Dick Winters wrote:...does it make a huge difference if the dimensions are a half inch off here or there? I assume the CSA, precise dimensions, are to ensure the “thermo siphon” works and the unit drafts correctly.
The vertical wood feed, then the horizontal burn tunnel, should be as close as possible to the same dimensions, for best draft performance. If one must be slightly less CSA than the other, make the one with slightly less CSA be the vertical wood feed.
Before encasing the J-tube combustion unit with Perlite/clay mix, wrap it in high-temp fiber blanket insulation, about an 1 inch thick. That will serve as both insulation and an expansion joint between the combustion core and the outer skin, which can consist of your Perlite/clay "encasement mix". On my build, I used two inches thickness of high-temp blanket insulation to encase the core, then over the top of that went straight to a decorative brick work (also serves as thermal mass) for the outer skin over the combustion unit.
There are several ways to build these combustion units. The high-temp blanket insulation technique is the best I've personally used for preventing heat expansion cracks from forming in the combustion unit's encasement masonry.
Dick Winters wrote:Perlite mixed with clay would be a nice base for it correct? On top of the cement board?
Insulation does not prevent heat transfer, it only slows down the process. Best practice when building on a wood floor is to have air circulation (air gap) under the J-tube combustion unit. I.e. lay down parallel and evenly spaced rows of common brick, the rows spaced far enough apart that they can be "bridged" with a layer of horizontally laid brick that form a platform the same square dimensions of the cement board base. No mortar is needed. Then build the combustion core on that elevated brick "bed". Have a careful look at the diagram on pages 43 and 45 of The RMH Builder's Guide by Ernie and Erica Wisner.
Notice in the diagram's insert on page 43 the exact detail, which includes a thermal reflective layer of aluminum foil on top of the elevated brick bed, then an insulating layer of Perlite/clay mix, then the combustion core brick work.
Hi Dick, neat start on the combustion unit. I'm not sure if it's the photo angle etc. but it looks like the burn tunnel may not be tall enough inside. From the floor of the burn tunnel to its ceiling, is it the same measurement as the span / diameter of the wood feed opening?
Thanks. I live in the tropics. High humidity. Mould.
So I just have an interest in Earthships. That they are feasible and we can draw inspiration from actual practice just like that famous double roof house in the Philippines.
But what bugged me most is this: "Every building has its own renewable “power plant” with photovoltaic panels, batteries, charge controller, and inverter. The key step in making these systems affordable for residential use is to “design down” the electrical requirements of the home before the solar system is sized. Super efficient lighting, pumps, and refrigeration help lower the load, as does the lack of any need for electric heat or air conditioning."
This requires a lot of electronics most of which are not evergreen like the 741 Operational Amplifier. Don't forget the supply chain problems around the world and chip shortages. Unless you have your own silicon foundry ...........
Earthship Biotecture has an interesting Earthship design specifically for the tropics. Have you checked out their youtube channel? Especially of interest is their project in Haiti. I'll place the link here: https://www.youtube.com/c/earthship/videos
My own experience with mold, and preventing it, has to do with keeping indoor air moving, and fresh air flowing in the living space. Moisture combined with dead stagnant air, and a coarse substrate that holds moisture, dust and dirt, guarantees mold growth. The Haiti Earthships also have buried ventilation tubes, to keep the living space air refreshed etc. The DIY masonry roof design is appealing to me personally, to get away from using expensive materials like roofing "tin".
Hi Dick, are you locked into building a 14x24 house? I.e. using standard 2x4 foot sheets of plywood, OSB, siding and etc., it's not much more expensive to build a 20x... (5x sheets of 4x8 ply/OSB/etc.) instead of 14x...
Building in cold climates obviously means close attention to construction and insulation methods. Are you planning on doing "double-studded" wall construction? An exterior wall R-value of 38 is easily obtainable with a 12" thick "double-studded" (to limit thermal bridging) 2x4 wall construction. Some neat info is available on this site comparing single vs double-studded exterior wall construction and typical R-values: https://www.bpcgreenbuilders.com/what-is-double-wall-construction
I built an 8" RMH into a typical small 900 sq. foot stick-frame (2x4, R11 exterior walls) house and have been working on shoring up the insulation, in order to extend the thermal mass heating flywheel effect. Winters are fairly mild here, and on a day where the lows hit the mid 'teens (F) and highs into the mid to upper 30s', the house will stay around 70 degrees (F) for 12 hours after the fire is out, and I usually fire the RMH for 4 to 5 hours to warm up the 40-inch wide x 10-foot long thermal mass. My RMH is patterned after Ernie and Erica's Cabin-8, modified to use a Peter van den Berg 6" batch-box combustion unit.
Carl Nystrom wrote:
Edit: looking at the picture again, I realize that the colored wires on there just attach with spade connectors, dont they. So they could easily have been switched. Check for a resistance between the center and one of the other legs. A value in the hundreds of ohms would be your LED circuit.
Another thing to keep in mind is that LEDs' only conduct in one direction. So if the few hundred's ohms resistance is not found, reverse the ohm-meter's probes and test again. Some multi-meters have a DIODE TEST FUNCTION, and it's helpful for finding the connections to the switch's built-in LED.
Without an ohm-meter, and if the switch wiring diagram is suspected to be in error, it can be confirmed as follows. Sometimes it is faster this way too, i.e. use a small resistor, anything in the range of say a few hundred ohms to a thousand ohms, and 1/8th watt rating will suffice.
Attach the resistor in series with a test wire (or a couple of red clip-leads) and connect one end of this to the positive terminal of a battery or DC power source. Attach a second wire (black clip-lead) to the negative of the battery or DC power source. With the switch under test set to its ON position, randomly connect the free ends of the test leads to the switch until a combination and polarity of a particular connection arrangement is found that results in the switch's LED illuminating. This will verify or indicate the common / negative connection for the switch's LED, the terminal to which the black clip-lead is attached.
EDIT: I see this issue has been sorted out, whilst I was hammering out the above:o) Yeah, the disregard by certain overseas MFGs for proper wiring color codes is 'nuff to have one engaged in hair-pulling frustration. I've run into the same thing many times.
Jeff Pollari wrote:Wow!
So, my deck is only 4 feet off the ground. Stacking concrete on a slab is not a bad idea but IDK how hard it would be to build such a slab considering I'd have to hang out under the deck to dig and pour it.
In that case, it's easier if the deck is opened up first, then you have a nice open hole in the deck in which to work standing up straight etc.
Jeff Pollari wrote: My house is already heated with a big stove which is on the other side of my coming addition. I'm wondering if I can just convert the present stove (pictured below) into a thermal mass rocket stove which connects to the addition through the wall behind it.
A doorway or two in the wall behind your present woodstove will allow the adjacent addition to be heated, assuming it's not a huge room. As far as converting a steel box-stove into an efficient, say batch-box, you basically just end up using the door off the thing, if you want to do it right. It can be done, has been done, but it takes a good deal of work to do the conversion, metal working skills, and etc. and really doesn't save (replace) much in the way of masonry materials. Easier to just build a batch-box / bell brick masonry stove. Have you seen Peter van den Berg's website ? --->
Jeff Pollari wrote:I've hired a contractor to build an addition for my home on my deck and I need to know what rocket mass design would work best for my needs...
Which begs the question: how high is the deck above the ground? One option being, reinforced concrete filled CMUs' resting on a concrete pad, rising up to the floor level of the deck via another (elevated) concrete pad to support any version of RMH footprint desired. A compact 6" system size batch-box with thermal mass bench can easily fit onto a 44 to 48 inch wide by 10 foot long concrete pad.
Since 12-volt powered HF transceivers have fused DC power leads, your battery bank's DC power distribution can be as simple as using a pair of bus bars hardwired to the battery bank. Each bus bar (Recoil BB46) contains multiple bolt terminals (for high current devices) and screw terminals (for low current devices). A typical 100 watt HF transceiver will have a maximum DC current draw, on transmit, of around 20 amperes peak (receiver current drain is very low, usually 1.5 ~ 2.5 amperes or less).
Back to the bus bars, these units by Recoil will do nicely where the total current for all connected devices to be powered is 150 amperes or less.
Leigh Tate wrote:Ben, that's very helpful. I'm particularly interested how you set it up on solar. My current solar experience is with AC appliances; I have a chest freezer and converted chest freezer->fridge with a 705-AH battery bank. I have a sine wave inverter connected to the charge controller to plug the appliances into. So, powering DC will be new for me. Obviously, I don't need an inverter. But what do I need?
The 12-volt DC powered SSB HF Transceivers I've referenced earlier are supplied with a 10 to 12 foot long DC power cord, as are most all 12 volt powered amateur radio transceivers. Simply connect that power cord to your solar charge managed battery bank, I.e. directly to the battery bank.
If your battery bank is something other than 12-volts, say 24 volts for example, you may "tap" the battery bank at the 12 volt position. To do that, the negative lead (black) of the transceiver's power cord will connect to the battery bank's common / negative (ground potential) terminal. The power cord's positive wire (red) will connect to the positive terminal of the first 12 volt battery (the battery with its negative terminal at ground potential).
Before connecting the transceiver's power cord to the battery bank, I would recommend measuring the battery connection points with a DC volt-meter to insure the polarity and voltage are correct. Also verify the battery banks ground potential connection is actually at ground potential - there should be zero DC voltage measured between the negative battery terminal and the chassis-ground of the AC inverter. Obviously, this can alternately be verified by visually tracing the inverter's negative power wire back to the battery bank's negative / ground terminal.
Hi Eric, this (link below) greenhouse insulating system looks interesting. I wonder just how much R-value would be added with such a system, and how much it would reduce your heating energy requirements. Looks like something that you could probably fabricate yourself in the interest of saving money:
A true 10" to 12" system size batch-box can easily push a 75-ft. long "half-barrel" horizontal thermal mass bench. I picture one running straight down the center of your greenhouse, as the secondary (mass) heat exchanger following the "bell". Details of Matt Walker's half barrel bench can be found here:
I do see a potential problem with making an arch over the firebox. In thinking about the super clean burning high performance design of Peter van den Berg's batch-box, and that it depends upon straight-line primary airflow through the fuel wood pile to the port. Ideally, and as recommended, the fire is lit near the port and burns from the port end of the wood pile back towards the primary air intake. Incoming primary air flows through the wood pile (firewood loaded lengthwise having one end near the primary air intake and the other end facing the port, which is why the fireboxes are naturally constructed long and skinny). Combustible gasses don't linger inside the firebox, but are essentially "piped" directly to the port for secondary combustion inside the riser.
Adding the arch, increasing the ISA of the firebox, introduces extra area for the unwanted buildup of combustible gasses. The danger is when those gasses flash off inside the firebox, pressure instantly builds, to much for the port to handle, thus the stove belches smoke and fire out of the primary air intake. In the worse case, the processes repeats itself in a cyclic manner.
Better to stay with the intended batch-box design, go for a hotter faster fire, and store the excess heat in masonry mass. That's really to best way to conserve on fuel-wood.
Eric, that is a beast of a batch-box:o) I've burned yellow pine in my 6" batch-box, and it burns crazy hot compared to hardwood. Pine just doesn't last as long, but it generates a whole lot of heat very quickly, tending for the whole load to be catching fire all at once when the stove is already warmed up to operating temperature. Might try a third to a half load of pine to see how it does first, while keeping a close eye on just how hot the steel top loading lid gets.
About the front loading door, mine is cast iron and gets uncomfortable to sit within 4 feet of whilst the batch-box is running full tilt. I noticed how Matt Walker constructs his front loading doors with CFB for insulation. I'm thinking of making something similar for my batch-box.
Gert Gerard wrote:
Anyone have experience with some similar long bench and short chimney?
Gert, your planned system will be almost identical to mine, when I account for the drag of two 180 turns inside my bench. I've never needed a bypass.
Your natural draft can be tested before the stove and bench is built. I.e. lay the 10m long duct on the floor and connect it to the 5m chimney. You should be able to feel air flowing into the open end of the 10m duct. The draft may even be strong enough to pull the flame off a lit match or lighter, when the room is warmer than it is outdoors. A very good sign to say the least.
The Yaesu FT-991A is an "all your eggs in one basket" (MF ~ VHF frequency coverage) miniature mobile class MF/HF/VHF/UHF radio. The more eggs in the basket, the greater probability of breakage. And reading through the reviews on eHam.net, lots of folks are having problems with the company and the FT-991A:
Anne Miller wrote:The signal booster is in my laundry room which the only access has a large water tank and our well pressure tank so there is no room to get a vehicle in the area.
It is entirely okay to use any convenient length of common electrical extension cord, either plugged into the 12 VDC to 120 VAC Inverter Unit's output, or a computer system UPS's 120 VAC outlet, in order to get that emergency power 120 VAC source to where it is needed, i.e. your laundry room:o)
If your vehicle / automobile is available during the power outage, then you have a portable 12 volt DC generator (vehicle running at idle) right there. In order to have AC power, a small 12 volt DC to 120 volt AC Inverter (i.e. from Radio Shack) can be hooked up to the vehicle's battery, or plugged it into the vehicle's cigarette lighter socket, or DC ass'y jack, which will easily power a phone, or cellphone booster.
When the power fails here, I use my computer's "extended backup time" UPS system to run 120 VAC powered items. The UPS uses a pair of sealed 35 ampere "outboard" batteries. During a power outage it will run a desktop computer for 30+ minutes along with my phone system.
The Icom IC-718 is what I'd consider an entry level HF fixed station radio, though it is small enough to go mobile too. For an HF SSB transceiver budget of around $1000, and considering this will be that once in a lifetime purchase, the recommendation most often given is the Icom IC-7300, a feature packed multi-mode rig that covers 160 meters to 6 meters and includes a built-in "antenna tuner". It is such a good rig that even Hams that have been in the hobby for 40+ years, favor the IC-7300 as their primary everyday radio. It is that good in both performance and dependability, and you get a lot for your money.
Leigh Tate wrote:So, in order to have long distance communication, my understanding at the moment is that I need to be looking into an HF transceiver and a relatively tall dipole antenna. Setting up up DC is no problem.
That's a start!
Yes, HF SSB transceiver. SSB = Single Side Band, the most commonly used shortwave voice mode, and HF = High Frequency aka Shortwave, those frequencies from 3 to 30 MHz, generally speaking. Most HF SSB rigs also include the 160 meter Ham band (1.8 ~ 2.0 MHz) which is great for local communication within about 50 ~ 70 miles radius during the daytime, and several thousands of miles after dark. The band opens into Europe late in the evening, close to midnight for us East Coast Hams.
Your dipole antenna does not have to be ridiculously high in the air. Also, the easiest configuration for a dipole is the Inverted-Vee. I.e. hoist it up a flag pole, or from a rope over a tree limb, supporting the antenna only from it's center with 1/8" diameter Dacron rope. Each end of the antenna is then stretched out in opposite directions and tied off a few feet above the ground to whatever is available; bush, ground stake, nearby tree, etc. The height of the ends is not critical, only that the angle between the two "legs" of the antenna (which form the upside down V-shape) are 90 degrees or more, but never less than 90 degrees. The center support height can be fairly low, i.e. 20 to 30 feet and the antenna will still perform well.
The antenna lead-in will be coaxial cable (type: RG-8X is a popular size) which connects to the Inverted-Vee's center "feed point". Running a search for "80 meter dipole" and "Multi-band Fan Dipole" will bring up some pictures. If making your own antennas sounds interesting, and it will save you a ton of money, have a look at Ed Noll's 73 Dipole and Long-wire Antennas book http://www.on5au.be/Books/73%20Dipole%20and%20Long-wire%20Antennas.pdf
Wire dipole type antennas are inexpensive and can easily be constructed from readily available materials. I'll bet your 1000 mile distant Ham friends, if they are into operating on the previously mentioned shortwave bands, have already put together a wire dipole antenna or two.
The 12-volt DC powered HF SSB transceivers I've mentioned are universal in that they are made to be operated both as mobile and fixed station, and are among the least expensive group of multi-band shortwave Ham radios. Using 12 volt deep cycle battery power is common practice especially for emergency / AC-mains power outages.
Here's an example of a basic HF SSB transceiver in the least expensive class, one that has good owner reviews:
Leigh Tate wrote:
For communication distances, we have friends who are hams and live about 1000 miles away.
Hi Leigh, your most welcome.
1000 miles is easily done, depending on the band (80/75, 60, 40, and 20 meter bands) and time of day. That's the great thing about shortwave radio, long distance communication. However, the antenna's for these bands tend to be rather long, for wire dipole antennas, varying in length from 120 feet (75 meters) to 33 feet (20 meters). Obviously, due to the antenna size requirement for efficient operation on the shortwave bands, we're talking "fixed station". Mobile operation can be done, with special "loaded" short antennas and higher transmitter power, but that's a whole other topic.
The "handhelds" are VHF/UHF radios with exceptionally limited range, being good for "local" communication, i.e. 20 miles radius, depending on "repeater station" capabilities and multi-repeater link-up options for your area. Obviously out of the question for communicating with friends that are 1000 miles away.
Hi Leigh, what kind of distances are you desiring to communicate over? I personally use the 80/75 meter band (3.5 ~ 4.0 MHz) to stay in touch with friends within a 400 mile radius, operating SSB voice during the early morning hours, and also evenings. That frequency band (and other HF Ham bands) requires at minimal a General class Ham license. With the study guide containing the actual exam questions and answers, passing the exam is a piece-o-cake. Look around your area for the local Ham radio club. They are very helpful to new aspiring Hams and may even be administering licensing exams, or can steer you in the right direction.
Off-grid would be a none issue for the common 100-watt multi-mode HF transceivers, which are 12 volt DC powered by design. Obviously that 12 volt DC power can be supplied by the electrical system in the average automobile, or in the house / ham-shack via 12 volt battery system, or an AC mains generator powered 12-volt DC power supply.
Under ideal operating conditions the MTBF service life of a typical 100-watt class multi-mode HF transceiver runs about 10 to 15 years, before it needs to go in the shop for servicing and or repair. With that, I'd recommend against buying a secondhand radio unless you know of a good local electronics shop qualified to service your particular make/model Ham rig. This is coming from someone that's actively servicing Ham equipment professionally for the past three decades.
Hi Eloise, if your 600 sq. ft. house is constructed and insulated to Net-Zero standards, using a passive solar design, I imagine the living space will remain above freezing with nothing more than the waste heat given off by the refrigerator, and etc., according to what I've read on them. Given that, a Matt Walker style full size masonry cook stove with thermal mass bench will not only heat such a highly insulated space, but would have one opening a door or window in the dead of winter to keep from overheating the space -- or running the energy recovery ventilator in cooling mode. Here's a short video on one such Net-Zero dwelling in Canada that goes into the construction details, next best thing to an Earthship IMHO:
Properly constructed and configured, an Earthship will maintain a 70° nominal living space temperature all year round, summer through winter, in temperature extremes from summer's high 90° F, to winter's below zeros, by way of the building's thermal mass / thermodynamics / passive solar. That one thing is what caught my attention. Zero utility bills, with the exception of gas (LP/Propane) on demand hot water to back-up the solar hot water on cloudy days - as per Mike Reynolds designs (see https://earthshipbiotecture.com for more details.
About the Rocket Mass Heater, my stove is essentially Ernie & Erica's Cabin-8 but modified to use a Peter van den Berg batch-box combustion unit; see Cabin-8 link below. The whole nine yards, bench and all, is 9.5 feet in length. No cob used, I chose to use brick instead (see photo below). The trusty steel drum is painted with 1200° (F.) stove black for good looks. During an average winter, on the coldest days, overnight temperatures at my place will tend to drop into the low teens, rarely but sometimes near or below zero. Daily highs usually go above freezing to as high as the 40s.
The stove is typically fired each morning from 4 to 6 hours straight, bringing the immediate room temperature up (from 62° to 65° F) to between 72° and 75° F. The mass heats up and provides a nice 24 hour flywheel into the following morning. The overnight drop in room temperature could be made less by better wall insulation (currently R11). I do have R38 in the ceiling. I imagine, that in AK, code will dictate insulation R-values. For mass heaters, more insulation is better and I would shoot for at least R30 in exterior walls and R50 for the ceiling, and an air-lock entry doors arrangement.
The 8" system size RMH will easily heat my entire house and addition, with the use of a couple of fans pushing air at floor level from the furthest rooms towards the stove.
Just realized, that TedX video is from 12 years ago. In it Reynolds mentions that Earthships are built into the ground. However, the latest models of the Earthships are built on grade, and then have the dirt etc. berm pushed up to and formed around the building and perimeter insulation as the building goes up. Here's an example and tour of a late model Earthship, the "Encounter" model:
I'm using a RMH for 100% heating a fairly insulated stick-frame 900 sq. ft. house with 450 sp. ft. add-on, but actively heating the main (open floor plan) living space of some 680 sq. ft. While my RMH is the best woodstove on the planet (for my use) and has cut firewood usage from 3.5 cords (for a steel box stove) to between 1 and 1.3 cords (RMH) per winter, it is still a chore gathering and processing fire wood on a continual, year after year, for the past 20+ years, basis when one has other things one would rather be doing:o)
I'll pass on some personal opinions, and you're welcome to take or reject anything useful or not so useful etc. Depending on just where in AK you're building your house, you may want to consider building a house (or incorporating some of the ideas) where the house both heats and cools itself, collects, stores and filters its potable water, provides its own electricity, and grows food for you too. And you can still have a Matt Walker masonry cook stove (for auxiliary heating plus cooking) and/or an electric (solar powered) induction cook-top.
When viewing the following video, keep in mind that these dwellings can easily, actually more easily, be built from traditional materials. I.e. instead of labor intensive beating of earth into auto tires, build the walls using steel reinforce MCUs (concrete blocks) filled with concrete, and forget all those cans and bottles too, unless you have a source that will walk itself to the build site. Do keep the four foot wall thickness (thermal mass), perimeter insulation, and moisture barrier - obviously.
Sorry, I'm of no help there as far a moving heat passively. Everything is on one floor level here, with a more or less centrally located, along one wall, RMH (6" batch-box + thermal mass bench) in 680 sq. ft. of open floor space, standard 8 ft. ceiling, in a small stick-frame house. Still grid-tied and using electric fans, to spread the warmth to the furthest back rooms. Would like to build an Earthship one day, but no dirt packed tires (steel reinforced MCUs instead, since I am a traditional builder at heart).
There are other ways to "drive" the cooling tubes effect besides that really nice glass front built-in greenhouse:o) As far as the cooling tubes being sloped at a more aggressive angle, I have no idea if that is of concern or not. Only that cooling tubes that are sized in the range of 20 to 40 feet in length are commonly made to 10 inches in diameter (according the Earthship plans / drawings).
To cool a dome structure, the way Earthship Biotecture does it, is via an opening in the center of the top of the dome, and a short but large diameter pipe (chimney of sorts) fitted at that point and extending above the roof-line. Hot air inside the living space rises and flows out of the ceiling opening through that section of pipe. The sun heats up the pipe's rooftop located convection umbrella, raising the air temperature at that location, thus further driving the thermosyphon effect, and obviously pulling fresh cool air into living space from the buried cooling tubes.
Phil talks about this type of system at around 17 minutes (cooling tubes) and 30 minutes (convection umbrella) into this video:
Your most welcome. If Taos wasn't a 1000 mile drive, I'd show up for a tour of the Encounter model Earthship on the hottest day of the hottest month, armed with a pocket thermometer:o) I've heard summer temps get into the 90s' there just outside of Taos at the Greater-World Earthship community, high desert at 7000 feet. And that makes for a 20° or better temp. diff. with the Earthship staying close to 70° F in the living room.
The Encounter is the basic but still does it all design, the affordable Chevy Nova of Earthships. Here's a neat video tour with some elaboration on the cooling tube system:
Another good video worth watching is the tour given by Tom Duke, including his own early model Earthship he and his family have lived in for close to two decades now:
There are lots of attractive interesting energy saving concepts of the Earthship design, I've been studying to eventually incorporate into my home.
Amy Gardener wrote:Bumping this thread because I'm attempting to try something like the deep pipe in the ground for convection air circulating: thermosyphon. Does anyone have any recommended reads for the concepts at work here? Would Scott's ideas have worked? Anyone have a good source for experimenting with thermodynamics?
Hi Amy, have you looked into Michael Reynolds Earthship designs by chance? The Earthships his team builds in Taos, NM utilize fresh air intake cooling tubes of 10 inches ID, in the range of 20 feet in length, buried approx 8-feet deep. There's a diagram (drawing) on their website here: https://earthshipbiotecture.com/design-principles
The "cooling tubes" slant downhill away from the living space, to drain any condensate to the outdoors. From what I've read about the latest Earthship designs (Encounter & Global models) the indoor temperature stays 70° F all year round without supplemental heating or cooling.
Nice build you have going there Jordan. Yeah, for cleaning mortared in place bricks of excess mortar, a spray bottle of water and a small brush, even an old toothbrush works well.
Re. April's question about the sheets of cardboard, yes, it's used to prevent mortar from falling into the air-gap and bridging between the core brick and outer brick "skin". Such mortar bridges, while the stove is coming up to temperature, would transfer unwanted force between the hot and expanding core to the much cooler outer brick body, resulting in cracks and fractures, and possible combustion gas leakage in certain situations.
Back when Matt Walker had built the larger all masonry kitchen cook stove, the one constructed with standard size IFB for the core, I asked him about the output as a heater. Seems this was on Donkey's forum. Anyway, Mat mentioned the stove would serve as a heater, having enough extra output to drive a small thermal mass bench or bell. Contact Matt and I'm sure he can give you an idea of how much sq. ft. area the larger cook stove is capable of heating.
Steven Lindsay wrote:Thanks Jane, same here. My biggest issue is the 'mass' or overall size, but my heating needs are minimal and for a short season (and seeming to get shorter each year!) I may even be able to get by with solar gain and a trombe wall, but a 'small', efficient fuel heater of some kind would be handy.
For a barrel-less RMH, even for confined space, have you considered building one of Matt Walker's glass top masonry rocket cook stoves? More info. here in case you've not seen his stove designs before, including his all brick RMH design/plans: