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Glenn Herbert

Rocket Scientist
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since Mar 04, 2013
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Glenn Herbert currently moderates these forums:
Early education and work in architecture has given way to a diverse array of pottery, goldsmithing, and recently developing the family property as a venue for the New York Faerie Festival, while maintaining its natural beauty and function as private homestead.
Upstate NY, zone 5
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Recent posts by Glenn Herbert

One thing you could do to reduce the length of horizontal stovepipe to the chimney would be to switch from a ducted bench to a hollow brick box open to the main mass cavity. That would allow hot air to circulate freely, then you could run an exhaust plunger tube from the bottom of the cavity nearest the fireplace up and over just a few feet. If you want to force more circulation through the bench cavity, you could run the exhaust tube along the floor to the mouth of the bench.
3 hours ago
A single bell is better simply because it is easier to build. The small holes in the walls would be for hot gases to travel from one bell to the next. There would be only one fire.

I think the best way to use copper pipes to absorb heat for pumping to remote rooms would be to embed the piping in the cob mass of the bench, so it will still be getting heat when the fire has been out for a while. I would probably locate it near the top surface of the bench so that it doesn't get too hot, not near the hot interior which may be well above boiling temperature.

Another way which would keep all the piping accessible for inspection or maintenance would be to expose a largish tank of water to the heat, and run your piping through that to exchange heat for pumping. This ensures that the pressurized piping never gets to boiling point while giving it consistent heating.
7 hours ago
To your question about a future second floor and being able to heat it, that depends largely on your climate. How cold do winters get? Do you have a lot of warm and cold days, or does it get cold and stay cold for days at a time? 100 m2 is around 1000 square feet, so easily heated by an 8" J-tube in a moderate climate. A second floor on top of that might be a stretch to heat. You say the walls are adobe, is there any insulation added? Adobe is great for hot days and cold nights, not so much for long cold spells. You might need a larger system, or an 8" batch box instead of J-tube. (A batch box puts out about twice as much heat as the same size J-tube.) Batch boxes are more complicated to build than J-tubes. You might start by building the J-tube, and see if it gives you enough heat without needing long burns. Make the layout so that you could change to a batch box later if you need to.
"Set in stone" means a fixed, unchangeable plan

I see you have an existing house where you want to add a rocket mass heater. Thomas' idea of a single centralized bell or stratification chamber would be perfect for new construction, but obviously not for your actual situation. Do you want benches for sitting on, or would vertical masses work better for some or all of the rooms? Assuming you can't safely make more than small holes in the walls, you could do a series of bells one in each room with short channels connecting one to the next.

The J-tube location looks fine. Where would the wood be brought in? It is good to make the path from there to the fire as short as practical, to minimize carrying and mess.
2 days ago
Welcome, Christian! I had radiant floor heat for 15 years before I built my RMH six years ago, still have it for the basement as the heater is on the main floor. I love the radiant heat, but don't miss it with the RMH operating. The comfort is very similar. If you have a zone for the second floor, you can probably keep that running as needed while the RMH takes care of lower floors. Having a full backup system is valuable if you are ever away in winter.

I have found that the lofts above my main floor are warmed well too, even the one farthest from the heater. As long as your house is relatively compact and decently insulated you should be okay. I would have to see your setup before guaranteeing anything, of course. All the "shoulds" and "probablys" you hear are due to the many variables in houses; nothing works in all situations. Identify all features of your house that might be different from ones known to work and you can get a good idea of how yours would work.

For the basement installation, there are a few issues to consider.
Draft: What is the insulation and sealing level of your house? If the upper floors let hot air out, that might compete with the chimney and pull smoke back into the basement. You want the house well sealed to retain heat anyway. Is your chimney inside the house? That will have better draft than an external chimney (and also not waste heat to the outdoors.)
Convenience: Do you already use your basement regularly? If you don't have any other reason to go down there, you may have trouble keeping the fire tended. A J-tube needs to be fed every half hour or so for the couple-few hours per day required to build up heat in the mass. A batch box can give the same amount of heat with fewer loadings. Of course, some people have found that once they had a warm comfy spot in the basement it became a favorite hangout.
Heat distribution: If you have openings in the floor above or an open stairwell, you may get plenty of warm air migrating to the first floor. Another possibility is a tall bell in the basement that extends through a framed opening in the floor to give a radiant source in the first floor. You would have warm floors from the basement and direct heating. This depends on your ability to find a workable location for both floors simultaneously. A double bell on two different floors has also been done, but that is a more advanced project for an experienced builder.

Boom squish is a serious issue, but there is a well-tested style that is safe. An open (unpressurized) tank is heated, with a pressurized coil running through it and feeding the heating load. As pressurized water has a higher boiling point than atmospheric water, even if the tank boils the coil will not. A makeup water supply with float valve is needed to make sure the tank does not run dry.

4 days ago
Welcome to permies, Nate. Sounds like you have already been studying rocket science

You say you like the simplicity of the J-tube over the batch box, but it sounds like you are introducing as much complexity as a batch box with doors and air washing glass. If you have a chamber large enough to load with a batch of larger logs, the geometry of the burn tunnel and riser goes away and you no longer have the turbulence of the fast right angle bend. To bring that back, you could make a constriction... like a batch box has.

A top loading firebox may be fine for starting; what happens when the first load burns down and you want to refill it while it is still hot? You will have a strong push for hot gases and smoke to go straight up into your face as you are loading. Draft will not overcome that for a large lid. A front door will have less of that issue.

Peter van den Berg spent literally years and many dozens of iterations with professional testing equipment to optimize the batch box design. I believe all of the issues you mention have been addressed in his work.
4 days ago
A selvedge (along the factory woven edge of the fabric) will not fray, but will likely want reinforcement for attachments. The square sewn on and cross-stitched with an "X" is typical for commercial canvas.

Cut ends of fabric will definitely want a hem to avoid fraying.

I would advise taking the sewn-up roof with the last seam undone, putting it on the frame, and seeing how much more it wants with the edges adjusted for proper overhang. Measure at top and bottom as they will likely be different. You really do not want extra width especially at the bottom as that will allow sagging right where water might want to pond. Extra width at the top would pretty much just be sloppy and maybe allow flapping.
5 days ago
Who coined the term "Rocket Mass Heater"?
--?Probably Ianto Evans and his collaborators, maybe including Larry Winiarsky while they were first working together. (Check this with Ianto's book which I don't have on hand at the moment.)

When were RMHs invented?
-- ( " )

Why is it called a "Rocket"?
--From the "whooshing/roaring" sound made by a J-tube burning at full blast.

Will that big metal rocket barrel suddenly launch off in my living room?  If so, do I need to notify the FAA?

The mizzus hates the looks of that barrel.  What other options are there?
--Lots, from simply disguising or dressing up the barrel with custom fabricated versions or decorations, to styles which use masonry of various types in place of the barrel to surround the heat riser.

Is there a coffee table book or directory of nice, finished, Pinterst-y RMHs?  
--This thread, beautiful rocket mass heaters on permies has good examples.

I see there is a "ship-able core" called the Liberator for the rocket stove part.  If I buy that, how do I add the mass part?  Are there instructions online specifically made for the Liberator?

I don't want mud in the house.  Do I have to use cob?   What materials can I use for the "Mass" part of an RMH?  
--You don't have to use cob, even though it is generally free and very effective. Any material that is noncombustible and holds significant heat to release later will work. In particular, brick, stone, concrete and water are very effective, as long as any gaps are filled so there is solid material with no air spaces. Air spaces insulate, so sand is a mediocre mass by itself.

What is that ducting made of?  Can I use bricks instead of metal ducting?
--Ducting inside a mass is generally galvanized HVAC duct, as that is the cheapest new material for the job. Any smooth noncombustible tubing will work as long as the size is right and it does not have sharp corners. Flexible corrugated duct will not work well as it is too rough and will cause drag. You can use bricks as long as the round duct it replaces would fit inside the brick channel.

Do I have to use firebricks to build one of these?  What about "normal" bricks?
--You need firebricks or other highly refractory materials for the combustion core where temperatures can be 2000 degrees F (xxxx C) or more. For other parts, ordinary clay brick will work fine. Clay brick can even be used for the combustion core in a pinch, though it will not be durable there. Concrete bricks will not be durable in any higher heat areas, and may crumble or explode if used in the combustion core.

What are the main benefits of, or differences between a RMH and a Masonry Stove?
--A rocket mass heater is actually a subset of masonry heaters. The centuries-old attraction of masonry heaters is that they allow for a hot, fast fire and store the heat to release over many hours or days without a fire burning. The essential difference between a traditional masonry heater and a rocket mass heater is the super-efficient combustion core. Common differences are that an RMH is often built by the owner using local, scrounged or recycled materials instead of by a professional mason using expensive new materials.

Does the fire in an RMH always burn sideways?  Does it ever change its mind and smoke up the house?
--A properly built J-tube RMH once primed and burning will essentially always burn sideways drawing in fresh air down the feed tube. An improperly built RMH may be subject to smokeback either sometimes in unusual weather conditions or frequently due to poor design. Sometimes an unusual environment like mountains or forests will make draft reversal and smokeback happen despite optimal system design. This is why certain criteria of design and construction need to be followed, and not just freehanded or "similar" construction.

Are there free clubs, non-profits, professional associations, or business organizations near me who can help teach me how to build or operate one of these?  What about helpful individuals?
--This is highly localized and cannot be answered in general. There are lots of helpful individuals who will give free advice here on permies, and some around the world who may be available to hire or consult. List of Rocket Mass Heater Builders
6 days ago
Brick flues versus standard metal ducting: It is reported that a square channel offers about the same resistance as a round channel of the same dimension, so replacing 8" duct with an 8" x 8" brick flue should work about the same. I would be generous as there is also the roughness factor added, though only a small part of the square cross section will actually contact the main flow for roughness to apply.
6 days ago
I have a LOT of experience with temporary roof covering I like to use scrap lumber with a couple/few screws to batten down exposed edges. Also, for a very strong covering, you can get used billboard vinyl tarps. A 14' x 40 or 48' one can be gotten for around $100 or so.