Dale's, Cob Igloo Home Heating System --- Replaces forced air furnace --- uses same ducts

A little history --- This is an adaptation of an idea that I devised over 30 years ago to heat a house with a bee hive kiln. When I first learned about the Russian fireplace, I was intrigued with the efficiency but knew instantly that I did not want to develop the masonry skills required to produce one. I haven't. I figured that I could do a cob kiln with an inner liner of firebrick. Picture a cob oven that is big enough to walk into it. Down draft snorkel stoves were popular at the time for heating hot tubs. Their major drawback was that the fire could never get hot enough for an efficient burn. I knew from my investigations into wood fired kilns, that the trick would be to use a firebrick feed tunnel. When sticks are poked into the feed holes of wood fired kilns, they vaporize in seconds. Later on, I read many thing about the rocket stoves produced by Aprovecho. Neat design - cool burn when compared to a kiln. --- In the winter of 99/2000 I heated a home in Nanaimo B.C. using two small electric pottery kilns that were cycled so that when one was cooling, the other was heating. Toasty warm, and the need for heat forced me to be quite prolific for a brand new potter. --- Then , in the spring of 2011, I discovered rocket mass heaters while searching YouTube for thermal mass heating using a kiln. The idea of using a metal barrel to create a pump was new to me. All of my previous plans involved a fairly normal chimney. For over two years, I have watched with interest and sometimes dismay, the evolution of the RMH. Most of what I see presented as innovation seems dangerous or silly. I planned to put one in my cottage two years ago, but the tenants who will be leaving soon, were deathly afraid of them after speaking to some random expert they met at a gas station. The firebrick needed for a good sized unit has sat there waiting.
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There are many existing bungalows which are broken up into so many rooms that heating with a RMH would present some heat circulation challenges. In my area, these homes often have a full height concrete basement with an oil or gas forced air furnace in the middle. I'd like to see those replaced with wood burners.
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This thread will examine the possibility of using a big, kiln shaped furnace room with a RMH built into the wall. (Let's try to limit it to how this can be done. There are innumerable ways to screw things up. I'm already aware of all ten thousand examples.) I say wall because this igloo shaped structure has only one circular wall. It could be built square, octagonal... Picture an igloo shape made of 2 foot thick cob. At about 95 lb per cubic foot, a unit 8 feet in diameter would come close to 20 tons, which seems about right for whole house heating. The RMH would be built into the wall and the feed would be on the outside. The dome would have a door which allows air to enter and be heated. It would get quite hot in there. There would seldom be a reason to go inside. When a furnace is ripped out, we are left with 2 sets of duct work. Hot air is normally pumped by a fan through one plenum and the other ducts are the cold air return. I say we use them all for warm air and don't have a fan. Floor vents can return cold air to the basement. Many older homes used to have convective hot air systems that were called an octopus. They employed larger pipes than are common with forced air systems. By using both the hot and cold air pipes for warm air, we should be able to move plenty of air by convection. I lived in a home in Newfoundland that used a wood furnace with no cold air return. It worked quite well. The floors were warm.

The top of the igloo would be built around the openings of the duct work so that air at the hottest upper portion of the dome could flow up into the ducts and to the rest of the house. A vent in the lower portion of the door to the igloo, would allow cool air to flow in. Stacks of bricks or urbanite could occupy the floor if more thermal mass is desired. A unit that is six to eight feet tall will produce plenty of draft. It would be pretty easy to install a humidifier if one is needed.

Force air heating is not ideal. Most of the downsides have to do with humidity and temperature stratification. The constant on an off of the fan and subsequent temperature swings would not be a problem here. The outer wall of the igloo will keep the basement and the floor above, toasty warm. Any room on the first floor will be heated by the air coming through the ducts and by the heat radiating from flooring materials.

If some cob structures are desired for the upper rooms, they could be built over the vents. Cob window benches would add a nice touch and they would moderate the heating, by absorbing heat which would become radiant.

One advantage to this sort of warm air system is ease of control. Individual room temperature could be managed by opening the vents to maintain the correct temperature. A central baffle could be placed at the top of the igloo, to shut off the heat, or the incoming air to the igloo could be blocked. Either choice would greatly restrict the amount of heat leaving the igloo. This system assumes that we want a warm basement and floors. The outside of the unit could be insulated to prevent heat loss to the basement. I would not build this unless basement heating is desired.

The furnace room could have cob walls and an insulated roof built of flame proof materials. Steel studs, fiberglass insulation and drywall seem simple enough. The plenum would pass through this lid.

We'd want to run airflow that is somewhere between 110 F and 160 F.

A carbon monoxide detector is advisable with any fuel burning system.

It would be wise to allow full air flow while firing. The barrel will pump slower if the inside of the igloo is quite hot. This would cause a greater proportion of the heat to go into the mass. Even with full air flow, some of the heat from the barrel will be absorbed by the wall and some will go immediately into the ducts.

We might want to have some sort of alarm that goes off if plenum temperature reaches a critical point. The flash point of wood is about 575 F. Those ducts are attached to wood. I don't think we ever want to go over 300F. At that temperature, the system would induce massive air flow. I don't see overheating as being a major concern. The igloo should probably be managed at under 175 F. The only issue then, would be the heat coming off of the barrel, since it is in a confined space. That's why we want full air flow while firing.

It would be easy to use this as a clothes dryer and as a dehydrator. The moisture could be vented outside with a little ducting. A rack that can be rolled through the door would work. Don't do it while firing. The barrel will scorch things.

I don't anticipate running up and down the stairs to fire this thing. The basement is where guys do guy things. Fire it while engaged in those things. Tell her that it takes 8 hours to fire it, if that's the amount of "you time" that you can get away with.

Edit --- I just read this thread. https://permies.com/t/23328/rocket-stoves/rocket-mass-heater-forced-air We have a cold winter here, so once the system is fired up in October, I doubt that it would ever cool down completely until May. This should make draft problems less of a problem. Erica also advises basement heating only if the basement needs heat and is regularly occupied in the evening. For me, that means a big screen TV and a computer need to be down there. A workshop would be nice too. Most basements are already a bit of a workshop and many are rec rooms. I see no reason why 2 completely independent 8 inch systems could not be built into one igloo. They would not share a chimney. At the beginning and end of the season, only one would be fired. During the colder months, both could be fired at once so you aren't forced to spend more time than desired down there.

I was 17 when I lived in that house in Newfoundland with the wood furnace in the basement. I suggested to my friend Perry, that a big kiln like affair built over a firebox would store heat and dish it out more evenly. He looked at me like I was nuts. The thing has been taking up brain space ever since. Every time I've lived in a place where the temperature goes up and down every 5 minutes, I come back to it. Hopefully I've cleared out room for other things now.
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The first photo is of a nice glazed furnace room that looks remarkably like an inverted coffee cup. Notice the duct on top.

The second photo is of my professional looking drawing. No attempt was made to draw the RMH properly. It's a regular 8 inch unit with no bench. The thick wall(s) of the furnace room are the thermal mass.

Dale Hodgins : One of my first 'jobs' ever was climbing inside huge old Coal furnaces that had been converted to burn fuel oil and drag a flex-metal hose after me to remove
a years worth of soot from the Octopus type of heater you are describing, Having had my hands full of tools to clean out its Carbon black neither regions, you could say that
I am still intimately familiar with them !

In our region of Northern New York we had many small mining operations that provided Company Housing (and a company store !) for mine employees,This type of Bungalow
often had a dirt basement floor, but was actually often copied by other people as a practical and cheap small house, most with later added Kitchens and mud rooms/wood
sheds ! So again, I have an idea of the types of buildings and Heating options, to include furnaces whose basic design was laid down over a Century ago.(the Octopus)

I too can think of a lot of ways to make a mess out of the very simple plan you are proposing, Certainly having the ability to run down into the basement and open a damper
to put more heat in the back bedroom when 'Company comes' is a major plus !

A few Issues to be worked on, Stack effect within the whole house, much of that could be dealt with by proper sizing and location of a well built vertical Chimney!

While a very tall Heat Riser could be created, especially if a floor vent grate* was installed above your Igloo/furnace room, allowing for easy barrel removal you are limited by
the Ratio of the height of the Heat riser to the length of your burn tunnel, and thereby how close to the center of the room you can get your barrel ! I think this is important
because I am visualizing everything coming up to temperature, and the Skin of the barrel not being able to radiate off enough heat, and the pump not just slowing down, but
stoping ! Here there may be certain benefits to the bell system that strongly appears to have an advantage Here, In fact you are creating a type of bell with the igloo !

While i've been on the computer, Too long ask my wife, life has come along with a -10dF type problem, I'll have to think on this some more ! Big AL

*In the bungalow houses everyone dried clothes over the 4'x4' floor grate and had one or more wooden clothes drying folding racks, And often the kids were washed in a wash
tub, needing only to stand up to grab a toasty towel, the coal fire below making the grate to hot to stand on in bare feet !
toasty towel,

I suppose that if there were a problem with getting draft, a vertical chimney could be added.

A heat riser could be built completely within the wall. Then the flue gasses could spiral around the unit a couple times, exiting 5 ft. off the floor, and continuing to a chimney. This would eliminate the barrel altogether. Again, similar to how a kiln is fired, except that the hollow space contains only clean air. The whole system becomes one long chimney. Romans had similar heaters. The barrel has always seemed the most troublesome component. A long, spiraling chimney would have nothing made of metal. It's closer to the Russian fireplace in operation. Firing would not produce an immediate heat surge. Instead, the unit would be fired to keep the cob temperature in the desired range and heat delivery controled by vent adjustments. A simple rope and pulley system could allow the main plenum baffle to be adjusted from upstairs or it could be thermostatically controlled. Firing times could vary quite a bit. The greater the thermal mass, the less concerned we need be about exactly when it is fired.