Using an existing platform as a RMH bench in barn

Hello, everyone!

We had an idea for using an existing platform in the barn as a mass for a RMH. I would appreciate any and all thoughts on this idea (i.e. would this thing work well as a bench or is it too big, would it be stupid expensive to get enough stovepipe in here, do we need gravel or can we just fill it with dirt, etc.).

The barn is probably a hundred years old and clearly had chickens at one point in time. Currently we're trying to clear out all the trash and put in some supports to keep the roof from collapsing. After we get the roof a little more supported we want to put a RMH in there to keep the chickens and their water from freezing. There is an existing platform in the corner that had some roosts on it. I think the idea was that you lift up the roosts and sweep up all the poop from the platform without having to bend down and shovel it off the ground. I think it's just dirt inside but it could be trash knowing all the other things we've found in this barn.







Thanks!

It looks like the platform might be about 4' x 6' (or 6' x 8' if the studs are 2' on center), which is certainly not too large. You could make a J-tube with a serpentine duct, though the number of bends required would severely limit the length of duct you could use. The rule of thumb is that a 6" J-tube can support about 35' of duct, and an 8" can support about 50' of duct... but you need to subtract 5' for each elbow before you turn up to the chimney. Six 90 degree elbows to get four 5' runs of duct doesn't sound very effective. The upside of this is that if you can get old discarded duct, you can use dirt fill for a very cheap and easy build. For an auxiliary use such as this, using recycled old red bricks for the combustion core could further reduce the cost in dollars to almost nothing.

The other way you could do it is to swap the serpentine duct for a "bell" - a hollow brick box inside the platform where the hot gases enter, heat the walls and ceiling, fall as they cool, and exit to the chimney from the bottom of the bell space. This would take more bricks (any kind you can find) and some ceiling material (old corrugated sheet metal would do for this application), all surrounded by packed dirt. You would want to seal the gaps in this box; ordinary mortar would work, or clay mortar without gravel or stones if you can find that locally, for zero monetary expense. A 6" J-tube would want an ISA (internal surface area) of about 40 square feet or so, and an 8" J-tube would use around 60 square feet. ISA counts only walls and ceiling, not floor. Your 4' x 6' (?) platform could hold a 3' x 5' x 1' high bell cavity for about 30 square feet, more if you can stretch the masonry larger than my guesstimate.

Thank you so much for the detailed response!!!  Great job on guessing the dimensions BTW, it is in fact 6'x8'.

We haven't actually made ourselves aware of the batch box concept until you brought it up.  Looking around on permies that style definitely sounds like it will better fit our use case!

I want to make sure I understand some of the nuances of your proposed design.  My husband made a doodle of what we think you are suggesting, and would like to know if we are off our rocker on this.

everything drawn over the box is intended to be inside of the box.

Also, would it be possible to make this exhaust chamber out of cinder block? Or should it really be done with brick? Asking because we know a source of an over abundance of cinder block.

red --> brick
grey --> the burn chamber
black --> Bell and exhaust
maroon --> dirt

That looks like a reasonable schematic. Don't confuse "bell" with "batch box"; one is a heat extraction chamber, the other is a combustion core. They are generally used together as batch boxes do not like the friction of duct-in-mass, but bells can equally be used with J-tubes. A J-tube is simple to build and reliable if you get close to the right dimensions, while a batch box is more finicky and would probably better be saved for your next build once you have some experience... unless you are already an experienced mason or builder.

Cinder block should work fine for the walls of your bell. The higher heat areas need to be at least old red brick, or firebrick or other refractory material to stand the heat.