Pea gravel versus sand

[Rick Thornton]

Hi Folks,

I've been lurking here on and off for over a year and am finally going to build an RMH for a temporary application. Because it is temporary I will not be cobbing what will serve as a heated bed platform, but instead will be encasing the heated piping in either sand or pea gravel. What is the preferred medium?

Thanks for your time and your help.

Rick

[satamax]

Rick, neither.

In my opinion, use a half barrel system in your bench, set up as a bell, with cob at the bottom to make it gas tight. Then cover that with fist sized rocks. Instead of having lots of air trapped between the pea gravel or sand, which would act as an insulator, at least your heating the air and having some convection movements between the rocks, heating theses.

[matthewwalker]

I wouldn't be scared of the cob with regards to it being temporary. If you just use local "premix", as Donkey calls it, it probably won't be all that difficult to break up when it's time. Build it the same as you would with gravel, i.e. fill a box with mud instead of gravel. The only downside is the moisture that you will bring into the space that needs to be driven out with the stove. I actually think it would be easier to remove, since it will come out in chunks. It's certainly better with regards to sealing among other things. Definitely don't use sand. It's a great insulator once dry.

[Rick Thornton]

Thanks, Satamax and Matthew. The insulating capabilities of both pea gravel and sand, as you point out, would not make these mediums good heat batteries. All my "lurking" did not not inform me of this, but now I know.

While I don't relish introducing into the living space a lot of cob that needs to dry out I guess I have no choice. I'll make liberal use of stones to cob into the mass to limit the amount of cob.

Matthew, while I have you, I'm following your Youtube videos for casting the core and heat riser, and I just want to thank you for providing those.

So, if I go the half barrel/bell approach I'm going to need some help envisioning connection details. My lurking hasn't put me in contact with concept sketches, although I have seen Matthews outdoor half barrel RMH on Youtube.

Thanks, again, guys.

Rick

[Rick Thornton]

I've been perusing the various threads and am trying to get an idea what I need to do to insulate the bottom of the bench which will be sitting on a wood floor. On another thread I saw references to rock wool and superwool. Wouldn't the extreme weight of the bench compress either of these materials?

Rick

[satamax]

Well, dunno if insulating the bench is that important.

[morticcio]

I used 4" Thermalite blocks as a base for mine - provided insulation and added some height to the bench.

[Donkey]

Most importantly, ESPECIALLY on a wood floor, create an air gap below the combustion chamber and the first few feet of bench. Be sure that the air gap can circulate and take heat away from the area. No matter how well you insulate, heat will still build up below it.

[Rick Thornton]

Max, if I go with a half barrel bench over a wooden floor you're saying it would be unnecessary to provide insulation between the bench and floor?

Schematic in my head: Existing wood subfloor; old floor tiles (possibly containing asbestos, according to a friend); Pergo floating floor with vapor/cushion membrane underneath.

Now for the bench:

On top of the Pergo I would put down perhaps roof in felt paper for a vapor barrier; 3-4" of cob; half barrels; stone and cob over and around the barrels with a wood frame boxing it all in and finally a piece of plywood on top to carry a twin size bed mattress.

Reasonable? Unreasonable?

Rick

[Rick Thornton]

Hi Donkey,

I'm planning on placing a Matt inspired cast core with riser and barrel outside the structure and placing the bench in a small bedroom, so no combustion chamber in the house. The half barrel system/bench is all that will be on the existing wood floor.

Rick

[matthewwalker]

Rick, glad you are finding the videos valuable, thanks for letting me know. I appreciate hearing that. As for your mass on the floor, I doubt the bottom of the mass under the barrels would ever get terribly hot, but I'd still probably put cementitious board, like hardibacker or durarock, raised up on some bricks, under the whole thing to get an air gap under there. Those boards are only about $10 a pop, so it's cheap insurance. It will insulate the mass so you get better performance, keep moisture from getting trapped under that pergo, keep heat from building up where you can't monitor it, and mostly just give you peace of mind since you will know what's going on under there. That said, I've done a half barrel mass on wood floor in an intermittently used shed and used perlite/cob as an insulator underneath the barrels. I'd still recommend that approach even on a raised platform.

I would encourage you to reconsider putting the burn chamber outdoors, for many reasons that you have probably read before.

[Rick Thornton]

Tell me more, Matt. What reasons would make placing the combustion chamber/heat riser inside the living space a better idea?

I should probably share that the "living space" is what remains of an old bungalow that is now surrounded by a straw bale house that I'm completing. The remnant bungalow is now just a 15'x24' space in which I have kitchen, bathroom, small bedroom and small extra room. The bungalow walls are now uninsulated and free of the vermiculite that once filled the stud bays. Same is true for the ceiling--no insulation to hold the heat in the living space.

The straw bale structure surrounding the incrementally de-constructed bungalow has not been fully weathered in and lacks insulation in the ceiling. There is also a wood burning stove in the 15'x24' basement that the old disappearing bungalow sits above.

So, with space inside the living space of the bungalow at a premium as well as a desire to heat the larger space created by the surrounding straw bale structure, I hatched the idea to place the combustion chamber/riser on the outside of the bungalow wall and the bench, of course, inside the living space.

Did I just confuse you?

While I have you Matt, I'm about to complete the box and burn tunnel form for the cast combustion chamber you've kindly posted to YouTube. For the heat riser you state that it requires 1 bag of fire clay and 2 c.f. of perlite. For the burn chamber you don't give an amount of fire clay, just a proportion of 14:14:1 for fire clay, perlite and refractory cement. Not knowing the amount of fire clay needed to complete the combustion chamber I bought 3 bags of fire clay, one for the heat riser and two for the chamber. Will the heat chamber require two bags? More? Less?

Thanks for your help.

Rick

[matthewwalker]

Rick, I was able to cast my chamber with two 50lb bags of fireclay, but I received a comment on Youtube from a viewer who ended up short with that amount. It's hard work mixing it, so I'd suggest tackling the two casts in different sessions. You might get by with just the three bags total, but some folks are finding they need more than two for the core.

These heaters are unique in one very significant way from either wood stoves or strictly masonry heaters. They have both an intense radiant component from the radiator barrel, and then the gentler radiant/conductive component from the mass. One thing that to my mind makes them so wonderful is that while you are in the space, you have a cheery fire that you feed every 45 minutes or so. The radiator puts a whole lot of that energy into the room and quickly heats the space to a very comfortable temperature. When you are warm enough, or leave, or go to bed, the fire goes out quickly and the mass takes over keeping the temp from sloughing off. If you put the burn chamber and radiator out of the living space you will find yourself out there feeding it often, and since you are missing out on the intense radiant component inside, you will burn a whole lot more fuel trying to get the mass to bring the space up to temp rather than just having to hold it there.

Additionally, it's just dang nice to sit on them next to a crackling fire with rosy cheeks from the hot radiator blasting away at you. I strongly recommend you embrace the whole thing in the living space, you will absolutely love it. With it outside, I imagine you would be less thrilled with it overall.

[Rick Thornton]

Thanks for your time and help, Matt. You are greatly appreciated, and your advice is well received.

I may be forced to locate the burn chamber outside the bungalow due to space concerns, and while I'll be the poorer man for it, as you ably describe, the barrel will be able to heat a space that will be weathered in over the next weeks and months and I'll need to heat that space. If I didn't confuse you, while the burn chamber is "outside" it occupies a space that is inside the straw bale structure. I'm sequentially expanding my habitat and the whole works is still under the same roof.

Thanks again, and thanks for the fire clay info. I'll procure another bag.

Rick

[Rick Thornton]

Following Matt's excellent Youtube videos I have built the box and the combustion chamber mold, prepared the heat riser barrel by burning off the paint and cutting the 8" hole, and have cut the first of a few more 55-gallon drums in half lengthwise. After I finish this post I'm going out to mix up Matt's recipe for the cast combustion core. I fear my questions will go unanswered before I'm ready to proceed, but such is life.

But already the gracious and generous nature of this forum has armed me with better ideas which have seen me decide to locate the burn chamber and bench together down in the basement. My questions are thus:

With a concrete floor, is it advised to lay the half barrels directly on the floor and cob them in?

If I'm seeking to make the half barrels perform as bells, what would the holes in the ends of the barrels look like in terms of shape, size and location?

How would I connect the half barrels? Butt them and just cob over?

As per Matt's idea, I'm building an 8" riser. How many half barrels would I want to link together?

There is an existing wood stove in the basement and I'm tempted to exhaust the RMH into the 6" metal flue that serves the stove. Are velocities and volumes going to permit exhausting into the existing 6" pipe that has about 20' of vertical distance before it exits the roof?

And then there are questions about trip wires and P-channels, but then I started a new thread to ask those questions.

Now it's time to mix Matt's core recipe. Wish me luck, and thanks so much for your time and help. It is greatly appreciated. I'll post pictures when things start coming together.

Be well,

Rick