Peter van den Berg

Can you use a rocket mass heater/stove to provide the heat for an in floor heating system?
Historically the Chinese and Koreans used an in floor heating system using a fire outside the home that heated flues under the home.

The systems you mention here are broadly following the lines of a roman hypocaust. Essentially a hollow floor where the gas stream to the chimney flows, (or chimneys, like hypocaust) through channels or under the entire floor. All systems like that has a raised floor.

Streching this idea a bit, you would need a pit, cellar of basement where the combustion core is located, feeding the hollow floor. This could be done, with the use of a reversed Shorty core. This is a latest-generation batchrocket core sporting a very low riser that won't exit at the top but at the front or back instead.
Have a look at the workshop report from this spring in France. The core that was used there happened to be a reversed sidewinder, and it worked flawlessly.

Looks correct so far, don't forget the clean-out.

Hi Thomas, I think I spotted the first mistake. The exhaust box, for want of a better term, is too cramped, it should project at least half a brick further to the rear. As it is, there's no chance that the projected stove pipe could go straight up, the outer skin's thickness would prevent that. The picture of the dry stacked layout shows this very clearly.

It might be that the current stove pipe position allows it to go straight up. In that case, the whole of the inner skin should shift (at least) half a brick to the front.

Cerbu Ulea wrote:is there the roar you know? https://www.youtube.com/shorts/KF6lO16Ve1I
the smoke /steam is low https://www.youtube.com/shorts/64-sAlZ1mkw , gas temperature in the open damper around 50 C, the bell heats fine, the terracotta wall reached about 40 C(i did not install outside control yet, left the bypass fixed a thin crack open)

It's a low rumbling, could be louder, I'd think. Gas temperature in the chimney could be higher, 58 ºC is the bare minimum to avoid condensation. Steam from the chimney is (mostly) absent, which means the heater is getting dry now. The system clearly works, congratulations!

Cerbu Ulea wrote:LE: the end of the burn temperatures , it was already warm from the days before https://www.youtube.com/shorts/H6zZ81-n59E , I think it requires the second  skin, splits are prepared

The outside of the heater is reaching 70 ºC and more, a second skin is required, in my opinion. Maybe not even splits, I would go for full bricks. What are the outside temperatures in a normal winter, where you live?

Cerbu Ulea wrote:there where 2 degrees C  , It was transparent to white , I just think it is steam as I increased the fuel and the bell finishes drying , tomorrow will be 2 thirds full. I will record in silence to listen if the roar is good.

This amount of visable  water vapor is quite normal while drying a masonry heater, given the low temperature outside. Keep in mind, fuel in the firebox loaded not higher than the port. And a bit away from the rear wall, this will give the best results. To my eye, the fuel could be more compact. The roar of a Shorty core is not as loud as compared to a first-generation batchrocket, although it's still very much there.

Cerbu Ulea wrote:Please tell me if a k probe inserted at the chimney's vertical start is good, or where else to measure , there is no pipe exposed, all flue is in-wall bricks

A k-thermocoupler in the start of vertical is good, normally I use a pen thermometer and place it at eye level though the chimney's wall. If you have a digital 2-channel thermometer, another one high in the bell is also a good indication.

Heather Arvensis wrote:The dimensions in the photo are not set in stone, by any means…. I would also prefer to extend the length of the right side arm a bit if you think that would be ok for friction?
(I circled the exhaust with red.)

The shape you are drawing now is doable, as long as it is within the guidelines for ISA. Lengthening the right side arm could result in a front corner that won't get warm at all, because the gases won't get past the corner. The exhaust into the bench should be still as wide and high as the bench internals, I can't picture how you would achieve that in this situation.

Heather Arvensis wrote:There are windows and doors everywhere… a tree trunk in the middle of the room… and the exhaust is already installed so lots of variables to work around. What if we did something like this quick, not to scale sketch?

Possible, this design has the better papers.

Heather Arvensis wrote:Since we are building mostly with cob I can do a bench wider at gas receiving end and narrower at the other end. I would maybe need a baffle or wall between barrel area and exhaust since this design would have barrel in front of exhaust?

Provided the following is met: the entire bench the same width throughout. There will be a warm gas stream along the ceiling of the bench and a colder one along the floor in the opposite direction. The less friction (again!) the better this will work. The division between colder and warmer gases isn't at the same height all the time, this varies quite a bit. Most people think like you: trying to "steer" the gases in the right direction. However, this could also be done another way, by making use of the temperature difference of these same gases. The warmer ones are against the seat, as long the forward velocity of the gases is slow, shortcutting to the low positioned exit won't occur. Feeding into the bench should be as high as the bench internals, and width as large as you can manage. I.e. not by a single piece of stove pipe.
Details how to construct this later.

Heather Arvensis wrote:(I’m not sure on exact measurements of width yet as that can be adjusted to hit the desired ISA. I would also need to get creative with supporting such a wide bench top span. I do have rebar, angle iron and bricks for towers!)
I do like the backrest bench design you did! We are still pondering that idea.

Seeing the space where the heater should be placed: the French bench, couch, divan or whatever opens up other possibilities. Placing it in front of one of the windows looks downright silly to me, because of the high backrest.
But what do you think of this: the combustion core in the middle of the room and the bench itself at right angles with the wall?
The door side of the firebox could be placed in any of three directions this way.
Plus the possibilty of a ceramic glass window on top of the riser.
The only French couch to date was a single-sided one, what about a shorter but double-sided one?
Maybe even placed at another angle instead of 90 degrees?

Heather Arvensis wrote:Thanks for the tip on including the barrel in the ISA… does that apply to jtube builds as well?

Yes, heat is absorbed and radiated out by the barrel, so it's part of the heat absorbing surface area.

Cerbu Ulea wrote:nice , mine being a sidewinder affects somehow the position of the top lighting pattern , or the position of the air slits?

Both of those aspects are the same as in a normal straight Shorty core. This has been tried and tested thoroughly, already over a year ago. In general, the aerodynamics of this core are very specific, it will function even without a door at all. Even the reversed sidewinder showed exactly the same behaviour during the workshop in France this spring. Much to my surprise, I might add.

Coydon Wallham wrote:The exhaust pipe is constrained some on the sides around it. Would cutting a vertical slit in the pipe and bending it back to enlarge the opening on the side toward the firebox be enough to increase the draw ability, or will it be necessary to break the cob, cut and reposition the whole pipe?

If you are able to reach into the bench, cut the pipe and bend the ends open, it might help. Breaking cob and redoing the exhaust thing I would keep as a plan B.

Cerbu Ulea wrote:I'm splitting an oak and a horse chestnut tree I was given this January, would this piece be too big ?

To build a fire in a lukewarm or cold heater, in that case it's too big. When fed into a hot firebox with somewhat smaller pieces around it, it would be consumed in short order. The following is an old video, taken from a first generation batchrocket. Nevertheless, the Shorty design is equally capable of this.