This thread is for folks who just didn't get enough of each other's company at the Missoula-area rocket mass heaters workshops in October 2013.
We thought you might like a chance to discuss the digital handouts, ask any remaining Burning Questions, or swap emails for updates on top-secret research projects.
- What type of ciment and goop did we use again?
A: That would be one of those top-secret research questions. Please check your email for private communications.
- Something about floor support...
- Temperature ratings on brick, clay, perlite, etc?
A: Best to look up the product specs on your actual material if possible.
Household building brick or "common" brick can handle up to about 2000 F but sometimes cracks from heat shock.
Firebrick usually handles 2500 or better.
Perlite is good up to about 2300 F, but in practice it's so insulative that layers below the surface rarely reach these temps.
Steel melts around 2900 F and does some serious warping starting much cooler (like around 1000 F).
Clays differ from each other, and the type of firing process affects whether a clay will crack, powder off, fire into a very hard ceramic like bricks or pottery, or vitrify (glaze and perhaps even melt). Most clay is inert up to about 600 F, then may weaken with abrupt temperature changes above that. Over about 1500 F, to maybe 2400 F, various types of clay will become permanently hard.
Portland cement and concrete pavers are not suitable for high temperatures - they tend to powder or spall from 600 F to 800 F depending on the mix.
Thanks for all your great help and questions!
John Ewan wrote:I hope this is the proper place to ask! I just got Paul's daily-ish email that summarizes the workshops and boy does it raise some questions. Which designs failed? Which succeeded? Also, I'd looove any photos of the finished products!
I wanted to get into more detail than is appropriate for the public forums, so I posted that one to the participants by email.
A few other burning questions:
- The long box - insulation - concern with plastic on the wall.. should we be?
Masonry heater code allows a 4" air gap to combustibles if you have at least 5" masonry thickness. Where there is less than 5" masonry thickness, clearances would be 36" from combustibles as for a non-certified woodstove. Woodstove clearances can be reduced with specific types of heat shielding; the smallest legal clearance with excellent heat shielding is something like 12".
The whole wooden-box-full-of-pea-gravel thing is experimental, but we are not seeing higher surface temperatures than expected. So far the bench itself is performing pretty nicely, right in the range we expected, no problems with the clearances to the woodwork at least around the heat-exchange ducts. I am not concerned about the insulation that is 4" to 6" back from the box. The only concerns I still have are about the fireboxes in the 2 installs where there is wood underneath the firebox - those will bear watching until they have been proven in use.
The barrel on the other hand, definitely needed heat shielding. I like to follow woodstove clearances around the top of the barrel, which gets very hot. The lower part of the sides is less of a big deal, as it only gets up around 250-300 in the lowest third of the barrel.
Big thanks to Wilson for helping me install the heat shield during one of those late-night sessions. There is now a simple metal heat shield with 1" air gap between the wood posts, a full 4" to 6" from that plastic-sheathed insulation.
- Rules for length of ducting - I think we covered this, no? Most 8" systems are 20 to 40 feet total length, with 2 to 6 elbows. We start with a max. of 50 feet, then every 90 turn takes off about 5 feet due to drag. Corrugated materials take off something like 10 feet for drag for every foot of corrugated material.
- How to make it all nice and purdy - Any masonry cladding can be used, so you could make it brick, stone, plaster, or tile finished using our standard cob-style designs. Soapstone gives some nice cool-color options if you don't like the earth-tones that come with brick. Tile-stoves can be made with custom tiles, spendy but gorgeous.
The wood-box designs are still experimental, but once the safe clearances are established then your normal cabinetry techniques would apply.
- Soapstone as the firebrick material: I have used it in fireplace and thermal mass, but I don't know how it would hold up as the firebrick for the firebox itself. Worth a try. I would sooner use it as facing tile myself, since firebrick is pretty cheap and soapstone is pretty pretty.
- Could you pour mud into a mold (on top of the auditorium pipe run) for more efficiency than pea gravel but simpler than cob?
I think so. Certainly we've had reasonable results with moistened, tamped earth for greenhouse or brick-box infill. Some people also want to do concrete this way, which is a more permanent choice but seems like it could work. I'd be tempted to place little barriers in the pour so that it could come apart in bricks if needed for later removal / repairs.