I was lucky enough to score a Jotul 118 for free locally a couple of months ago. Instead of repairing the cracks in the stove, I decided to make a rocket mass heater out of the stove. I have seen this video from Ernie Wisner about such a combination:
In contrast to the design in the video above, I have insulated the firebox and heat riser with insulating firebrick (IFB 2600) from Ceramaterials. As can be seen in the pictures below, I still need to put bricks over the top and make the riser out of stacked bricks.
The dimensions are box W8"xH9"xD15", port W3"xH9"xD3", HeatRiser L40.5"x6" (and see pictures below). I am hoping this insulated hybrid design will burn cleanly. I still have to decide how to get a P-channel incorporated into the design.
One main question I have is with regard to extracting heat from the stove. Typically, the mass part of a rocket mass heater is a cob bench or something similar. The hot gases pass through a snaking horizontal chimney and heat up the cob/mass on its way through. In this set up, the mass is at room temperature or warmer as it heats up, so the gases provide heat to the mass due to the temperature difference (Tgas-Tbench). However, I am looking to avoid such a large bulky bench. If I put a few barrels stacked on top (20' ceiling) and a regular 6 inch chimney on the side, similar to this setup:
https://permies.com/t/40084/Peter-Bergs-Batch-Box-Beast then put fans behind the barrels/chimney throughout the burn time, would the heat transfer be similar? In this case, the heat transfer would occur over a similar surface area, but the heat transfer would be a function of temperature difference between the gases and the air itself (Tgas-Tair). I have about 600 cubic meters of air in my house which weighs around 720kg. Can I get a good enough heat transfer with fans so I can use the mass of the air in the house as the thermal storage medium instead of a cob bench? The house has a fairly tight envelope.
Good looking project! It will be interesting to see how well it works.
Have you studied the information at batchrocket.eu? According to the table there, the port for a 6" system should be 2 3/16" wide x 9 1/2" high, so yours is a bit oversize and may not develop proper turbulence. I might slide the port sides together a bit to make it 2 1/4" wide instead of 3".
You could use the stacked barrels for heat transfer surface, but this would give only an instant heating capacity, with no real stored heat. Air is not a very good heat storage medium. You would probably get much better results with a vertical masonry bell, which would take minimal floor space, be as tall as you want, and hold and radiate heat for numerous hours after the fire is out. You do need a strong floor to support the concentrated weight of the masonry bell.
My batch box Rocket mass heater has progressed to the point that I hope to install it in time for our New Hampshire winter in a couple of months.
Below is what the stove now looks like without the barrels on the top and to the side which will serve as radiating bells and mass with rocks in the side barrel. Insulating firebrick lines both the firebox inside the stove, bottom top and sides, and forms the vertical 36" riser section just by dry placing them there. In fact, I do not plan to use mortar in this stove at all so that I can easily disassemble it in the future.
I put 8 to 10 two inch thick 14 inch long pine sticks as fuel for the initial burn. The current air intake is on the bottom of the stove and comes up through the fuel past a quarter inch stainless steel screen that the wood sits on. The plan is for this external air intake to go through the house floor drawing air from the outside. There is also an air channel in the bottom of the Jotul stove that allows air to travel under the bottom layer of bricks to a secondary port at the beginning of the heat riser at the bottom.
I started the fire with this little electric firestarter which only took about one minute to blow hot air through the system to get the draft going, then light the pine on fire, then fan the flames so that everything gets going:
https://www.amazon.com/HomeRight-Electro-C900046-Charcoal-Chemical/dp/B019H0DF50/ref=asc_df_B019H0DF50/?tag=hyprod-20&linkCode=df0&hvadid=198091107028&hvpos=1o1&hvnetw=g&hvrand=5984592362485449790&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9002420&hvtargid=pla-320475141745&psc=1 Here’s a video of the initial burn. No smoke after the first 20 or 30 seconds, and I was amazed with the insulating firebrick that the metal of the stove itself did not get warm for the first 5 to 10 minutes of the burn even though I was getting the rocket effect up through the riser. The insulating firebrick lining the firebox really did its job. The fire burned hot and violent for about 25 minutes, then the pine sticks had collapsed somewhat to about half their initial height whereupon there still was no smoke, but there weren’t flames in the riser much either. If I threw another stick on, it would immediately burst into violent flame again in the rocket effect could be heard. After the 25 minutes of rocket style burning, the sticks kept putting out decent heat for about 15 minutes, then there was smoldering for another 30 minutes or so until there were just a few coals and a bit of ashes left. During the 15 minute section there was a little bit of black smoke compared to no smoke during the 25 minutes.
I put a stainless steel quarter inch mesh screen both to protect this soft insulating firebrick lining the firebox into help the wood not block the 3” diameter air intake port at the bottom middle of the firebox. This seemed to work well to provide plenty of oxygen both at the main port and that the auxiliary port at the beginning of the heat riser.
The plan is for this to become a two barrel system similar to the Dragon Heater derrick build kit using barrels:
http://www.dragonheaters.com/8-rocket-masonry-heater-derrick-build-kit/ I bought the curved collars from a Vogelzang-type double barrel stove kit to attach some 6 inch black stovepipe between the barrels and from the second barrel out to the chimney. I have some ceramic wool to form gaskets at junctions between collars and barrels. Here’s a picture of what the end system will look like, again with 1000 pound of 6 to 12 inch diameter rocks filling up the bottom side barrel.
As with many DIY projects, this project has had its good phases and its troubled phases. One trouble was that the junction between the first barrel and the additional third of a barrel that I put on top of that first barrel let through smoke too easily. I had to do away with that extra height on the first stack using ceramic wool and self tapping metal screws as a lid for the first barrel.
Another issue I found was with the draw. Once the fire got going the draw was okay, but most of the time there was not enough draw and the smoke was getting in the house. So I added an in-line duct fan at the top of the chimney that draws 140 ft.³ per minute through the stove. And then I put that fan on a one-hour timer to coincide with the length of a normal burn. This fan is not meant for regular woodstoves and so has a thermal shut off at 140°F. Nevertheless, my smoke exhaust has always stayed under that number since the fan has never turned itself off due to thermal protection.
I also discovered that I certainly did not have enough air intake for this rocket stove. It needs quite a bit of oxygen! The hole I cut in the bottom leading to outside air was entirely insufficient for the oxygen needs of the stove. So my very simple but perhaps non-optimal solution was to simply open the front door of the stove and give it inside air in addition to some outside air through the stove floor. Now it runs much better although according to my calculations, for this one hour burn in the stove, it uses about 1600 ft.³ of inside air in the end. Considering that I have 40,000 ft.³ of air in the house total, this 1600 ft.³ can be considered part of the normal air exchanges during the day. It is not what I planned, but maybe I can figure something better out next year. As a side note, I should have scraped or stripped the paint off the first barrel/bell before moving the stove indoors. The first few burns were smelly with burnt paint.
Rocks in the second barrel stack (see pic) helped quite a bit with heat retention, and I put a box fan a few feet away from this stove to carry more heat away through air convection. My worry there was that he was going up the chimney due to the fan too quickly. Temperatures on the first barrel go up to about 600 F during the burn, then come back down. Temperatures on the chimney 10 feet off the floor never seem to exceed 120F.
We now have a system where it seems to work for us. We haven't encountered very cold days yet of course, but these one hour burns of about 8 pounds of dry pine seem to warm up the house fairly well so far. The process is as follows:
- load up the firebox with 8 pounds of dry wood (pine branches, 2x4 cutoffs, etc.)
- open the chimney slide damper and turn on the chimney inline fan on a one hour timer
- put an old iphone to chime in 58 minutes as a reminder to shut things down
- start the fire with a Homeright C900046 fire starter
- leave the door half open for the 58 minute burn
- at the end of the burn, close the chimney slide damper, let the fan turn itself off, shut the stove door
Here is an update how the winter is going here in New Hampshire with this Jotul 118 converted to rocket mass heater. Overall it is going well, and my estimate is that it is three times more efficient with wood than my old outdoor 5036 central boiler.
I have learned a few things in the first few months of this first winter. For example, I burn mostly pine, either bunches of 12 foot long 2" x 2" pieces of pine bundled into a 3' x 4' x 12' bundle for nearly free or free lumber cutoffs from a local truss company. But, because the vast majority of my burn is burning the creosote in the gases with the second combustion as well, when I inspected my stove a couple of weeks ago there was only a bit of soot built up on the inside of the system. Not enough to worry about until cleaning next fall.
I've also learned that the extra oxygen provided by the P-channel is quite important. My P-channel runs under my firebrick inside the cast iron stove entering from a hole I cut on the bottom of the stove. But because my P-channel hole comes out from the bottom of the riser, it can fill with ash and get clogged. I have to clean it out every week or so just to make sure I'm getting enough oxygen in the riser for a proper second burn.
I want to give an accurate picture of how much fuel I use and how much attending this fire needs. I am burning indoor air right now, so my door is left open at about a 45° angle. My firebox is effectively about 16 inches long, but if I fill it only at the back near the riser with 12 inch material, then no smoke gets into the house even with the door open. The burns do not last for about an hour as I listed in my last post. If you stuff it completely and just start the fire it might last for an hour, but I end up putting more wood in about every 15 to 20 minutes in order to maintain a good clean secondary burn in the riser. I set a timer on my phone and come back to put more wood in on this 15 to 20 minutes schedule. My house is decently tight and reasonably well insulated, but it is a large house with a 64' x 32' footprint and two stories with a flat roof, somewhat like a brick. But on a cold day like today where the high was 15°F in the low will be about 0°F tonight, I need to keep up a burn for about four or five hours total in order to keep the house at an average of 63°F (56 degrees in the mornings, 68 degrees when I go to bed). This ends up being about 60 to 75 pounds of wood (15 pounds per hour), and if I assume I'm getting about 5000 BTU per pound, then this is about 350,000 BTUs from this stove for the day.
There is still too much heat going up the chimney for me however. I do have maybe 600 pounds of rocks in the bottom left barrel, but they can only hold so much heat. I have put three box fans on high pointing at different parts of the system, and that helps quite a bit to transfer heat into the room. But the chimney above the barrels is still hotter than I want to be. I'd like to add another barrel on each side another fan as well. Also would like to burn outside air through an intake in the floor.
The duct fan didn't last too long, but I found I didn't need it once it got a little colder and the system had a better draw.
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
Ben, what i would do, is locate the entry point of the barrel tower at the bottom. And the exit to the chimney too. Mortar stones on the barrel walls, to make, may ne four inches each side. Doesn't matter if pointy bits stick in. You could mortar the stones outside too, for a bigger bell. Ans more mass.
It is a little bit difficult to see in the pictures, but the exit from the second barrel stack to the chimney is about as low as I can get it. The exit is only about 4 to 6 inches off of the floor level.
I may go to bricks inside the barrels instead of just natural rock from my fields. I'm not sure how to conveniently get rock on the outside of the barrels and still have the system be reasonably amenable to being taken apart and modified in the future. I don't want to do cob or mortar or things like that since I may modify or move this thing eventually. So just dry laying things would be better. I've thought about chicken wire and stones on the outside of the barrel for heat retention and surface area improvements.
And Matt Walker from Walker stoves clued me into a stainless steel alloy from rolled alloys, RA 330, that is supposedly resistant to oxidation and temperatures up to 2100°F. Perhaps I could make a water heat transfer system that could withstand riser environments. If this alloy is good enough, it might even be able to be the riser in RMHs.
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
The first winter of using only this rocket mass heater to heat my house was a success! I did indeed end up using about three times less would in my outdoor wood stove: about 2.5 cords compared to 8 cords. When I opened it up to clean it before the current 2019 – 2020 winter, there was only some fine ash around the rocks and the internal chimney. Easy to clean, and there was no creosote on any of the walls or chimney. As I cleaned, I did notice that several of the refractory bricks in the riser had cracked. This wasn't that much of a problem since they were all stacked with gravity and maintained their positions, but I was a bit surprised by about half the bricks being cracked.
The extra oxygen for the riser now comes through a hole in the bricks at the back bottom of the riser itself, not through that metal screen. That seems to work fine and the rocket effect starts very quickly, usually less than 30 seconds, after starting the fire. It is always a pleasant site to see that second burning back in the riser.
An extra barrel on the top of the riser allayed my concerns that the riser terminated too close to the first barrel top. I didn't want the hot gasses burning through that first barrel. The second barrel adds more surface area and better heat transfer into the room with the box fan right behind it. Another improvement was that I replaced all the rocks in the left barrel with some free local bricks stacked much neater.
I still have plans to create an outdoor air intake that feeds right into a glass door held on by magnets.
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