Guidance on RMH build in a light wood construction

Hey Permies! We’re going to be starting the next phase of building on this light wood construction on a cabin that is roughly 532sq ft. One bedroom, one bathroom, open kitchen/dining area and a loft.
It’s raised up 2ft off the ground on poles that are embedded about 2ft+ into the earth, the round poles are about 4in in diameter. The floor joist frame has 2ft x 2ft square spans, plenty to support the 120mm SIP panel floor we’re planning on installing. There will be a ¼” composite floor on top of that.

There’s a few buildings on the property built this way, though with a more traditional plywood + barrier + floor sandwich. The cabins are lightweight, inexpensive, and DIY.
And they are only lightly insulated/sealed and can get quite cold, though never freezing in the winters. We’ll be using light claw straw in the exterior walls of this building to insulate.

Some have conventional wood burner stoves and we don’t want to use that in this house - too inefficient and I’ve been wanting to build a RMH heater for awhile now, but this would be my first.

The climate here goes up to 90F in the summers and down to 40F in the winters, so not so challenging in terms of the cold, though it does have regular strong winds from the South West, the direction in this photo.  Elevation 1,500ft. Flat terrain, mountains to the east and ocean further to the west. It rarely is humid here, so moisture and evaporation are not a challenge. The goal is really to make this lightweight cabin a home that’s cozy and warm all year around. This in in central Chile and there's also frequent earthquakes. The lightweight construction dances and flexes well in these conditions.

I don’t intend to build the RMH or the bedroom wall over the SIP floor/compositing flooring, we would create a different structure over the joists to support the higher mass density of the RMH.

I’ve been reading the forums, read two of the books (RMS Superefficient and RMS Building’s Guide), and checking out videos and I see that some have built onto wood floors with some reinforcement below or in between the joists. I’m thinking something simple and maybe smaller than what’s common since the temperatures don’t get so cold in this climate. My biggest uncertainties about building a RMS in this building are with are weight, isolation and positioning

My first preference is to build the RMH below the stairs to the loft since this space is not particularly useful for anything else and we could use the lower half of the bedroom wall as thermal mass if we build it out of cobb. We could even use the cobb bench as the lower landing for the start of the stairs.
We can frame the rest of the wall over the cobb mass. If it’s at all possible to keep the weight under 1 ton and still have enough mass to have safe temperatures on the outer surface of the cobb to the touch. The maximum space I think I can work with in this location of the building is 9.5ft along the length of the bedroom wall and 3ft along the exterior wall until it comes close up to the exterior door. The stair landing would be built over that corner near the exterior door. The exhaust pipe either needs to go up through the roof, or if I can squeeze in a third row of exhaust, it could go out the side of the building. The eves extend 2ft out from the wall. With only two rows of exhaust, that only leaves about 18ft of pipe.
My concern is this will be too little of mass to absorb the radiant heat before it exits the mass.

None of the RMH in these images are the correct scale and detail yet - at this stage I’m mostly just playing with the overall space consumption. Once I decide where to build I will draw up a scale model with the correct proportions of burn chamber, riser, insulation, etc.

What do you think of that idea? Too small of space for the needed mass? Too risky with the stairs above the RMH?
None of this except the frame/roof has been built yet so we can modify the design and materials of the stairs to fit over the RMH in the safest way available.

If I were more experienced I think I would also go for bringing outside air intake through the floor to counter the negative pressure.
For this build it seems it has enough challenges that I’m thinking a conservative 6” design, J bend, Cobb mass, with an exhaust pipe going straight up through the roof. Unfortunately I don’t see a way to put the exhaust pipe up through the gap in the roof ridge, since the bedroom door is right in the center.

Other ideas I’ve had are extending the mass through the bedroom along the wall to the end of the house and exhausting out there. This would leave a narrow, low bench all along the wall and not interfere too much. I might even be able to get away with very little reinforcement along the single pipe bench.

Or probably the most reasonable option is just to move the RMH to the living room and occupy that space in a way that I don’t prefer.

Or forget the J Bend + Cobb Bench and build a shorty core with LOTS of reinforcement under the floor in that location?

Or a CottageRocket? https://permies.com/t/150536/Uncle-Mud-CottageRocket-Paper


Other ideas?

Also, if it seems totally silly to put a RMH in a house that doesn’t even reach freezing temperatures or if it looks like I’m just using this as an opportunity to build a RMH when it doesn’t belong here, please tell me:)

Thank you for your time and energy, I’m so glad this community is here.

Hi Mathew;
40F with wind is plenty cold enough to have an RMH. Add some rain/ sleet, and you will really appreciate it.
I suggest you consider not building a bench with pipes.
Most new builds utilize a brick bell (stratification chamber).
A much better use of space than a long immovable bench.

Shorty Core is a superb choice; being "short," her bell could fit under your stairs.
Construction is more technical, but still well within most folks' capabilities.
A 6" Shorty utilizes a bell apx 4' x 5', easy to add a few extra supports underneath, weight with bricks should be apx #2000- #2500

An 8" J-Tube is very easy to build, though it requires headroom for the tall riser, and the space under the stairs would not be ideal.

Also, there is a big difference between using a barrel versus using a brick bell.
The barrel will rapidly throw heat in the room, and can quickly become too hot.
A brick bell takes some time to heat the bricks before they can heat your room, but once heated they will hold and share that heat for many hours

I fully agree with using a bell instead of a piped mass. I do think that with your climate and likely intermittent or light heating requirements, a J-tube would be quite practical, and easy for a novice to build. With the core and riser in the location you show, you can build a single-skin brick bell to clear the riser and have enough space to the stairs above to be safe. (I would not feel safe with a barrel under the stairs at all.)

I would consider a narrow tall bell (just big enough for good airflow around the riser) with a bench-height extension to the left. This would all need to be sized for the right internal surface area. Batchrocket.eu gives the complete information on this; an 8" J-tube has been found to be similar to a 6" batch box for bell sizing purposes. If you can exit nearly straight up, I believe a 6" chimney will serve you well. An 8" chimney would be bigger than you need, and you would have lazy exhaust and possible condensation issues. I would not be concerned about oversizing a J-tube, as you can burn just one load a day if your heat needs are slight, and not overheat the house.

The stair location looks ideal for evenly heating your house.

For structural support, I would suggest adding a couple of posts between the existing ones under the bell, to equalize the loading. I also like your idea of making the mass exposed on the bedroom side.

With your climate, please strongly consider a bypass

Thank you! This is exactly the guidance I was seeking!
So we'll add thick posts under the floor between the joists, supported by concrete to hold the load. I can build some of the thermal insulation into the structure of the cavity beneath the heater where we won't lay any SIP panel to bring it flush with the top surface of the adjacent SIP panels.

Either a narrow brick Bell J-Tube or a batch box with a bypass!
I'll research the design and sizing next using the resources you've linked.

Thoughts on sending the exhaust out the side of the external wall or out through the roof?
I'd need add a bend towards the front of the heater to clear the stairs if I go straight up, so probably a little bend away from the bedroom wall towards the front of the heater to get the exhaust from the rear to the front to clear the stairs to get up to the roof. Not visually appealing, but maybe more sensible.
Or I could move the whole heater further out from the bedroom wall so it's only half under the stairs to make it easier to pass the exhaust pipe straight up.

If I were to go out the side of the external wall I would need to position the exhaust under the landing of the stairs and near the bedroom wall, which might not be possible.

Since we're starting without a floor does it seem like a good opportunity to include outside air intake from under the house or is drafting too inconsistent? Here the wind always blows in from the SW, so I'm theory I could build a little wind catcher under the floor that would prevent negative pressure from the intake, but it would still have variance in the airflow compared to pulling inside air.
Maybe I'm totally missing something in the compatibility with an outside air intake from the floor and this style RMH.

Hey Matt;
Definitely through the roof, as straight as you can.
Through-the-wall generally requires a tall, insulated outdoor pipe and is only recommended if through-the-roof is not an option.

Installing a bypass pipe during construction is always a smart move.
Unless you choose a Shorty Core, you do not need to connect it.
If you have draft issues OR might want a small fire on a cold, wet summer day, having the option of a bypass is nice.

Fresh air supply.
With a tight, newly constructed home, a fresh air supply is important.
However, unless you live in extreme cold temperatures, I suggest cracking open a window rather than a fresh-air pipe.
Your RMH, no matter the style, will only be lit for a few hours a day, unlike a traditional wood burner, which is kept going 24 hours a day for months on end.
As soon as your fire dies out, you close off all intake sources (leaving the exhaust open). This restricts the natural draft from pulling your stored heat up and out.

thomas rubino wrote:
Fresh air supply.
With a tight, newly constructed home, a fresh air supply is important.
However, unless you live in extreme cold temperatures, I suggest cracking open a window rather than a fresh-air pipe.

Can you explain, please? I'm curious why you'd want to lose already-heated air from inside the house rather than keep the whole supply/exhaust linkage external.

One other thing I wish was provided more often with discussion of non-[salvage+cob] RMH builds in general, is a transparent cost comparison of time & materials vs. a conventional heating system. Recently I watched this Youtube video (https://youtu.be/56L-_YsnFcU?si=F6DRuEnSIeCdAVLG) of what looked like a similar build to the one being described here, and for instance the guy mentioned using these heat-resistant fiberboards that cost $100 each, for what looked like a 2'x4' piece of material...he used layer after layer of them, and that was just one component out of many expensive ones, including a custom weld job that for most people would probably run thousands of dollars...and I'm thinking "You'd have saved money and time with an electric furnace, at least for the first 15 years or so" but who knows. Without the cost comparison I can't tell.

Hi Ned;
Sounds counter productive does'nt it?  Opening a window in a home you are trying to heat.
The answer is easy.
When you are heating with bricks, the amount of gentle heat that fills your house is so pleasant that you will want a source of cool air, for you more than the stove..
Hard to believe if you have only lived with a metal box stove.

Then there is this.
Consider installing the fresh air pipe. A two or three-inch PVC pipe is entering your home.
This may be an easy install, or it may be more involved than you expected.
Now you have a cold pipe coming directly into your home.
Remember, this pipe must be insect, mouse- & snake-proof.
Now it's six am, you want a fire, first thing you must do is open your fresh air supply, no problem, right?
You build your RMH fire, and it warms your mass.  Great! Except you must remember to shut that fresh air supply when the fire coals out... no problem, right?
After all, you also must remember to shut your air intakes on the stove itself, or your nice heat will draft right out of your mass.
Day after day, all winter long. But what if you are not home and someone else is in charge? Will they remember?
Uncle Joe or cousin Mary has been burning fires their whole life, but not with an RMH.
If they forget, your air intakes and fresh-air pipe could be open the entire time you are not home...
They will not be impressed with this RMH that you have been bragging about, not nearly as nice as you made it out to be...
Now let's talk about children.  Nothing more fascinating than an open pipe that you can drop things into...

A cracked open window is something Uncle Joe can understand; it will not be forgotten.
No creepy crawlies are entering unannounced, no children will be dropping things in the open window.

I guess, in my opinion, adding a pipe for fresh air is extra work that's not needed.

Ok! Thank you. What if my fresh air supply is simply a hole in the floor that has a door/plug I can open and close right near the base of the RMH door air intake?  
When I want to pull in outside air through the windows I can just leave the plug in and when I want to limit the amount of outside draft coming in I can just pop the plug out of the floor near the RMH intake?
Think it will do any good or will it not make much of a difference?

As a beginner to RMH heater building and with no experience with masonry is it too ambitious for me to build a 5” batch box with a bench as a bell? I really like the idea of it and I think it will fit better in this space than any J-Tube I could design, even though I keep reading that J-Tubes are easier for beginners to build.
I do have help, people who know how to lay brick, who can weld and a few engineers in the family, but none of them have built a RMH before, so they are also beginners.
6” batch box is what is recommended, but I’m tempted to go with 5" to keep the size and weight low.

Is the reason 6” is recommended is that the 5” will not reach the temperatures to efficiently burn the wood fuel? I do see the 6” batch box designs are more common and frequent and 5” seems rare.

Here’s the idea so far from what has been suggested. I'm still trying to do the calculations on what the actual dimensions on the outside will be. The information to calculate the inner parts is excellent, now I need to research the size of the bricks that are available to buy/salvage in this region to calculate what the total size of floor space that the single skin bell bench would occupy.

Hi Matt;
A hole with a plug is a much simpler idea than a pipe.
But I still say that with a stove that only runs a few hours a day, it is not necessary.
Builder's choice for sure!

Now, let's talk about construction.
Scary Scary right???  No, not at all, it is very simple stuff once you get started.
To begin, you do not need any cement, refractory, or basic.
All construction is done with a clay/sand mortar.  Easy on the hands, cheaper than refractory.
If something does not look correct, it is simple to stop and back up, a rubber mallet to pop bricks, a bucket to scrape the mortar into, and some water to rehydrate the mortar! Try that with concrete.
So you might be thinking, " Oh, that could just fall apart, without a permanent bond... not so, once dry, clay mortar does not just fall apart.

Next, nothing wrong with building a 5" batch rather than a 6".  Most new builders think they need an 8"
Here is the bottom line about sizing.  The only difference is the size of the firebox and how long it will last at full burn.
Yes, the "ISA" (internal surface area) of the bell varies, but the core unit itself is identical in shape and function.

Looking at your design,  do you know the ISA of your bell?
Although a warm bench to sit on seems like an outstanding idea, in reality, they are hard and require cushions, as well as taking up a large portion of your space.
In your design the bench portion is short and cramped. Although it might become a short-term seat to quickly take off the chill, and remove your boots, I doubt you would be spending much time hanging out there... but I could be wrong, as always, it is the builder's choice.
I suggest forgetting the bench and going for as large a bell as the ISA permits.
ISA size information can be found at Peter Bergs website   https://batchrocket.eu/en/

I offer a hard copy or a PDF book on first-generation Batchbox construction on my website https://dragontechrmh.com/

You're building an empty brick box with a Batchbox core inside, and the chimney vents from the bottom of the brick box.
It's so simple a caveman could build one... well a progressive extra smart caveman...

When you think about the size of the core, I‘d say bigger is better, because it needs less firing. In other words, with a big core you might be fine with one fire per day, while a smaller core will ask for two fires per day, to keep your house warm.

On the batchrocket site you will find a way to calculate the energy consumption of your house and which core size will fit.

But the limiting factor is the chimney diameter. The core size may not be larger than the chimney diameter.

Another factor is the floor construction, how much weight can it carry?

I’d also advise to build a simpel bell without a bench. And take a look at the shorty core!

Do some maths first, you have to know for example about base number and ISA of the bell. Then when you have decided in which ballpark you want to have your core, take a look at the refractory available to you. It makes the build much easier when you choose a core size, that asks for only some few refractory cutting. The less cutting the better!

I agree that a simple bell will be considerably easier to build than one with a bench; if you can fit a simple rectangular bell with the right ISA under the stairs, do it. My main thought with the "bench" extension was to get enough ISA under the sloping stairs.

A small chair or two that store under the stairs and can be pulled out when you want to sit close to the fire would work nicely. We have a couple of old wooden little child-size school chairs, very sturdy and perfect for sitting and feeding the fire.

Thank you Thomas, Glenn and Matthias!

There are readily available 4.5", 5" 6" and 8" exhaust piping here.  There will be about 4.3m of exhaust to reach the roof ridge exit out of the building.
I was considering going with this 4.5" design, it's incredibly detailed and the author is really generous in walking through each step of the build. It's too tall to fit under the stairs, so I could make the bell shorter and expand it's with and depth to match the 3.2m2 ISA.
https://roquetinho.eu/en/The_stove.html

I do think I have space for a 5" (125mm) and to meet the ISA of the bell am considering a bell ISA of 65 x 65 x 126cm (3.7m2) bell to fit under these stairs.
I'm thinking a straight brick core since it's my first and this is the most simple, though I could be tempted to try a shorty core, but the bell sizing is a bit more guesswork for a shorty core, yes?

For floor support we worked out a plan to add concrete and bricks under the floor to the ground to support the structure of the RMH. It's only 50cm up from the earth so not so difficult to build up.

I am realizing I need some advice on exhaust positioning before I commit to sizing. The exhaust pipe needs to ascend vertically near the front of the RMH to clear the stair railing and roof rafters that are directly above. Do you think it's best to add a bend in the pipe from the rear to the front near the top of the bell or even just under the stair where it's coolest?

Or could I create a sort of sidestep at the bottom of the bell that would allow the exhaust to be positioned near the front and go straight up from the bell step? Or is too close to the burn chamber and will cause airflow issues since it might soak up heat from the burn chamber?
I've attached an example.

Hi Matt;
I like your design with the step out for the exhaust; both my Shorty core build and the Studio Dragon build use one to bring the exhaust stack into proper position.
Your core heat will rise before sinking; you should not have any issues.
A straight stack is always preferable. Be sure to position the pipes so that all moisture stays inside, and I suggest using metal duct tape at each pipe connection.
Shorty core is easy to build, except for the three slabs needing to be poured.
I believe Peter, reduced by ISA 10% from the first-generation Batchbox design.
It does require a bypass pipe to ease cold starts, but with a stepout for the chimney, installing a bypass is pretty easy.

Great! I'll go with the exhaust step.
What I haven't found clarity on is do I include the exhaust step and walls of that box (directly below the heat box) in calculating the ISA of the bell, excluding the top plate of the flue step since it's unlikely to absorb heat as it is directly below the heatbox?

Or is the bell ISA strictly the area of the 5 sides (4 sides and top plate) above the top plate of the heat box? Therefore, the higher I raise the position of the heatbox from the bottom of the empty cavity, the less vertical space available for the bell to absorb heat, reducing the ISA of the bell unless I expand in either width or depth?

Hi Matt;
The exhaust box is not counted.
It is very small, located at floor level with an opening of apx 8" x 12".
The walls and roof all count inside the box, unless insulated.
The height of the core will change, where the highest heat is concentrated.
It does not affect the output of the Batchbox.
Peter calculated the correct ISA for each Batchbox size's output.

Thank you, so I won't include the exhaust step in the bell ISA calculation.
Here's the part that I'm not clearly understanding from the instructions:
"Equally, if the firebox is built into the bell then the surface area of the firebox within the bell won't play a role in calculating this area as no heat is absorbed there."
Is he referring the SA of the firebox itself or the walls of the bell that surround it?
A visual to compare.
Do I calculate ISA from the SA the bell only above the firebox? Figure 1, selected in blue
Or do I calculate ISA from SA of the bell above and surrounding the firebox, including the face of the door as SA? Figure 2, selected in blue

The door wouldn't absorb heat, so it's not clear if it should be included.

Imagine the bell as a cube of 1x1x1 meters. The ISA simply would be all (inner) surface areas minus the floor. So in case of the cube 6 minus 1 equals 5.

For loading the core with wood it is much more comfortable to have the core a bit higher in the bell. About half a meter above the floor is a good hight.

Hi Matt
All four walls, floor to ceiling, and the ceiling itself are measured.
The core and core door are not measured.

Matthew Galloway wrote:Here's the part that I'm not clearly understanding from the instructions:
"Equally, if the firebox is built into the bell then the surface area of the firebox within the bell won't play a role in calculating this area as no heat is absorbed there."
Is he referring the SA of the firebox itself or the walls of the bell that surround it?

Just because people tend to be confused about what should be included into the ISA, I tried to simplify the method. Pretend there isn't a core at all in the bell, just calculate the wall and top and that's it. Reason behind this: when painstakingly calculated, the door opening should be left out, and the core should be included for about 20 to 25 %. When started from cold, the core will take up some of the produced heat, but shortly after that it will emit heat instead.

Calculated the simple way, the piece of wall where the door opening is, will be on a par with what the core will extract. So please, don't overthink this, just take the four walls and the deck, all internal. The result will be exactly fitting for the intended core.

Oh thank you for explaining - it makes sense - I just want to double check if I was missing something.
So if I'm going for 5" and just a little under the bell ISA of 3.7m2,
I'll do something like 65 x 65 x 125cm internal bell size with a step out the side for exhaust. I still need to factor in the brick sizing here - it may make more sense to size it a little different if it works better with the proportions of the bricks here. Both clay and refractory are common here in 6x15x30cm.
Now I can get into the fun of designing it.