How to support rocket mass heater weight

So here’s the conundrum:

We want to build a small cabin with a low carbon footprint, so we want to avoid a concrete slab (HUGE embodied energy/emissions)

We live where earthen floors get damp and moldy and frost reaches many feet deep in winter. The only attempts I’ve seen near here, even above grade, have failed, so earthen floors seem inappropriate (even though they may be great for drier climates.)

So we are thinking of building a standard wood floor (well insulated) on small poured sonotube footings.

We want a rocket mass heater for the long cold winters…but how to support such a heavy, sprawling thing when we don’t want to use (much) concrete to reinforce the cabin floor, need to be careful with moisture issues, and need to insulate from extreme cold?

Any ideas??

We’re also considering a more upright masonry heater so a poured support wouldn’t need such a big footprint. Then, if we had to resort to concrete, it at least would be less of it.

How have others dealt with this sort of issue?

Clay bricks (so also firebricks) have much higher embodied energy than concrete.

Cristobal Cristo wrote:Clay bricks (so also firebricks) have much higher embodied energy than concrete.

Did you mention this in relation to my comment about maybe using a masonry heater (lots of brick) instead of a RMH? If so, that’s helpful to think about, thanks. I didn’t consider the embodied energy of the actual masonry heater since it’s made with so many bricks. Another good reason for me to figure out how to support the RMH and go that route…

Elizabeth,

I just mentioned it as the interesting fact.
So let's say the brick embodied energy is 3 MJ/kg and an average heater will use 1000-1500 kg of bricks, so the total energy will be 4500 MJ. Let's add another 500 kg for other masonry materials to round it up to 6000 MJ = 6 GJ.
Let's assume a kg of wood has 14 MJ of energy and you need let's say 7 m3 (2 cords) to heat the house. By using properly built masonry heater you can cut it in half, so you save 3.5 m3 of wood - let's assume 1000 kg. This way you already saved 14 GJ of energy - over two times more than the embodied energy of the heater material. Just in one year. By building properly - from tried out materials, the heater will last longer and savings will be even greater. By having proper foundation, it will be more stable, less prone to cracking and required maintenance, so it will serve even longer.

12 mm rebar mesh on 20 cm grid, on both sides of 20 cm thick foundation slab, the end of rebars bent to form L-shape. Distance of the rebars from the surface - around 20-30 mm. 20 MPa concrete (3000 PSI), and you have the perfect slab.
Stones plus lime based cement with pozolan would also work, but would be more expansive to make and ingredients not easily sourced anywhere. In this case it would be more difficult to add reinforcement and I saw cracked slabs without reinforcement too many times.

This is how I see it.

Hi Elizabeth;
I suggest a Batchbox-style Montana masonry heater (RMH)
Depending if you build a single skin or a double skin bell and what size core you use.
You are looking at a rough footprint of 4' x 5' with a weight of 2500# to 5000#.
Not that much extra support would be needed; perhaps two extra soni tubes with a wood or metal cross-beam would be sufficient to beef up the floor joists.

Hi Elizabeth, Following on from Thomas' recommendation above to build a batch box RMH... for your floor/foundation I would suggest you pour a concrete pad perhaps 18" wider in all directions than the footprint of your RMH. Making the concrete at least 6" thick so that the concrete is your foundation. You'll need to determine the design of the pad depending on your soil conditions and elevation of the floor above grade so having some form of piers or small diameter Sonotubes that are deeper into the ground for stability may be required. I would then install some insulation around the perimeter of the slab below floor level as well as underneath. This is the principal that I used when building my house area that has in-floor hydronic heat. I hate styrofoam, but 2" thick construction  grade sheets laid underneath and around the perimeter of the on-grade slab help significantly to retain heat in the slab. The slab will then become a part of the heated mass and provide you with a solid foundation from which to build the RMH.

I can tell you from the experience of my 6" BB-RMH system that is built on a 6" thick concrete floor in my shop area that does not have the insulated hydronic floor as noted above... the concrete 18" out from the perimeter of the RMH is roughly 18 deg F warmer than the rest of the floor in the shop. If you were to surround the concrete pad with insulation it will better retain the heat and likely run well more than 20 deg higher than the ambient air and become part of your thermal mass.

thomas rubino wrote:Hi Elizabeth;
I suggest a Batchbox-style Montana masonry heater (RMH)
Depending if you build a single skin or a double skin bell and what size core you use.
You are looking at a rough footprint of 4' x 5' with a weight of 2500# to 5000#.

Do we know how much the pebble-style RMH in the Fischer Price House weights?

Glenn Littman wrote:Hi Elizabeth, Following on from Thomas' recommendation above to build a batch box RMH... for your floor/foundation I would suggest you pour a concrete pad perhaps 18" wider in all directions than the footprint of your RMH. Making the concrete at least 6" thick so that the concrete is your foundation. You'll need to determine the design of the pad depending on your soil conditions and elevation of the floor above grade so having some form of piers or small diameter Sonotubes that are deeper into the ground for stability may be required. I would then install some insulation around the perimeter of the slab below floor level as well as underneath. This is the principal that I used when building my house area that has in-floor hydronic heat. I hate styrofoam, but 2" thick construction  grade sheets laid underneath and around the perimeter of the on-grade slab help significantly to retain heat in the slab. The slab will then become a part of the heated mass and provide you with a solid foundation from which to build the RMH.

I can tell you from the experience of my 6" BB-RMH system that is built on a 6" thick concrete floor in my shop area that does not have the insulated hydronic floor as noted above... the concrete 18" out from the perimeter of the RMH is roughly 18 deg F warmer than the rest of the floor in the shop. If you were to surround the concrete pad with insulation it will better retain the heat and likely run well more than 20 deg higher than the ambient air and become part of your thermal mass.

That's a good point about the concrete base becoming part of the thermal mass if insulated well from the exterior elements. Our current home has a foam insulated slab for in-floor hydronic heat, and it definitely does a good job. I just wish there was a different option than foam.

Sounds like you have direct experience with a BB-RMH. I've been hesitant with them because I've heard they're more finicky with airflow. Have you found that yours is okay in that regard? Anyone else want to weigh in on the finickiness of BB-RMHs compared to bench-style RMHs?

thomas rubino wrote:Hi Elizabeth;
I suggest a Batchbox-style Montana masonry heater (RMH)
Depending if you build a single skin or a double skin bell and what size core you use.
You are looking at a rough footprint of 4' x 5' with a weight of 2500# to 5000#.
Not that much extra support would be needed; perhaps two extra soni tubes with a wood or metal cross-beam would be sufficient to beef up the floor joists.

The weight figures are super helpful, thanks! Is this pic something you operate? Have you found it an smooth to run (airflow, backdraft, etc) as bench-style RMH's?

Hi Elizabeth;
Any style RMH may have trouble starting with a completely cold system.
I have had J-Tubes, First Generation Batchboxes, and a Walker riserless core, each has smoked back a time or two when cold.
I also recommend installing a bypass gate with all RMH builds to facilitate cold starts.

thomas rubino wrote:Hi Elizabeth;
Any style RMH may have trouble starting with a completely cold system.
I have had J-Tubes, First Generation Batchboxes, and a Walker riserless core, each has smoked back a time or two when cold.
I also recommend installing a bypass gate with all RMH builds to facilitate cold starts.

Remember all newcomers to wood burning stoves of any type or make, not just RMH's- the words of the first sentence (above) .  Any style may have trouble starting with  a completely cold system.   It is my opinion, that after the first burn and warm up, with the fact that most Mass based stoves, retain some heat by the next burning, which often cannot be said if standard types are allowed to go out, will lead to the last line here.

These massed based stoves, will most likely be good to go on the draw, every day.