By Eric Moshier – Certified Heater Mason, Third-Generation Mason, MHA Technical Committee Member, ASTM E1602 Masonry Heater Group Member
Solid Rock Masonry LLC in Duluth, Minnesota builds masonry heaters, which is the legal version of a rocket mass heater. The SR-13HCH and SR-18HCH were designed to deliver what people actually want from a rocket mass heater while being built to ASTM E1602 and the residential masonry heater code, which means they can be permitted and inspected, and gives an insurer something recognized to underwrite. Owner Eric Moshier is a Certified Heater Mason, MHA Technical Committee member, and ASTM E1602 committee member with more than 25 years in the trade. Both models pair a firebrick firebox with a steel cook top set directly above it and a bell chamber on the side that strips heat out of the exhaust before it reaches the chimney. Materials are firebrick and concrete block, not cob. Solid Rock Masonry Heater Kits start at $7,016 FOB Duluth and ship nationwide. Burn time is 2 to 3 hours per fire, and one fire heats roughly 700 to 1,800 square feet depending on model.
Want to skip ahead and buy one? SR Core kits start at $7,016 and ship nationwide from Duluth, Minnesota. Every kit includes firebrick, cast refractory, refractory mortar, insulation, gaskets, the SR firebox door and hardware, and course-by-course drawings you build from. See all models and kit prices, or call 218-343-2978 and I will tell you which one fits your house.
What is a rocket mass heater?
The rocket stove was not invented to heat houses. It was invented to solve a cooking problem. Through the 1970s the Aprovecho Research Center in Cottage Grove, Oregon worked on wood cookstoves for families cooking over three-stone fires, where fuel was scarce and indoor smoke was making people sick. Ianto Evans developed the Lorena Stove in Guatemala out of that work. Around 1980 Aprovecho’s technical director, Dr. Larry Winiarski, developed the design that became known as the rocket stove, building on earlier work by Sam Baldwin. Aprovecho is still going, still testing stoves in Oregon.
What Winiarski arrived at works through materials and geometry rather than gadgetry. The burn chamber is sized so the fire reflects heat back on itself. The riser above it is insulated so it stays hot enough to finish combustion. The fuel is small, dry, straight wood. The openings are proportioned to meter the air. The L or J shape of the passage forces the gases to mix. Get all of that right and the wood burns almost completely, which is the whole trick.
The rocket mass heater came later, when Evans asked what would happen if you stopped letting that heat go up the chimney and stored it instead. In the classic form the combustion unit sits under an inverted 55 gallon drum, and the exhaust runs from there through ducting buried in a cob bench or bed. You get a fast blast of radiant heat off the barrel while the mass charges, then hours of low steady warmth out of the bench afterward. Roughly an hour of tending the fire buys 12 to 24 hours of heat.
That is the same principle a masonry heater runs on. Burn hot and fast, store the heat in mass, release it slowly over the day. The difference is entirely in the execution, and that difference is what the rest of this page is about.
What is a rocket mass heater like to live with?
The most honest account of the downsides comes from the people who wrote the book. Leslie Jackson, co-author of Rocket Mass Heaters with Ianto Evans, lays them out plainly. The mass bench is heavy enough that the floor or foundation may need reinforcing. The inverted oil drum is a large and not especially handsome presence in a living room. The fuel has to be small, dry, and straight, which means processing and storing it separately from ordinary firewood. There is a tinkering factor, with maintenance, creosote, and the occasional burn-back to deal with.
The one that stops most people is attention. A rocket mass heater wants tending for the whole burn. You are checking fuel and feeding it in small pieces for roughly 45 minutes to an hour to charge the mass. As Jackson puts it, if you would rather light a fire, set the air, and walk away, this may not be your stove.
That is a fair description of the classic J-tube design, and it is why the batch box came about. Jackson points at the batch box herself as the likely fix for the attention problem, naming Lasse Holmes in Homer, Alaska as an early adopter. Which brings us to the question of what to actually build.
Which rocket mass heater design should you build?
I like rocket mass heaters, and I like the people behind them even more. Ernie and Erica Wisner have done more than anyone to turn this from a pile of internet guesses into a real body of work. Ernie was on the research team for the second edition of Rocket Mass Heaters. Between them they have built, supervised, or repaired installations across a wide range of buildings and advised on projects on all six inhabited continents. Their Rocket Mass Heater Builder’s Guide is the closest thing the field has to a proper construction manual, and I had a hand in it. Erica included a section drawing on my masonry heater work.
For a traditional J-tube rocket mass heater, their plans are where I would start. Learn their terminology before you talk to anyone about a build, because it will save you a great deal of confusion.
I prefer the batch box. It uses a real masonry heater door on a front-loading firebox instead of the J-tube feed, which makes it behave much more like the heaters I build every day and makes it far easier to live with. You load it, shut the door, and let it burn. The idea came out of an exchange between the two, with Lasse Holmes contributing the concept that Peter then developed and optimized, building in turn on Peter’s own earlier experiments.Batch Box Rocket, documented in detail and released open source at batchrocket.eu. He is meticulous, and his firebox geometry now turns up in wood burning appliances well beyond the rocket world.
It is worth paying attention to how these people talk about their own numbers. Peter’s development was done with a Testo 330-2 gas analyzer logging temperature, oxygen, and carbon monoxide through every burn, and his own site states that its conclusions are plausible rather than scientifically substantiated. He has said publicly that efficiency figures vary enough between analyzers and between national formulas that they are an unreliable thing to chase. Ernie and Erica publish an article specifically pushing back on inflated performance claims. That is what careful people sound like, and it is a good filter for everything else you will read on this subject.
It is also not the same thing as an appliance tested to a standard method by a laboratory, which is what a building official and an insurance underwriter are looking for. Both things are true at once, and that is where the next section comes in.
Do rocket mass heaters burn cleaner than masonry heaters?
Nobody can honestly answer that, and that is the real answer.
A masonry heater can be tested. ASTM E2817 is a published standard test method for fueling masonry heaters during emissions measurement, used together with ASTM E2515, and it reports results in grams of particulate per kilogram of dry fuel. It specifies the fuel loading, the firing intervals, and the whole protocol, so that two different labs testing two different heaters produce numbers that mean the same thing.
There is no equivalent for rocket mass heaters. The development work has been done with handheld flue gas analyzers by careful people, and that work is genuinely useful, but the developers themselves say the numbers are not comparable between instruments and are not scientifically substantiated. So when someone tells you a rocket mass heater is the cleanest burning wood appliance there is, ask what method produced that number and who repeated it.
The masonry heater data is not a marketing claim either. Testing across the appliance class shows masonry heaters operating well below EPA Phase II particulate limits for wood stoves, and Lopez Labs work suggests sustained performance below 1 g/kg may be achievable, though that research describes its own results as not conclusive. E2817 itself warns against comparing emissions rates across appliance types without studying how each one actually operates. I would rather give you that caveat than a number. I am a member of Lopez Labs, and I would rather you trust the method than trust me.
Both designs burn hot and fast and store the heat, which is why both burn cleanly compared to a smoldering box stove. The difference is that one of them has a standard, and a standard is what a building official, an insurer, and an air quality district can act on.
It is also worth knowing how long this has been going on. Stove tile production in Germany is documented as early as 1350, made by the stove builders whose trade name gives us Hafner and Hafnermeister. By the Renaissance, building tile stoves was a recognized craft with its own masters. The Russian stove took its familiar form in the fifteenth century once the brick flue was added. That is roughly seven hundred years of people refining how to burn wood hot and hold the heat in masonry. The rocket stove dates to about 1980. There is nothing wrong with being the new idea, but it is worth being honest about which body of practice is deeper.
Are rocket mass heaters legal to build?
The honest answer depends on how one is built, not on what it is called. The problem is the construction method, not the idea.
A classic cob and steel drum rocket mass heater is not a recognized appliance in the residential building code. The code addresses masonry heaters and points to ASTM E1602 as the construction standard. Cob, earthen mass, and a salvaged drum are not what that standard describes, so a building official reviewing one has nothing to measure it against and the permit usually does not get issued. That is not a judgment about whether the design works. It is a statement about what the code recognizes.
A batch box is a different situation. Because it burns behind a door in a masonry firebox, it can be built to the masonry heater code. Lasse Holmes’ work shows this is achievable. If you want a rocket mass heater that a building official can actually approve, that is the direction to go, and you should plan on brick rather than cob for the parts that matter.
So where are all the cob heaters actually getting built? Mostly in places where nobody is inspecting. The International Residential Code is a model code. It carries no force of law anywhere until a state legislature or a local governing body formally adopts it. Some states adopt it statewide. Others leave adoption to individual cities and counties, and plenty of rural counties in those states have never adopted it or have no one on staff to enforce it. That is where most owner-built cob heaters live. It is not a loophole anyone found. It is the absence of a code official.
Two things follow you even where there is no inspector. The first is insurance. Coverage terms vary by carrier and policy, and an unpermitted, site-built heating appliance is exactly the kind of thing an adjuster examines after a fire. Ask your carrier in writing, before you build, whether they will cover it, and get the answer on paper. The second is resale. An unpermitted heat source in the house becomes the buyer’s inspector’s problem, and then yours.
There is a narrow exception worth knowing about. Some jurisdictions have a formal process for reviewing building methods the code does not yet cover. Portland, Oregon runs one through its building bureau, an Alternative Technology Advisory Committee. Under Portland City Code 24.10.087, that review serves only in an advisory capacity to the City, and whatever the City does afterward through the applicable board of appeal is the City’s own responsibility. It is a route to ask the question, not a permit, and most jurisdictions have no equivalent.
What does Solid Rock Masonry build?
Let me be straight about this, because someone is going to ask. The SR-13HCH and SR-18HCH are not technically rocket mass heaters. There is no insulated riser. What they are is my answer to what a legal rocket mass heater would look like if you had to get it past an inspector: a small firebrick firebox with a steel cook top over it, and a bell on the side to pull the heat out of the exhaust. It is a simple masonry heater, and it is straightforward to build.
It does the same three things people want from a rocket mass heater. You get instant radiant heat off the steel cook top, the same way you do off a 55 gallon drum. You get heat storage out of the mass. And you get the efficiency of a bell. You can cook on it. If you like the look, you can finish the outside in cob, clay concrete, or 4″ of any solid masonry.
If someone wants something simple and reasonably priced that will heat their house and cook their food, and that a building official will sign off on, this is what I build instead of a rocket mass heater. Materials are firebrick and concrete block at minimum. Full model range and options are on the SR Core DIY masonry heater kits page.
SR-13HCH specifications
| Firebox | 13″ w x 18″ d |
| Core Footprint | 50-1/2″ long x 23″ deep (23″ firebox section plus 27-1/2″ bell chamber) |
| Finished Footprint | 58-1/2″ long x 31″ deep with 4″ veneer |
| Cook Top Height | 39″ above the hearth |
| Height at Firebox | 54-1/2″ |
| Overall Height at Bell | 59-3/8″ |
| Chimney | 7″ ID round clay tile or equivalent |
| Clean Out | 4-1/2″ x 4-1/2″ on the 2nd course |
| Fuel Load | 35 lbs. per fire, maximum every 8 hours |
| Burn Time | 2 to 3 hours |
| Heat Output (1 fire) | ~183,750 BTU per fire / ~7,650 BTU/hr over 24 hours |
| Heat Output (2 fires) | ~367,500 BTU over 24 hours / ~15,300 BTU/hr |
| Heating Area | ~700-1,000 sq. ft. with 8 ft. ceilings, Energy Star construction |
SR-18HCH specifications
| Firebox | 17″ w x 18″ d |
| Core Footprint | 54-1/2″ long x 23″ deep (24-1/2″ firebox plus 30″ draft box) |
| Finished Footprint | 62-1/2″ long x 31″ deep with 4″ veneer |
| Cook Top Height | 39″ above the hearth |
| Height at Draft Box | 54-1/2″ |
| Overall Height | 59-1/2″ including 3″ insulation and 2″ finish cap |
| Cook Top Plate | SR-18SCT steel cook top, 32″ x 31-3/4″ |
| Chimney | 8″ ID round flue or equivalent |
| Fuel Load | 60 lbs. per fire, maximum every 8 hours |
| Burn Time | 2 to 3 hours |
| Heat Output (1 fire) | ~327,000 BTU per fire / ~13,650 BTU/hr over 24 hours |
| Heat Output (2 fires) | ~655,200 BTU over 24 hours / ~27,300 BTU/hr |
| Heating Area | ~1,200-1,800 sq. ft. with 8 ft. ceilings, Energy Star construction |
Both models come out at the same 39 inch cook top height, measured from the top of the concrete foundation. That is the number to plan a kitchen around.
Clearances to combustibles
Clearances are what a building official asks about first. These come off the unit layout plans in the owner manual.
| Sides and Back | 4″ minimum to combustibles |
| Side with Firebox Door | 48″ minimum to combustibles |
| Above the Cook Top | 36″ minimum to combustibles |
| Clearance Envelope, SR-13HCH | 66-1/2″ wide x 83″ deep |
| Clearance Envelope, SR-18HCH | 70-1/2″ wide x 83″ deep |
| Foundation | 4,500 psi concrete or better, 8 inches deep, 1/2-inch rebar at 12 inches on center |
| Hearth Extension | 18″ masonry hearth extension |
Pricing
| Model | Configuration | Kit Price (FOB Duluth MN) |
|---|---|---|
| SR-13HCH | SR-13 firebox with cook top | $7,016 |
| SR-13HCH-BO | SR-13 firebox with cook top and black oven | $8,216 |
| SR-18HCH | SR-18 firebox with cook top | $8,769 |
| SR-18HCH-BO | SR-18 firebox with cook top and black oven | $9,969 |
Prices are for the core kit, FOB Duluth, Minnesota. Black oven and heated bench configurations are available on both models. Custom sizes are available, so call to discuss your project.
What a finished one looks like
The core is firebrick. Everything outside it is a free choice. Solid Rock Masonry has finished these in common brick, stone, stucco, and in handmade tile, and the same core accepts a heated bench or a black oven on the bell side.
The heater below was built for a client by Rochester, Minnesota. It is a see-through unit finished in stucco with handmade art tile set into the face, and it was built without the cook top, which is an option on the SR-13 platform for anyone who wants the heat and the code compliance without the cooking surface.
More finished work is in the project gallery.
Who is a rocket mass heater for?
- Off-grid homes and cabins where one appliance needs to heat and cook
- Homesteads and small farms heating 700 to 1,800 square feet
- Owner-builders who want thermal mass at the lowest entry price in the SR Core line
- Anyone who has researched cob rocket mass heaters and run into the permitting wall
Use LoadCalc.net to calculate your design heat loss before choosing a model. A masonry heater is a radiant heat storage appliance, so its output spreads across 12 to 24 hours. A peak BTU per hour figure from a Manual J calculation is not the number to size against.
Rocket mass heater questions
Can a rocket mass heater pass inspection?
A cob rocket mass heater generally cannot, because there is no standard to inspect it against. A heater built to ASTM E1602 and IRC R1002 can, because those are the standards written into the residential code. The SR-13HCH and SR-18HCH are built to both.
What is the difference between a rocket stove and a rocket mass heater?
A rocket stove is a cooking stove with no thermal mass. A rocket mass heater burns fuel the same fast, hot way but routes the exhaust through a large masonry body that stores the heat and releases it over many hours. Only the second one heats a house.
Will my insurance cover a rocket mass heater?
Policies vary, but most carriers will not cover a loss involving an unpermitted heating appliance. A permitted, inspected, code-compliant heater removes that problem. Ask your carrier before you build anything.
How much does a rocket mass heater cost?
Solid Rock kits start at $7,016 for the SR-13HCH and run to $9,969 for the SR-18HCH with a black oven, FOB Duluth. That is the core kit. The owner supplies the foundation, chimney, and finish material.
How often do you have to feed it?
One fire burns 2 to 3 hours, with a maximum of one fuel load every 8 hours. In cold weather most owners run two fires a day. Between fires the masonry radiates heat with no fire burning at all.
Can I build one myself?
Yes. The kit ships with firebrick, cast refractory, mortar, insulation, gaskets, door and hardware, and course-by-course drawings. Owner-builders complete these regularly. See the DIY page and the workshop schedule.
How clean does it burn?
Masonry heaters burn very cleanly because combustion happens fast and hot inside an insulated firebox rather than being damped down. See the particulate emissions testing page for the measured numbers.
Talk to Eric about your project
Ordering a kit. The SR-13HCH is $7,016 and the SR-18HCH is $8,769, with black oven versions at $8,216 and $9,969. Prices are FOB Duluth, Minnesota and kits ship anywhere in North America. You supply the foundation, the chimney, and the finish material. Go to the SR Core kits page to order.
Not sure which size? Send your floor plan and a heat loss calculation from LoadCalc.net and I will tell you which model fits. If you would rather talk it through, call 218-343-2978. If you are still deciding whether the investment makes sense, start with are masonry heaters worth it.
Solid Rock Masonry LLC
5347 Howard Gnesen Road, Duluth, MN 55803
218-343-2978
eric@solidrockmasonry.com
www.solidrockmasonry.com
