Rocket Stove Stress Test
When last we saw the rocket stove it had gone through core/chimney test firing, then had been wrapped with rockwool inside a hard backer-board shell trimmed with steel. It awaited the stainless steel drum.
Here the drum is in place fitted over the inner chimney riser, with two vents of stainless steel pipe securely welded near the bottom of the drum. The drum radiates heat to the room, and vents the comparatively cooler air near its base, creating draw on the chimney by dissipating heat quickly. The vent pipes are fitted with double wall stovepipe vent, increasing from 3 inches at the drum to 4 inch vent. This rear vent is the main riser to the house chimney, and the other will feed into and through a mass form (to be built in-situ at the ranch), this mass-venting line will link back into the main riser, ensuring a continuous hot draw out the house chimney, as well as a strong draw through the mass.
Running with no visible emission or smell of smoke for 40 minutes. I need to verify that the rockwool liner survives a high heat and keeps things air tight, that the outer drywall board remains stable, and that the metal frame stays cool. The stainless steel drum begins to turn copper color with heat, as it should, but the box is cool to the touch. Time to test to possible failure.
I pack the firebox with 1inch diameter pine and 3 inch branches, and the chamber, already over 1000F, devours the wood straight into a white light. Temperature at the chimney, with o2 blending into the gas reburn venturi-vortex, jumped to over 1500F or more- and suddenly a putrid heavy black smoke churned from both vents. It was similar to foundry fails, when molten metal is poured into a mold that hadn’t burned out completely, and the metal annihilates everything not burned out and gasses out the uncured mold- in a vertical jet of putrid flame. This wasn’t anywhere nearly as dramatic, yet I recognized the smell of things coming apart in high heat. The black chem-smoke would dissipate, and I would repack the box, and the black would bloom again. After 20 minutes the worst was over, and I pulled out the remaining big branch, and replaced it with a 1×2 x two foot run of oak. This hardwood will burn hotter, and burn very clean; soon things were running back at “zero emission” again.
I have already ordered ceramic fiber board rated to 3,000F, same as the unscathed core-box, to rebuild the chimney. I wanted to try the round chimney, even though it’s temp rating was only 1200-1500F as it allows a stronger spinning venturi-effect to cleanly re-burn all the gasses. Looks like I’ll be going with the square version made from fiber board. I’ll post how the retest goes soon.
Last night we had our biggest overnight snowfall in more than a decade, with 15 inches at the house and 20 inch drifts. In the mountains; 5.5 feet at Alta resort.
Nora and I walked to the park yesterday and it started spitting snow, but nothing stuck ’til around 5pm. I shoveled a few inches before nightfall, clearing everything before E got home from work (I’d just picked up and stacked another 1000lbs of bricks picked up from a nearby house remodel for redoing our winding pathway in the back yard this spring).
This morning I had a cup of Joe and headed out to tackle shoveling. It took me nearly an hour to clear a path from the truck to the street, as I had determined I couldn’t clear the path all the way back to the garage to access E’s 4×4 Subaru in time for her to leave for work, so I planned to drive her to work then come back and keep shoveling. A neighbor let me know all schools and gvmt offices were closed, just as E came outside. She checked in and sure enough, Snow Day! So I kept shoveling, and shoveling, and shoveling. I have an oversize shovel made to push up a big bite, then slide snow out of the way to dump elsewhere- which works great with this California Concrete. I just have to stomp out a path to dump the snow, then grab a big shovel full and cart it over and start ramping up; to five feet tall by the entire length of the driveway. And another big pile at the foot of the drive out by the street.
I thought I’d make my own super-efficient Rocket Mass-Heater for the ranch, to replace the dangerous old cast iron wood stove. A Rocket Mass-Heater burns about 80% less wood than the old stove, can run on pellets as well, produces no smoke, few gasses, and will keep the house warm all night with no fire burning. It provides a radiant heat source via a stainless steel bell, as well as a large radiant mass that warms to a few hundred degrees which then radiates heat for 8 to 10 hours after the fire has burned. It can heat the house for 12 to 24 hours for one hour of burning- depending on how well my mass structure absorbs and retains heat. I will build that up at the ranch, using aircrete; concrete blended with super foamed soapy water. This reduces the weight of the concrete mass by 60-75 percent, and allows the heat to permeate the mass.
The interwebs are awash with bad Rocket Stove designs. Many cores are created with bare steel using old gas canisters. At 1500 to 2000 degrees in an oxygen depleted environment, steel undergoes a process called spalling. Essentially it rusts without oxygen, or more precisely it is the effect of reshuffling the iron molecules wherein they lose covalent bonds and layer like sheaves of paper. Refractory material is the only media that can survive the temperatures in the core and stack. I found the best understanding of the forces at work were from Masonry Stove builders, in particular Walker Design, who offers a J-Rocket core design to keep us diy dinks from burning down the ranch house. My design is a hybrid, as the stove will stand alone, as will the mass-heater. This is borrowed from another solid innovator, The Honey-Do Carpenter. I’ll be using his specs to create the concrete foaming gun, and modifying from his prototype of a light weight mass heater separate from the rocket stove.
Lots more to do: support the stainless drum; put on low legs; create gravity-assist wood feeder of wide square-stock tubing that is removable; resolve the stove piping; tile it; maybe make a pellet feeder. Then design up the mass heater for assembly in Montana.
Spreading industrial grade perforated aluminum over the top of the blown-in-insulation has been on my radar for a few years. It is primarily an upgrade for blocking summer heat radiating through the roof: mitigating about 97% of the radiant attic heat. In warmer climates the foil is attached to the rafters, and has little pinholes to allow convection to move the hot air up to the roof peak. The foil I used is made to be laid flat in hot/cold climates; it has larger punched perforations allowing moisture/condensation to pass through. Laying it atop the lofted insulation stops the cold air from pressing down and sinking through. Cold always drives downward, and the house heat meets the down-driving cold and creates a fast convection circuit that rips heat out of the house. With the diving force of the cold air blocked, the house should be cozier. Mostly though, it is for our triple-digit summers.
I pre-cut sections of foil out on the deck, then used a 10 foot run of pvc with a nail taped to an end to pierce the foil and push it into place. Before pushing it I added upright tabs of aluminum heat-tape, and after pushing it into place, used another pvc run with a T ending to press the tape down onto the adjoining section of foil cover. It went pretty well, but standing on a little board by the hole for the ladder in a slippery tyvec suit and finding physical leverage to make things go where they needed was a bit like doing yoga-for four hours.
update: The new gas tank and connections were all fine, so I took her up Emmigration Canyon on Halloween for speed trials on the back bit of “flat” road off the summit and she ran like a whole different car- quieter, no hesitation, no bogging down when pushed; just a clean and smooth response all the way through her full range in each gear, and falloff was just as clean.