fire. Circulating air in the spaces will  let the heat get out to the room where it will be useful.

For an RMH with a tall narrow bell, I would want to know that the slab was thick or reinforced or both. Failing that, I would pour a reinforced slab on top of the original one with enough area to spread out the load over the total bell footprint or more, depending on the circumstances. A bench-style bell will spread the load enough that I wouldn't worry about it.

M Ferguson wrote:Would you advise to build a RMH (with bell) on an existing concrete floor, without any knowledge of how it was poured, only that there is no basement beneath?

I second what Glenn said above.
You could do a test drilling in the concrete to determine its thickness and use some rebar detector to check if it's reinforced. If it turns out to be thin, less than 10 cm and without rebars, I would still pour a slab, properly reinforced. For a heater with a bench, 10 cm thick would suffice. This amount of concrete and rebars is nothing compared to the cost of material and labor to build a good heater.
If the floor has some rebars, I would build on that.

I would rather have thinner slab with reinforcement that thicker with no steel. I saw many slabs with no rebars that cracked badly. Concrete without reinforcement is a waste of great building medium.

Glenn Herbert wrote:The spaced-out bricks to hold the mass up off the slab are a good plan, but you do not   want to totally fill the spaces with insulation (an inch or so would be fine.) Insulation does not stop heat flow, it slows it down, and without freely moving air, heat will build up over time. This  is unlikely to actually damage concrete in a residential-scale application, but a wood floor could be overheated and eventually char and catch fire. Circulating air in the spaces will  let the heat get out to the room where it will be useful.

For an RMH with a tall narrow bell, I would want to know that the slab was thick or reinforced or both. Failing that, I would pour a reinforced slab on top of the original one with enough area to spread out the load over the total bell footprint or more, depending on the circumstances. A bench-style bell will spread the load enough that I wouldn't worry about it.

So I understand, would you advise to put only 1 inch layer of insulation between the bricks even on a concrete floor, or you only advise that for wooden floor? (I decided to follow everyone's advice and not build on a wooden floor). Circulation of heat sounds desirable yes. If I pour a new concrete floor in the future, should I take this into account and raise the level of the RMH accordingly? Please see diagram to check I understood correctly:


1. bottom part of RMH and bell. 2. bricks. 3. 1 inch layer of insulation material, with airspace on top. 4. Original concrete floor, of unknown depth. 5 proposed future concrete floor, poured directly on top of the old, with floor heating.

So I decided to start by building a RMH in the kitchen, because there is already a concrete floor there and it is a simpler job for me to try first.
I am also wondering, is it really a big disaster if I DON'T pour any new foundation slab for the RMH, and then the original concrete floor cracks under the weight of a RMH with bell? This worst case scenario doesn't seem so terrible to me, because in the long term I will be pouring a new concrete floor on top of the original one, which will be heated. Or maybe in my ignorance I misunderstand the significance of a cracked concrete floor...

Hi M;
It is more than just cracking.
The concrete will spall, leaving rubble with no support for the core above it.
Even a few inches of perlite clay will stop or delay that.

There are two issues, structural and thermal.

If the old slab can't support the concentrated loading of the bell and cracks, parts will settle differently and you will get structural cracks in your bell. It might even collapse in the worst case.

If you don't protect the concrete, new or old, from overheating, it will spall and explode in the hottest spots. As those hottest spots will be inside the bell, you will get shrapnel which may destroy parts of your combustion core; this would happen while the core was running at full heat capacity. You can see how this would be a bad thing.