Rocket mass heater in basement with firebox feed above?

We would like to build a RMH but our little farmhouse is 100+ years old and the floor joists will not support the weight of the cob bench without a several thousand dollar upgrade, so I wonder… is it possible to build it so the wood can be fed from the kitchen (we currently have a kerosene heater cookstove and flue there we could remove or incorporate) but have the bench extend below the floor in the basement? Our floors here in Nova Scotia are super cold (and uninsulated) all winter so I thought this might help warm them from underfloor. Does anyone have any suggestions?

Could you build a bolted steel platform that sits under the existing floor and allows the RMH to sit in place?
As a Civil Enginner I can tell you its straight forward.

I am not quite sure if I understand your question, you want to have the fire upstairs and the mass downstairs or the want it all downstairs with some sort of  auto loading?

If you are considering using a long tube to feed your basement fire from the floor above you risk the very real potential of having the fire creep up the feed tube and reverse the flow of the heater.  Dumping smoke and flames into the upstairs living space.  I would not advise it.

I agree that it would be unsafe to have a remote feed like a chute from the floor above. If you cannot reinforce the floor framing from below and need to have the RMH in the basement, you would need to use a batch box which is loaded, lit, and left to run for an hour, rather than a J-tube which requires tending every half hour or so. It would be possible to have a bell in the basement which extends up through the floor so there is direct radiant heating in the main floor space.

Thanks guys!

I like the idea of a steel support, although wouldn't that be expensive?

Yeah, we were hoping the fire could be made above the floor as it is very difficult to get into our basement. It's an old rock basement with a dirt floor and lots of mice and spiders. LOL You have to enter through a trap door in the bathroom floor and down a small ladder as there is no stairway down into there.  I was worried about the drafting issue as well, which is why I asked. Sounds like this idea is not too feasible, nor is having to tend the fire down in the basement. If it weren't for the issue of going down there to tend the fire, I really liked the bell-above-floor idea.

Hi Meadow
Without actually seeing your floor joists and set up, it’s hard to make an accurate recommendation however, if you build your RMH close to the walls and/or floor supports and not make your mass too wide, you may be just fine.
Bells are hollow chambers which if spanned over a larger area could transfer the mass weight enough which could give you enough heat retention to suit your needs.

I think you can have your firebox or jtube at ground level and have a bell in the basement.
The bell could be built from the basement up through the ground floor.
The j tube or batch box would be supported inside the bell with angle iron.
Rocket exhaust would fall all the way to the basement as it cooled.
For exhaust,  run a system sized flue inside the bell from near the basement floor up and out of the top of the bell.
A tee right before it existed the top of the bell would be a good way to add a bypass, for easier start up.

With a dirt floor and no regular use, it sounds like there would be no impediment to pouring a small reinforced concrete slab and some concrete block piers up to floor level - all things you could do yourself with a tiny bit of knowhow or advice - and then cutting out a section of floor, framing around the hole to support any cut joists, and pouring another small slab as a base for the actual RMH. This is the most secure and complete version, you could simplify it if you know what you are doing or have competent advice.

Thanks guys, these are all great suggestions. I think the concrete pad and cinder blocks are doable with the bell type RMH. We have to decide whether we want to go down there and do all that work. Just getting materials down that small opening may be the difficult part.

It would not be expensive to create a bolted steel frame to reinforce the floor. And the bits will easily go through a trapdoor.
I believe you will need 6 or 8 uprights, [ 50mm galvanised water pipe ] with plates at the top and bottom to spread the load.
Then a series of angle iron cross bars and some diagonal bars between the columns.
If the top plates are screwed to the floor or the floor joist all the better for stability.
So they will look like a weeb truss.
Those x bars can be screwed with self drilling screws to the columns.

It just occurred to me that something like workshop heater that Peter built could be used in this situation.
The bell of this heater was  three(?) steel barrels stacked one atop the other.
I think the firebox was near the top of the first one, but you could put it up higher in the bell.
This should save a lot of money on materials compared to bricks or cement blocks.


To ease construction, you could start by opening up a hole on the floor where you want the heater to be, making it large enough to work in.
When you are done with construction, the area around the stove can be reconstructed with fire proof materials like steel studs,cement boards, and tile,

Definitely make the floor opening before taking materials down; it will make access much easier. The concrete/block construction is the Cadillac version. A steel frame supporting the floor framing (with good airspace and insulation under the RMH) would do the job fine even if it may not follow code. Some metal cutting and welding would be needed, which for some would be the easy part and for others would require professional help. I note that the basement floor is dirt, so some concrete pad would be needed.

Another consideration is that, depending on the layout of floor framing compared to desired mass arrangement, it may be possible to spread the load over a large number of joists near their supports, and not need any extra support in the basement at all. A good sketch of the house would help assess that possibility.

A bell starting from the basement floor could help warm the underfloor space while supporting itself solidly. The sunken shop heater idea is nice but would not give the mass to heat through the night in Nova Scotia. However, if the steel barrels were used as a form to either line or surround with bricks or cob, it could be a simple and effective construction method. Do you have access to either cheap old bricks, or clay for cob?

Good point on the mass.
Do you think the basement floor could provide the material for cob?
If so they could gain head space and obtain building material at the same time.

Maybe use slip forms and tamp earth around the the barrel?
I get lots of sheet metal panels from appliances.
These or something similar could be used in the basement as leave in place forms.
If the steel  barrels are cheap/easy to come by, they can also be a source of sheet metal for the outer forms.

Wow, more people responded and I had no idea! Great suggestions by everyone. How do I get alerts when people respond to the post anyway? LOL I have "Notify when a reply is posted" but I never got notified.