So here's a look at the combustion chamber I recently cast for my first RMH build.
I used 1/2" chip board as it was inexpensive to build the mold of the inside chamber. Built it so the outside dimensions where 6" square, that way when the wood burned away on the first fire, the inside diameter of the chamber would be 6"
Using some waxed poster board we had laying around the shop from our sign painting days, I covered up the turbulence curve below the heat riser.
Then built a box to hold the cement in a uniform shape when poured around the combustion chamber.
Here's a video of the first burn. We waited 24 hours for the cement to cure, then only fired it for about 20 minutes. On the second burn, we put 3 hours worth of wood through the combustion chamber, and the outside of the cement was merely warm to the touch.
John Elliott wrote:That looks pretty hot! Are you going to share your recipe for refractory cement with us? And how it holds up for more burns? Inquiring minds want to know.
Sadly I went the cheater route and bought castable refractory cement so I have no recipe. Used two 55lb bags of KS4V+ (rated to 2,600 degrees) and mixed in about a 5 gal. bucket of perlite to reach the volume needed as each bag of cement was roughly $60. I built a 6" system, but if you went for a 4" the two bags alone would be plenty, and an 8" you'd probably want 3 bags plus a little perlite as filler. Mixed just like regular cement, poured just like cement, and after 3-4 prolonged burns there have been no stress cracks or signs of problems in the burn chamber.
The beauty of casting your chamber the way I did is you have no pinholes or leaks. You get an extremely efficient chamber for a fraction of the cost if you ordered one, and you don't have to mess around with bricks and clay slip.
I'm interested in what you call Cast Refractory Cement, is that something bought from a kiln/refractory supplier? What temps can it handle?
I like that design, I think I could make something like that work for the design I was going for, but how long do you think it would hold up to the temperatures?
Are you going to cast a heat riser from the same stuff?
Yeah I purchased it from a local supplier of refractory and insulation supplies (EJ Bartells in Portland, and I know they have other locations in the Pacific Northwest). The cement I used is rated to handle 2600 degrees which is only 300 degrees less than your standard firebrick.
I think it will hold up well. So far no stress cracks, or any visible damage. The stuff is hard as a rock without being brittle. It's always a good idea when you cast something to take the leftovers and save them for testing. I had 3 balls of the cement left over and I chucked them as hard as I could onto a cement floor......didn't even phase the little test pieces.
My understanding is the heat riser is where the majority of the temps rise to crazy levels, so to answer your question: no I won't be casting the heat riser from refractory cement. Not only would it be WAY too expensive ($60 a bag, and 3 cubic feet to pour just doesn't add up) I'll use vermiculite board then put a tube around the heat riser leaving between a 2-3" gap, which will be filled with perlite and a light clay slurry mix to really insulate that sucker.
Jared McKee wrote:What is the thickness of the bottom of the cast? From the pictures I am assuming the board was attached to the cast to hold it away from the bottom same thickness all the way around?
If I remember correctly the thickness all the way around from wall to burn chamber is 1.5"
The wood on the bottom was part of the cast mold box, and we just left it on there through the burns so that after firing it for long periods of time we could pull the wood away and see if it was charred or burnt.
It's never done THAT before. Explain it to me tiny ad: