Rocket Mass Heater install!

Danger is as surprised as you are to see this contraption out of the truck and in place, with the old 300# wood stove moved off.

On the left is the J-Stove with its stainless steel barrel radiator, and on the steel bench is the mass heater. This will be their first time being connected, if all goes well.

While Danger test-fits stovepipe; The mass heater weighs about 130# and was lifted into the truck in SLC with an electric 1-ton hoist. To remove it, the steel bench was cinched to a dolly (flat) which reached the height of the tailgate of the truck. The truck was backed onto the concrete platform outside the kitchen, and the mass-heater was slid out of the truck onto the bench/dolly combo, then the bench and mass-heater were separately cinched. Then it was all rolled through the house. Once in position at the hearth brick, the dolly cinches were removed and the payload was tilted up and slid into position. E helped Danger by stabilizing the load as well as his mental stability.

Nora is so done with all of Danger’s stove tinkering. She had hoped it would all stay in SLC, but no, here it all is in Montana, and her big cast-iron stove is standing cold at her feet.

Butterfly on the main riser dampens the flow (it is open here), directing more heat into the secondary riser fitted to the mass heater.

Danger’s theory of on-site craftin’: bring bunches of stuff and fiddle till it fits, then return whatever is left.

Danger measured and measured and measured, making sure the steel bench would be just lower than the J-Stove. He welded big nuts with riser-bolts into the feet of the bench to raise it and align with the J-stove. Danger’s measures were high by 1/2 inch, and the J-Stove is now mounted on bricks and Danger is glad he put in 3.5 inch riser bolts, though it seemed overkill at the time.

Danger finds Lyle’s big wrench set and deploys the extension bolts.

The double wall pipe from the mass heater points a little bit down, as the concrete floated the tubing juust out of alignment- Danger thinks he can snug it together though.

Snugging the stainless steel pipe into the double-wall stovepipe. It all fits. Whew!

Danger recovers his mojo by making a creative solution to the too-long 8″ to 6″ reduction stovepipe coupled to a 6″ to 4″ reduction. He sleeves the 6/4 inside the 8/6 and it aligns perfectly to the T pipe on the stove, so he lines it with stovepipe caulk, then screws it together with self-tapping metal screws. This part is then calked and screwed into the existing 8″ stovepipe protruding from the wall.

The double-wall T connection with bottom dropout/clean-out is affixed to the stove.

Stove connected to mass heater: check. T connector bonded to stove: check. Chimney pipe connected to reduction: check. Now let’s tinker-toy it all together!

So close! I need just a little extra part of something…right there by the drill! (Left side mid photo)

All fitted out!

Now I loosen it all and seam inside the connections with stove pipe caulk. Then I wrap all the seams with heavy duty AL tape.

Time for a fire biscuit. Not for you Danger! Feed it to the stove.

I made a bunch of these little fire biscuits; formed of wood-chips (from the pet store), paraffin wax, a flat cotton swab, and perlite. They stay lit in wind, which is a must within the J-stove.

“My biscuits are burnin’! My biscuits are burnin’!” -Yosemite Sam-

Fire is go!

The fire will only burn at the bottom, within the stove, allowing long staves to slowly burn down into the beast.

Split firewood needs to be split small to fit in the inferno.

I brought all the piping parts to draw air from the basement, and set that up as well. The stove draws hard, and otherwise would pull sub-zero outside air through the house.

I added the steel throat, slung into the mouth of the stove at an angle to direct wood in and protect the firebrick maw. There are two air openings for the outside of the steel, pulling the air down to the low foot of the fire. There is another air port pointing down at the center providing air to a custom channel that leads to the base of the internal riser stack where superheated gasses are ignited. The stove is near to zero emission.

You might wonder; how can you split firewood down to size to fit the steel throat? Ask New Zealand, they have a great answer: this handy and precise manual kindling splitter.

The little mallet does 90% of the load, and the bigger mallet is the convincer for any stubborn bits.

E and I brought in a few wagon loads of broken discarded corral lumber, the Kiwi splitter made super-long staves of it- and it burned down nicely.

E makes fresh peanut butter cookies to keep me off the fire biscuits.

Xander and E both display symptoms of feline Stove-Syndrome.

After creating an air-baffle with rockwool around the air intake at the steel throat, we thought it might like some dressing up. E recommended “pressed tin” from Home Depot.

It matches the deepening bronze colors of the stainless steel radiator drum.

A tidy surround.

I had made an air-tight lid for capping the steel throat, and restricting the airflow to the draw through the tubing from the basement. It was dressed out as well, and I scrounged up this old ceramic insulator as the handle/weight.

Cap in-situ.

Like a fancy cake.

Well, this is all just getting a bit too fancy for the ranch.

Skinned all the way around.

Our near zero degree nights were pulling heat out of the mass in a matter of hours, leaving it stone cold by morning. So I made it this nice blanket of rockwool. That helped a lot.

The blanket got a temporary cap matching the stove, as the mass heater will eventually be skinned in stone and have a flagstone cap. Another tweak (not picutred): driving 28 giant corral screws through the mass heater (air-crete is similar to styrafoam in density, and screws jump right through it) to penetrate the interior piping, and sealing their heads with piping caulk. These became immensely hot, quickly transferring heat throughout the mass. This means less time burning wood to charge the mass, and much longer heat retention as the heat is more thoroughly dispersed.

80% less firewood. Near 0% emission. Immediate heat, with immediate feedback on level of heat desired with in-line butterfly and amount of wood. No carbon dioxide back-venting at night. No fire at night, so no danger while sleeping. No carbon build-up in the chimney. No smoke smell permeating everything in the house all winter. No visible smoke out the chimney, and little smell of smoke outside. Plus, it is fun to feed.

Mass Heater Bench

8 shorts from the end-cuts bin at Wasatch Steel, cut on my Evolution metal miter saw and welded up. This bench holds the Mass Heater from the previous post at the correct height to align stovepipe from the J-Stove.

All this pretty metal will be under the Mass Heater.

Spray-painted in “Oiled Bronze”

The rear ladder is a guard for the J-Stove, as I’ll store lengths of wood under the bench.

Each leg has a large nut welded within, and a bolt to adjust height to align with the J-Stove vent pipe.

E spotted 3 boxes of this stone veneer at 1/8 price.

Enough to cover three of the sides (short ends and this long side), all but the side facing the wall. I’ll cover the top with native flagstone.

Less Massive Mass Heater

Hooray for the learning curve. This iteration of a mass heater weighs in at just over 100lbs, and took a day to create.

simplicity. Mold form is a plaster hardy-backer board that is an aspect of the form, rather than the previous pop-away mold form. An external frame holds the thin board in place and will absorb the liquid pressure against the form. The seams are sealed with drywall paste.

The stovepipe ends are added in with AL tape and drywall paste on the bottom, and a salvaged board with a routed hole holds the top pipe in-situ. AL vent pipe taped together makes an “S” in the stove, using two straight flexi pipe connected to the stove pipe ends, the straight flexis meet at a short flexi forming the bottom curve of the S.

The drywall screws poking into the interior will help anchor the walls and the aircrete. This is my maximum size possible with a 3’x5′ panel; 14″ x 36 “floor, 13″ x 14″ front/back, 16″ x 36” sides.

Two days of curing in 75 degree temps, an inch or two of shrinkage at the top- and so I poured an un-airated concrete & perlite backfilled with cured aircrete “stones” from the demo’d mass. This will make a nice hard top for the form, and I may grind it flat for a reveal of the red aircrete backfill stone and white bits of volcanic perlite.

Rolled up in the old rubber pond liner to retain moisture in the desert wind, and build heat for a better cure. This form I can easily move into the studio for the weekend snowstorm, and has about 1/4 of the footprint and 1/3 the weight of the demo’d mass- so the trip to the ranch will be easy. Now I just have to weld up the little bench it will sit on to align with the J-Stove.

Weight reduction as pretext to demolition.

After a week of curing. At least 300lbs. The mold is unscrewed and pops away clean. Some settling and a bit of layer separation at the bottom where I had technical issues with the first few mixes and they collapsed as they cured. A lot more red dye in those first mixes.

I back poured some collapsed areas with quickrete/perlite- the gray area…

The underside is the bench seat, and it came out smooth and strong.

The bench was an easy weight reduction of 40lbs. The mesh caused issues of a hollowed out collapse in the body of the form. It deflected the mix in a few other areas as well.

And the narrow bit of wall sawzalled off like styrofoam (embedded with rebar). Still over 200lbs. WTF? you might ask. Well; too big is too big, and a fail is a fail.

I ripped the top layer of aircrete & mess away without too much trouble, exposing another layer of solid concrete back-pour into a big gap.

Salvage the expensive bits. This bit of stovepipe is worth more than the all the concrete.

Sawzall weight reduction so I can expose the bottom. The venting system was flawless.

Easily tipped on its side now, this would have been the top.

Stovepipe salvage complete.

The is the Gorn’s stash of throwing boulders for lobbing at Captain Kirk.

All tidied up for round two.

Too massive Mass-Heater

First, lets remember the J-Stove / Rocket Stove project: https://youtu.be/V2q4g-5P8Js

To capture the heat from the J-Stove, a secondary stove pipe connects into a separate mass, the pipe then exits the mass and rejoins the main J-Stove stove pipe to flow out the chimney. This is the mass form I designed and made. It will warm to a few hundred degrees durning a 40 minute burn of the J-Stove and radiate heat for 8 to 12 hours.

The side vent of the stove fits into the hole pictured below- the mass form is upside down and will have a welded metal platform to align it with the side vent.

This is the mass heater form / mold, inverted for pouring in the mass. It will have stove pipe connections and tubing added, along with rebar and expanded metal to add strength to the mass of aircrete (foamed concrete). This form provides a large mass and a bench for warm seating.

The holes are where the tubing will enter and exit, and the L shape is the bench. The form is made of one 4×8 sheet of Laminated 3/4″ MDF. It is screwed together, then hot glued, then silicone sealed along all edges and some exposed MDF was taped over as well to create a water tight form that could hold hundreds of pounds of aerated concrete.

The fins under the seating ledge provide support for the concrete weight it will bear.

It has to live through the pour, holding hundreds of pounds of liquid concrete, then after the concrete cures it has to be able to come apart and knock free of the internal mass.

The form had to wait a bit on the order of two 6″ long sections of 4″ diameter double-wall pellet stove vent that fit into the enter/exit holes. I thought I had the right pieces already…then tape on the flex tubing and add in a rebar cage with expanded metal to carry heat through the mass.

Another layer of expanded metal is added to bolster the seating platform (remember, the form is upside down / inverted for pouring.

All connections have to be secured from the outside, and water tight.

Hats off to the Honey-Do Carpenter for the aircrete cannon and aircrete recipe- just add warm water and a bit of shampoo (and my air compressor) for foam too thick to shave with. Then I use a “barber-pole” drill bit to lift the concrete mix from the bottom of the bucket into the foam laid over it, until all the concrete is suspended in the foam. It seems impossible, but it works.

The aircrete mixing station: 17 five gallon buckets will fill the form- at about 1/3 the weight of concrete. So about 300lbs, should have been 275 but the first three buckets were a bit off. Nora is next to the drill with the concrete mixer attached (black), the silver spiral between Nora and the drill is what I used to blend the concrete into the foam.

The main mass of the form is more than half full. I’m using a red dye in the aircrete.

This is all very tiring, and I’m only half convinced it will work- the form could blow out at the sides/bottom, the aircrete might not set, and even if it does the thing is a lot more massive than I had conceived.

4 hours after the pour is finished and not even beginning to cure out.

I wake up with a dread that the form has popped overnight, and the concrete never set, and it all flooded into the pond and killed all the fish and ruined the pump and waterfall. Now, 5 days later, and after 3 nights below freezing wearing a foam cap, it is solid and curing toward a respectable hardness.

I’m letting it cure into next week. There has been some small shrinkage/settling that will need another round of pouring- I may use a fast setting concrete to make a stronger platform for the mass, then I will remove the form. If all that is successful, I’m going to cut it down in size quite a bit. This is designed as the ideal form to have at the ranch that can fit into the back of the pickup, but it is too heavy to move without at least two more strong guys and a bit much for a 500 mile trip in the back of the little pickup. I’ll remove the bench and bevel an angle into the body of the mass, dropping around 100lbs. If all that works out, it will travel up to the ranch sometime this fall for installation and testing.

Real tests test to failure.

When last we saw the rocket stove, after test firing one… (descriptions are set to E.E. Cummings?)

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.

Test fire two. Stainless steel drum over chimney.

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.

Two vent pipes welded on, with stovepipe ends.

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.

40 minutes into the burn, running it hot.

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.

Tree branches vaporize in the chamber.

The connective welds (bronze weld to stainless steel) heat to rainbow color.

Oak end-cut for a clean burnout.

Chimney removed, core and rockwool are perfect

The chimney is a fail.

Chimney view. Shrunk, crazed, and vitrified: well over the 1200F rating at base and ruined, cooler at top and the ceramic is fine.

Outside SS wrap wire is fine, AL wrap thinned to parchment or gassed off, ceramic destroyed at base.

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.

J-Stove Rocket Mass Heater

Core Oven: refractory board rated to 3,000 degrees.

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.

This is the Walker J-Stove design. I bought the layout plan for $20 online. Brilliant.

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.

I added this air-flow channel, bringing fresh air to the riser to encourage the venturi and oxygenate the superheated gasses for a complete burn.

The air splits into three channels, that combine into one, with the three channels still delineated at the top. This opens directly into the stack.

I found this round refractory tubing rated to 1500 degrees, perfect for a venturi stack, or chimney. The Walker design uses the same media as the core, so that stack is square, which limits the gyre.

I wrapped the stack in heavy perf aluminum, and tied it with stainless steel wire. 

Now I wait for the fireplace brick splits order to arrive, to line the inside front of the firebox. This protects the delicate refractory media from the wood fuel.

The fire brick arrives, and I run a hot burn to temper / shrink all of the media, prior to spackle with high-temp mortar.

As the fire climbs to temperature, there is an initial column of smoke.

In a few minutes we are at temperature, and the smoke is gone. All wood is reduced to gas, and even the gasses are burned. Her core temperature is 1,500 to 2,000 degrees (hot enough to melt bronze and steel). I can hold my hand against the walls, they are warm to hot.

Operating at full burn. 

Only the bottom end of the wood burns. These board-remains were 4 feet long, and slowly digest into the jet of flame. The soft roar of the air to flame is why this is called a Rocket Stove.

The remaining cinders are the remains of the last few cooler minutes of burning, as the full incinerating force drops.

This is after the next step; encasing the delicate refractory core in a heat-safe and tough shell.

The refractory core is bedded in rock-wool, a fireproof spun silica insulation that allows no air movement. The outer shell is hardy backer board with a tough outer shell, ready for tile. I used an industrial cement and seam-webbing to join the board. 

20 feet of angle iron, and a few steel scraps, welded into a sturdy frame. 

The stainless steel 15 gallon drum goes over the chimney column. This acts as a heat radiator. 

I cut this perfect circle to sleeve over the chimney. Next I will cut a small hole on the outside for a stove pipe connection, and another to fit to the mass heater (another build for another day). 

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.