Rocket Mass Heater Vs Kachelofen

Does anyone who knows enough about both

Rocket Mass Heaters

and

Kachelofen (a type of wood fired storage heater common in Germany, Austria, Switzerland)

to do a side by side comparison?

Also  - anyone who knows if we are even ever allowed to do a RMH in Germany, or who has succeeded in getting it past a Schornsteinfeger inspection . (I have the permission for at least 1 Kachelofen that I do have already, there were 2 in the property looking at the floor wear and that there's most on the bits of one in the barn... so maybe I could restore that one)

From what I have seen in this forum, I doubt very much that a rocket mass heater is legal in Germany.

Even for a Kachelofen there is only a "Bestandsschutz" (permission due to antiquity) when the stove was installed pre 1950 (edited to add: and that has been running since then - so your barn find that is in pieces cannot be put to use again).
All others must pass the required emission standards.

My parents had to inactivate their Kachelofen because they did not want to do an expensive upgrade to the latest threshold numbers.

And as you might know, there is no such thing as going under the radar in Germany. The Kaminkehrermeister/Schornsteinfeger (official chimney inspector) will find you! Fines are quite high if you infringe the emission laws.
On this page there is a gallery for Kachelöfen (apparently called cockle stove):
https://en.wikipedia.org/wiki/Masonry_heater
Edit: Hopefully this link works: https://en.wikipedia.org/wiki/Masonry_heater


If you read the article you might find the reason why RMH would not be an upgrade to the existing very efficient Kachelöfen. The new generation of Kachelöfen is so efficient and clean that they are also built into modern houses and can keep a house nice and warm in winter (with the required insulation standards for walls and windows).

Hi Kirsty,

We recently completed installation of a "Temp Cast" Masonry heater in our unheated basement and we've been enjoying it since November.  We burn about 40 lbs of wood every night or day and can cook meals in the bake oven (black oven) for basically free.  

We started installation in June and it has basically been a summer long project.  I got help from a friend who is a contractor who has both RHM and masonry heater expertise, but I did the majority of the work myself.  He installed the chimney through the roof and helped with initial masonry, and I learned masonry from him and finished the job.  All told the budget was almost a $30K including all parts and hired labor.  We had been heating our basement with electric space heaters-- 3KW continuously which made the electric bill very high in winter.  Now we pay ~$750 to have our first season's wood delivered.  Where I live in upstate NY there are plenty of standing old big dead ash trees and expect to get many more seasons of wood for only the labor I need to get the wood myself-- an activity I enjoy which helps keep me physically fit.

I had been intending to build a RMH for years.  I have Ernie and Erica's book and was ready to break ground but discovered the MH option last year.  For our lifestyle, budget and location a MH was more appealing for these reasons:

1) less floor space.  We didn't want a "bench" sprawling horizontally in our limted floor space which is split half living space and half storage.   A MH is more of a vertical footprint and is centrally located in our basement to provide radiant heat-- a heat which has a large vertical 8000 lb column of bricks and will hold heat for several days.  Even after two days of no fire it is warm to the touch.

2) Batch style heating much more attractive than piecemeal RMH constant feeding.  We load the wood, light it and it burns completely without needing further attention.  We would't ever go down into the basement to sit on the bench and since we both work at home we couldn't afford to interrupt our schedules to tend a fire.

3) Attractiveness: The fire is behind a large glass door and makes an attractive additon to our nightly eating and entertainment space.  We mulled over the "steel drum in our living room" and both my wife and I were ready to make that compromise, but when we discovered the MH and realized we could afford it it was an easy choice to make.

4) MH is a known quantity for code/building considerations.

5) Bake oven is an attractive built in feature.  Pizzas are wonderful and fast.  Baking bread in a dutch oven is also fast and delicious.


Here are some comparisons of RMH vs MH:

https://permies.com/wiki/207126/Carbon-Footprint-forms-heat

https://www.reddit.com/r/Permaculture/comments/jn4w8v/rocket_mass_heater_vs_masonry_heater/

https://www.hearth.com/talk/threads/why-dont-we-all-want-rocket-mass-heaters.160043/

https://permies.com/t/32821/Rocket-Mass-Heaters-Scandinavian-Slavic

I'll be happy to answer any questions.
Joe and Mary

Cindyl541
There are some beautiful "Masonry Ovens" in google images but do they burn as efficiently as an RMH? Using 10% of the resources a wood stove would use? I didn't know Germany was so strict. Yikes.

I haven't found a direct RMH vs MH efficiency source but the MHA has many articles on efficiency.  Here is are a couple I had bookmarked:

https://www.mha-net.org/video_categories/masonry-heaters/

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjijd-ToduDAxWUFzQIHfVKDE0QFnoECBAQAQ&url=https%3A%2F%2Fwww.mha-net.org%2Fdocs%2Ftemp%2F180419%2520Masonry%2520Heaters%2520are%2520Massively%2520Comfy.pdf&usg=AOvVaw0a9sCuehN97Z9Z0VJdWm2r&opi=89978449

I expect the design of each will be the largest factor.  In general from what I've learned I think a RMH can be the most efficient, with a MH as a close second, and a wood stove a distant third even when used most efficiently (not choked to make a fire last all night).

Joe;  
That is one beautiful masonry stove!
You will get a lifetime of heat from it. and pizzas as well!
Your costs aligned with what I have been told to expect.
Thank You for sharing!

I wanted to tell you that the newest RMHs use a vertical bell (no bench)  they can be shaped to fit your space.
They use a horizontal batchbox core that you load up and walk away from.
Some have a window for viewing but no black oven.
Batchbox design by Peter Berg averages 95% clean burning.
The cost is significantly less.
Here is a link to Peters's website, you might find it interesting to read.
https://batchrocket.eu/en/workings

Thanks Thomas.  The "Temp Cast" kit was a good choice for us because it has a good track record and the Finnish contraflow design is well known.  If I were more of an experimenter I would have been willing to try different configurations, but the Temp Cast solution comes delivered on 3 pallets and you assemble the large refractory core bricks in "lego" style-- 3K lbs worth and then build a brick "veneer" around that in whatever style you like.  My wife is a professional artist and painted the tiles at the top of each face.

Temp Cast has some videos on line and watching them I felt it was more of a DIY project I could handle, and I believed I would be making a known investment in our property-- unlike with a RMH which seemed like me just playing with mud and trying to make something I know probably wouldnt' last and would probably take a lot of trial and error to perfect.    That was also the opinion of my friend who built a cob style home with RMH inside where he lived for several years with the RMH as his only heat source through our NY winters.

It seems with emerging technologies like this, trying to find "the answer" for oneself is always a moving target, and we did appreciate a turnkey solution.  Ernie and Erica's book didn't mention much for a masonry option-- which I really wanted, but didnt' know existed.  I know they wanted to focus on cost savings primarily, but in our case it wasn't a priority.

Anita Martin wrote:
On this page there is a gallery for Kachelöfen (apparently called cockle stove):
https://en.wikipedia.org/wiki/Masonry_heater

I was very interested in seeing these, to consider if they might work in a small house in the US. However, the link comes up with an error message on Wikipedia.  Could you please check and fix the link?
Thanks!

I think the comparison between kit built Temp Cast  to a home built RMH is not necessarily useful.
Many people do not own their homes outright, so even if there are not air quality regulations to deal with, insurance companies tend to frown on homemade wood heaters.
Temp Cast vs the Liberator could be a better comparison, since both have regulatory approval.

Freyda Black wrote:

Anita Martin wrote:
On this page there is a gallery for Kachelöfen (apparently called cockle stove):
https://en.wikipedia.org/wiki/Masonry_heater

I was very interested in seeing these, to consider if they might work in a small house in the US. However, the link comes up with an error message on Wikipedia.  Could you please check and fix the link?
Thanks!

The correct url is https://en.m.wikipedia.org/wiki/Masonry_heater

Kachelofen (plural “Kachelöfen”) are solid wood fueled thermal storage heaters made of firebrick, (Chamotte / Schamottestein) clay (“Tun”), and facing tile (“Kacheln”)

Kachel (plural “Kacheln”) are the ceramic tiles used for the facing of a traditional mass heater in Germany and Austria. (In German, they refer to all of these types of masonry mass heaters as “storage stoves” (“Speicheröfen”) since the fire’s heat is stored in the thermal mass of the firebrick and facing tile. Variations include Grundöfen or Kachelgrundöfen, and many types that provide for heat storage as well as cooking (stovetop) or baking.

In Sweden and Denmark, the name for this type of [masonry heater / thermal mass storage stove] became synonymous with the name for the tile (kachel) so they’re now regionally as known Kakelugn, Leemkakel (clay storage stove) etc.

Kachelöfen are still built today in Germany and Austria, they just have to meet the latest emissions standards, which are much more strict than in the US’s EPA, and even more strict than the rest of Europe. Austria is home to the institutes that set emissions, efficiency, and construction regulations for Germany and Austria. https://kachelofenverband.at/en/kov-service/

Interesting to note that compliance is handled through the chimneysweep’s trade union. All chimneys have to be professionally swept on a regular maintenance schedule, and the permitting to build a mass heater is handled through the chimneysweep and what we in English would call the local “authority having jurisdiction.”

I don’t think the American rocket mass heater would meet the German regulations for design safety, because the combustion chamber is “open” to the room. (Design standards often determine the parameters for a firebox’s shape and materials; the amount of thermal mass required for a given heater is determined by the weight of wood which can be burned in a single loading of the firebox.)

German and Austrian law also requires a “Hafnermeister” (master stove fitter / builder) to make the calculations for the chimney draft pressure (in pascals) at each stove installation’s chimney height, pipe diameter, the stove’s elevation above sea level (to determine local ambient air pressure,) its flue gas exhaust temperature (based on the length of the flue “trains” through the mass, and thus the heat harvested from the exhaust) and the caloric energy content of the weight/volume of wood (the amount which fits in a single loading of the given firebox size.) All the calculations are required to be mathematically modeled for precision efficiency and the lowest possible emissions before construction can begin. In Germany, it is possible to build your own thermal mass stove / Kachelofen under the guidance of a licensed Hafner, after getting local approval through your chimney sweep, of course.

Jessica Steinhäuser is a Kachelöfen maker in Canada whose work has been shown at the Masonry Heater Association, and her stoves have been built in many English-speaking countries in the last few years. She is an extremely gifted artist making some of the worlds most beautiful stoves. I don’t believe there is anyone in the western hemisphere more talented than she is.

https://shko.ca

As she notes, she builds the traditional style of Kachelöfen, and is glad she moved to Canada, because modern Germans and Austrians typically don’t prefer the style of the traditional Kacheln, instead opting for a much more modern / minimalist look of Grundöfen

Kirsty,

Tile covered heater is just a heater covered with heater tiles.
The firebox is a separate story and can be of any design.

I would just disassemble the existing masonry heater, build the batch rocket core and put the tiles back. You have to either spend quite an amount of money for someone to do it, or learn how to build yourself.
Heater of that type will have an outside skin made from firebrick on which the tiles would be laid.
The tiles look like the boxes. They are laid on clay mortar like bricks, first. Then they are connected vertically and horizontally with metal ties. Joints can not be larger than 2 mm, all joints should be struck with a jointing tool to recess them 2 to 3 mm, so the expanding tiles will not get the enamel cracked. Then the cavities in the tiles are filled with square or rectangular pieces of firebrick with the clay mortar.
Then the inner skins is built from bricks laid as shiners.

It's quite specific, but not something that can not be done.

Anita Martin wrote:The new generation of Kachelöfen is so efficient and clean that they are also built into modern houses and can keep a house nice and warm in winter (with the required insulation standards for walls and windows).

Anita, this is very interesting. Can you expand on this? I have been wondering how a wood fired mass heater can be introduced into dense urban zones and meet emission requirements (for the good of all).

Douglas Alpenstock wrote:

Anita Martin wrote:The new generation of Kachelöfen is so efficient and clean that they are also built into modern houses and can keep a house nice and warm in winter (with the required insulation standards for walls and windows).

Anita, this is very interesting. Can you expand on this? I have been wondering how a wood fired mass heater can be introduced into dense urban zones and meet emission requirements (for the good of all).

Here is the English-language version of the German law regulating efficiency, particulate matter, Carbon Monoxide, NOx, and dioxin/furans — if anybody wants to try to compare with measurements taken for Rocket Mass heaters.
(The US EPA standards don’t set regulatory maximums for all of the same emissions that the Germans do, so the available data aren’t completely comparable.)

https://www.bmuv.de/fileadmin/Daten_BMU/Download_PDF/Gesetze/1_bimschv_en_bf.pdf

Taken from
https://www.bmuv.de/heizen-mit-holz/verordnung-ueber-kleine-und-mittlere-feuerungsanlagen

If I recall correctly, the efficiency numbers between German and US regulations are not a 1-to-1 equal comparison, either. Has something to do with heat lost to evaporation of moisture content or the required amount of dilution air, I think. Supposedly an American 90-100% efficiency is calculated as a German 83-90% efficient, iirc.) If someone wants to crunch the numbers themselves, this document details how the measurements are to be taken and calculated during testing for the German standard.

Anita, Forsythe,

THANK YOU SO MUCH.

And thanks for the much better summary of the situation in Germany for our overseas friends. As Brits, we fitted a normal modern high efficiency wood stove in one room, connected to the existing chimney (which is split inside to 2 sections for 2 stoves) with no idea we were doing anything "dodgy". Fortunately our local inspector passed it on his last visit. I had kept the energy certificate when I bought it.

My Kachelofen is pretty efficient, two fills will keep it hot all day and night, it only burns for about an hour or 2 to do so, the rest of the time it just radiates heat. It takes About 1 15 litre buckets worth of wood per fill.

Mine is just a simple tall "box" shape. it draws like CRAZY in the burn phase (at least now that 20 years worth of ash has been cleaned from the inside). The chimney sweep did this but now I saw him do it, I could do it myself - every 5 years. There's 2 round tiles you remove to get access.

The chimney itself is, as you guys mentioned, inspected and cleaned every year, along with the oil furnace (efficient for it's age but it's nearly 30 years old). I believe that will need mandatory replacement soon. The local preference for that swap is a wood gasifier as it can "just" "plug in" to the existing radiator system. There's likely a grant for it, and in any case, one can deduct up to €6000 per year from overtime tax for workman's bills on house works. To heat the 60m2 ground floor and the one large (5m x 5m?) room upstairs takes about 1000l of oil per winter (€1200 at current prices). Also hot water but I barely use any. It also keeps the largeish "boiler room" (utility room)  that it is in warmish (above 12C) and dry as a bonus

I'm so grateful to hear there are modern approved versions of Kachelöfen that I could consider fitting.

We have a local big sawmill for wood but I haven't priced it up. I am using fallen cherry and fir trees from my property and wooden building waste - unpainted floorboards and debarked trunks that were (are) used as beams. I suspect I can "cull" more of my overgrown windbreak firs for about 10 years to come, and they are so densely planted I suspect that was the intention. My neighbour got a whole chestnut that fell on public land opposite his house by just asking the local council if it was ok if he chainsawed it up. This happens within a km 3-4 times a year. Old trees are generally protected though, another neighbour must get special permission to fell one that was digging up their house, and only if they planted TEN more new trees!

Local conditions and rules affect costings, that's for sure!

A small bundle of wood, about the size for one load, at the supermarket is €5!

*income* tax, not "overtime tax !

Forsythe Instauratur wrote:Kachelofen (plural “Kachelöfen”) are solid wood fueled thermal storage heaters made of firebrick, (Chamotte / Schamottestein) clay (“Tun”), and facing tile (“Kacheln”)
[...]

Well, that was a detailed, knowledeable post! I couldn't have written with that much in-depth knowledge. Thanks for this!

Douglas Alpenstock wrote:

Anita Martin wrote:The new generation of Kachelöfen is so efficient and clean that they are also built into modern houses and can keep a house nice and warm in winter (with the required insulation standards for walls and windows).

Anita, this is very interesting. Can you expand on this? I have been wondering how a wood fired mass heater can be introduced into dense urban zones and meet emission requirements (for the good of all).

I would say that the post by Forsythe Instauratur has all the information you need, plus the 33 pages leaflet. Germany efficiency and German bureaucracy in beautiful reunion, lol.
Here are some pictures of modern style Kachelöfen:
https://www.ebersberger-ofenbau.de/kacheloefen
(when you scroll down you see some more traditional cockle stoves but with modern heating technology)

Our young neighbours who built their house meeting latest heating standards have one of those, in addition to a pellet heater and solar panels on the roof.

We recently had a Kachelofen built for us by the aforementioned & very talented Jessica Steinhäuser, who just happens to live just a couple of hours away from us, and are thrilled with it.

One point that I don't think has been raised yet in this RMH vs Kachelofen discussion is cost - obviously a Kachelofen will cost substantially more than an RMH. However, in terms of cosmetic appeal, functionality (ours has a separate pizza/bread oven), and fuel efficiency, not to mention that it will long outlive my husband and I (early 60s), there really is no comparison.

One last additional point re cost... We used a Pacific Energy woodstove in our old house for some 20+ years (replaced once). Knowing what I know now, and if we could've afforded it at the time, it would have been more cost-effective to install a basic Kachelofen when we first moved in. Even though we had our own woodlot two hours away, and thus free wood, by the time you add up the additional costs of chainsaws, trailer, fuel, unpaid time/labour etc. for all the wood our woodstove consumed each winter, I think we would've broken even on the cost of a Kachelofen years earlier.

Some lovely heaters featured on this thread but it seems some people are comparing a Kachelofen with a basic J tube RMH but, that design although still popular, is years behind the times compared to the latest RMH designs!
Surly a modern designed batch box in a brick bell would be the better comparison ?

William Bronson wrote: I think the comparison between kit built Temp Cast  to a home built RMH is not necessarily useful.
Many people do not own their homes outright, so even if there are not air quality regulations to deal with, insurance companies tend to frown on homemade wood heaters.
Temp Cast vs the Liberator could be a better comparison, since both have regulatory approval.

I have never heard of the Liberator. Could you please give some direction to where to read about it please?

Fox James wrote:Some lovely heaters featured on this thread but it seems some people are comparing a Kachelofen with a basic J tube RMH but, that design although still popular, is years behind the times compared to the latest RMH designs!
Surly a modern designed batch box in a brick bell would be the better comparison ?

I couldn't agree more with Fox James, as making a list to compare a older style J tube with a Kachelofen stove is fairly easy.  They both burn wood.  Not much more than that.

Joe Hallmark wrote:https://rocketheater.com/

I was curious as I could not find, what kind of BTU's per hour is this considered?  

It does say this  "A single bag of pellets will help you weather the storm for up to 20 hours"  But I don't know what a bag weighs or I could figure it out backwards.

thanks to anyone that knows.

A 40-pound bag of softwood pellets has about 8,000 BTU's/lb- 320,000 BTUs per bag.

The reply from Forsythe Instauratur is most relevant.  Have a read...

RMH products are considered "masonry heaters" as defined in CSA B415.1 of 2022, a recently updated standard which is accepted by the EPA, though Canada hosts the technical committee.  In Europe generally there are ways to construct new heat retaining appliances. The most important activity is in Austria where there is an officially approved spreadsheet that will calculate whether or not the design is allowed.  You can build a unique one and and don't have to have it tested for first.  There is quite a high fee for using it but it is possible.  There is an initiative by a couple of people in the Masonry Heater Association in North America (it includes Canada and the USA).  The main guy is in France actually.  He is attempting to write an accepted public domain version of the calculations so it is available to everyone.

It is true that the requirements for the EU are tighter than those in the USA, but the masonry heater guys have demonstrated repeatedly that they easily exceed all of them.  It is far easier to burn 60 lbs of wood rapidly under predictable conditions than to burn the same amount of wood at a varying rate - hence the heat storage.

At present Canada/USA do not accept predictive models of performance.  Build, test, approve, disseminate.  Fireplaces and masonry heaters are not regulated at present, neither are coal stoves. However anything burning biomass is covered in B415.1 with at least a test method and suitable metrics.  Only some products have requirements.  It also covers stoves with TEGs for electricity.

I see a comment that RMHs are not allowed in Europe because the fire is open to the room.  I have not seen that reason given specifically for any country.  It is possible but there are ways to provide air and an enclosing portal for fuel so it is not a universal objection.  A door over the fuel hole with adequate air holes is not different from a fireplace with a door and vents.  What matters (a lot) is draft and various points where minima apply.  

I understand that the relevant European standard is EN 15250 "Slow heat release appliances fired by solid fuel. Requirements and test methods".   A copy might be available from www.kamicenter.ru/content/news/index.php? ELEMENT_ID=672

Kami Center is the effort on one man Mr Batsulin and he has done an amazing job of training and certifying many builders of masonry heaters.  He has been working based on the old USSR Standard GOST 3000-45 "Heat capacious heaters, methods of testing."  The "G" means it is a national standard'.  Same as China.  Warning, the legal construction method of a brick chimney passing through the second (wooden) floor of a house has been updated to widen it a little.

RHM:  It would be necessary to add a bleeder to the top of the heat exchanger leading to the chimney to prevent CO being trapped there.  I have used this routinely on downdrafting heat exchangers, for example on the Kyrgyzstan KG4 and KG5 series of heaters.  There is a German word that I can't remember for this deliberate "leak" up to the chimney.  This can guarantee the chimney draft is adequate even if the heat exchanger has over-extracted heat and created a dangerous situation (such as a gust of wind reversing the chimney).

Stoves with a safe downdrafting heat exchanger:
https://www.newdawnengineering.com/website/library/Stoves/Kyrgyzstan/KG%20Model4-Coal/KG4.4.12%20Updated%2016%20Dec%202017/ Go back one folder to see other models.
Polish copy, water heating version: https://www.zebiec.pl/kotly-co/szafir-kociol-na-wegiel/?lang=en
Document on the development of this culturally relevant stove (you have to click on "Download") https://openknowledge.worldbank.org/handle/10986/31282

It may be a challenge in Europe to build a heater that has any portion of the gas path with a positive pressure channel made of clay.  

Serious technical questions? Contact me.

Burn 'em clean...

Crispin

Member: TSC, CSA JB121.1
ISO TC-285
CSA JB127 for ISO-238
SABS TC-1054
New Dawn Engineering Inc, Alberta

Gerry Parent wrote:A 40-pound bag of softwood pellets has about 8,000 BTU's/lb- 320,000 BTUs per bag.

I have to admit that I would have lost that bet, both on BTU's per pound, and that it comes in 40 lb bags..  and with the moisture content so low,  it doesn't make much difference between hard or softwood output per pound.   Good info.

Dragon heater's Castle Build kit is meant to be tiled like a kachelofen. They sell pre-fabbed rocket cores and ovens that all work together. They have this flier available to describe their system. Castle Build flier

Crispin Pemberton-Pigott wrote:
I see a comment that RMHs are not allowed in Europe because the fire is open to the room.  I have not seen that reason given specifically for any country.  It is possible but there are ways to provide air and an enclosing portal for fuel so it is not a universal objection.  A door over the fuel hole with adequate air holes is not different from a fireplace with a door and vents.  What matters (a lot) is draft and various points where minima apply.

Maybe take the following with a grain of salt, but: a Hafner specializing in Grundöfen selber bauen (consulting/advising German homeowners building DIY Grundöfen in their own homes) told me a couple years ago that, as he understands it, the BIM SchV (the latest Austrian / German regulation above) effectively prohibits installation of a J-Tube, because the law distinguishes between “open-hearth” furnaces (without doors,) “hand-stoked” vs “fill-fueled” (per single firebox load) and “type-tested” firebox inserts for Kachelöfen and Grundöfen for use under the “single room heaters” usage/design-type (vs other types like hydronic-heating or boiler storage stoves.)

Various states within each country apparently treat the provisions of the law a little differently from each other, but according to him, most regulators seem disinclined to allow “open-hearth” or “open fireplace” installations to be used for room-heating. (The law says that open fireboxes are only allowed to be burned “occasionally” for an ambience or a traditional Christmas holiday “Chestnuts-roasting-on-an-open-fire” type thing.)

And, according to that Hafner, the regulators wouldn’t pass a type-test of even a standardized “shippable-core” J-Tube —where the amount and shape of fuel sticking out of the feed chute has a direct and significant impact on the amount of combustion air admitted into the burn tunnel combustion chamber at any given time …Whereas a closing/latching firebox door with an air flap has definite, measurable, and repeatable parameters for the volume of air admitted into the firebox, which guarantees cumbustion performance within the minimum and maximum settings of those built-in air slots in the door or door-frame.

Granted, the Grundöfen style he consulted DIY builders on were designed with door enclosures the person could build themselves to design specs — or buy the type-tested door from him — or buy the whole type-tested firebox core from him… and so he likely had little to no interest in investing what sounds to be a laborious, costly, and potentially fruitless effort in getting J-Tubes approved for his clients, when what he considered “the better and safer options” were already approved and readily available for a cheaper and faster DIY installation.

I guess that doesn’t necesarily mean it’s impossible to get a J-Tube approved under those German regulations… just that if there is technically a way to do it, it wouldn’t be worth the DIYer’s time and money to fling themselves into the mire of Teutonic red tape over it. 😂

RHM:  It would be necessary to add a bleeder to the top of the heat exchanger leading to the chimney to prevent CO being trapped there.  I have used this routinely on downdrafting heat exchangers, for example on the Kyrgyzstan KG4 and KG5 series of heaters.  There is a German word that I can't remember for this deliberate "leak" up to the chimney.

Yep, the “Gasschlitz,” literally: “gas slot.”

There is an initiative by a couple of people in the Masonry Heater Association in North America (it includes Canada and the USA).  The main guy is in France actually.  He is attempting to write an accepted public domain version of the calculations so it is available to everyone.

I hadn’t heard about that effort to make the Austrian computer modeling calculations into a free opensource spreadsheet! That is so cool. I had tried to reverse-engineer the gist of the modeling from some public info I found online, but I’m obviously missing some datapoints or logarithms to arrive at the same figures the regulators’ software does. Oh, how I would love to be on that email list when the opensource thing is ready. Any chance they’re taking donations or kind words of support for their effort?

A J-tube could have a hinged lid with vent mounted on it. There should be no wood sticking up out of the feed anyway. I wonder if gravity would be accepted for closure or if it would still require a locking latch.

Jeremy VanGelder wrote:Dragon heater's Castle Build kit is meant to be tiled like a kachelofen. They sell pre-fabbed rocket cores and ovens that all work together. They have this flier available to describe their system. Castle Build flier

with just a slight amount of pre-planning the same could be said for any RMH bell as far as being, tiled, stoned, bricked or what have you.  In my case, I tried (successfully I might add) to see what combinations worked the best.   in short it was to be expected with the thinnest (ceramic tile) that heated the fastest, Granite stone laid on edge  from 4" to 20" high,  granite stone laid flat about 2.5" deep and lastly for the most mass, granite stone laid flat but 4" thick. The latter of course has twice the mass of the 2" material and Way more than the other two.  Ironically, once everything is about 150 degree's exterior temp, then when I go up to 160-175 it is all about the same, same for cool down.   it would take a lot of sensors to know just how the heat dissipates % wise.  

I am not to concerned as long as it works the way it does.

You are right, Scott. Any bell can be built from or enclosed with masonry. That was an impressive amount of experimentation! Thanks for moving the craft forward by testing all of those permutations and documenting them!

Jeremy VanGelder wrote:You are right, Scott. Any bell can be built from or enclosed with masonry.

The “bell” design brings up an interesting point about a subtle difference in firebox design and internal construction between traditional Kachelöfen/Grundöfen on the one hand — and rocketstove batch-boxes on the other.

Both systems use gas pressure resistance to thoroughly mix and hold the pyrolysis gases in the primary and secondary combustion zones for longer residence time and thus more complete combustion.

They each accomplish that in different ways of creating restrictions to the gas pathway; Grundöfen and Kachelöfen accomplish that gas resistance by use of 90°-to-180° angle turns in a “train” (“Zug”) of flue pathways which gradually taper down in cross-section through the whole mass as it approaches the chimney end…

…While batch-box rocketstoves rely more on the port(s) in the combustion core to create the resistance. (The “riserless” rocketstoves typically rely on two ports: one at the firebox, and one “exit port” at the end of the horizontal secondary burn chamber.)

The difference in combustion core design usually means that (in order to prevent both slow, insufficient draft and too-rapid overfueling) a rocketstove batchbox typically needs to have a bit of an open-area (a gas-pressure-buffering expansion chamber) — the bell — after the riser [or secondary burn chamber], while the Kachelöfen/Grundöfen typically need that long, continuously tapering flue train after the combustion core.

I don’t think one is inherently better than the other, and they each have their own advantages and disadvantages in terms of heat distribution (and areas subjected to the greatest thermal expansion stresses) within the mass.

So maybe I’m missing something here, but the main premise of rocket heater’s efficiency is the extreme temperatures in the riser burning all of the gasses completely. But the Kachelofen, as I understand it, has a primary and secondary combustion the same as a typical epa stove has. No extremely hot riser. And that has always been my understanding of masonry heaters. Nothing insanely hot happening as with an rmh.
So… if that’s true, and yet European standards for emissions are even stricter than USA, how are the Kachelofen burning so clean? And by extension, burning possibly even more efficiently than an rmh?

Julie Reed wrote:So maybe I’m missing something here, but the main premise of rocket heater’s efficiency is the extreme temperatures in the riser burning all of the gasses completely. But the Kachelofen, as I understand it, has a primary and secondary combustion the same as a typical epa stove has. No extremely hot riser. And that has always been my understanding of masonry heaters. Nothing insanely hot happening as with an rmh.
So… if that’s true, and yet European standards for emissions are even stricter than USA, how are the Kachelofen burning so clean? And by extension, burning possibly even more efficiently than an rmh?

You bring up a good question and I have not really heard any answers in the past.  But there are a few things that I can tell you, that often don't get talked about but certainly seem to be true.

1)  The simplicity of a square, rectangle or even round bell,  is huge when comparing it to any type of Kachelofen/Russian stove.

2) if your using in your planning stage, a Internal Surface Area, (ISA) of a known amount, excluding the floor, it is fairly simple in a Rocket stove bell. Somewhat more complicated in the other.

3) in shear cost of added brick (internally) plus the additional labor could sway a person's plan.

4) Done as I did, you can basically make any Rocket bell as fancy (tiled) or thickness as your supporting base will allow. (this is true for both)

5) I have determined, that the difference of BTU's adsorbed by the mass of the bell per hour of burn is basically the same PER HOUR. Of course this means if you have a really thick mass,  And extended burns at the beginning, you can ramp up the total stored heat with time. (read not the size of the fire)  This is certainly true for both.  But the Russian type, are generally what they are when your done.  A bell, if you have supporting structure, can be added to, as time goes on.  For example, my basic stove was fired up Nov 4th of 2023, since that time, I have added perhaps an additional 1000 pounds of mass and will continue to do so until I either run out of space or free mass.   In my case, it will be when I run out of supporting ledge, as my "free mass" far exceeds my space.

6) This nit picky, but still nice to know. and temp probes could tell you on both just as easy, but when my top part of the bell reaches 150, my exit temp is 150, if  I take it up to 175, my exit goes to 175, and I presume it would continue on a linear fashion.  ( I personally stop at 175)   it is much easier to check this correlation if you have a bit of exposed flue pipe, this is often not the case if one has a really tall Russian type stove.  To mean this is strictly a efficiency measure. and to really know if it is true or not, it would certainly go back to your initial question to which I do not have an answer.

7) Back to the labor thing,  I simply say, I can build two Rocket mass bells in the same time I could build one Russian type. While I will never have that chance, it is my honest belief. Take it for what it cost.  Nothing.  

Best of success!

Greetings Julie

Your description of these designs is somewhat incomplete (and there is nothing wrong with that - I appreciate your questions as very useful).

I speak of modern versions of all devices as comparisons should be up to date when possible. Here goes:

In terms of combustion efficiency, there is very little difference between the following technologies:

Rocket Mass Heater (RMH) rooted in the work of Ianto Evans
A Masonry Heater as exemplified by the multiple works of the Masonry Heaters Association - exemplified by one of their Technical Committee members Norbert Senf and Alex Chernov https://www.stovemaster.com/html_en/gallery.html
Advanced-but-low-tech-materials biomass stoves such as the Kabuga stove, a report recently posted here:
https://www.newdawnengineering.com/website/library/Papers+Articles/CAU/CAU_FREPDC_2023
Any of a number of pellet stoves with fans (too many to show)

Combustion efficiency is a measure of how completely the fuel burns, though it is usually described in terms of combustion inefficiency because it is easier to calculate directly.  There are two versions: CO/CO2 expressed as % and CO/(CO+CO2) expressed as %, which the EPA calls the Modified Combustion Efficiency (MCE).  I am proposing a new version which is CO/(CO+CO2+CxHy) so that it covers gas appliances as well (especially propane).

For indoor cooking using a fuel like ethanol gel, the MCE should be under 2% to pass national regulation in some countries. Germany, South Africa and others.  Most small biomass burners that achieve good combustion are under 4%.  All the technologies listed above are under 0.5%.  When performance is in that range, there is no significant gain available  in terms of heat or smoke of fuel consumption - it is so clean that it becomes a numbers contest, nothing else.  Anything that can burn at an MCE of 0.2% is extremely clean. That means it is burning 99.8% of the carbon to CO2, not CO.  The unburned portion is near zero. I have seen coal stoves at 99.995% MCE.  For example here:
https://www.newdawnengineering.com/website/library/Stoves/Kyrgyzstan/KG%20Model4-Coal/

There is nothing inherent in wood that produces CO - it is manufactured in the fire. There is no performance difference between a RMH and a Masonry heater if both are properly built with one exception.  The masonry heater is more likely to have a well controlled level of excess air, compared with the RMH.  The reason for this is inherent in the design: the air entrance is not controlled, or not AS controlled.  Masonry heaters are constructed with primary and secondary air supply in a balanced manner.  The RMH (and Rocket Stoves in general) tend to have high excess air which cools the flame.  Not all the time min you, but between sometimes and usually.  The root of this is the ideology of the Rocket Stove design (originally developed by David Hancock in 1983-84, no matter what you hear on line or in blurbs) which holds that a fast, clean burn needs a lot of air.  Because the RMH at high power is very hot and clean is not a justification for having high excess air at other burning levels.  Excess air (EA) is expressed in % and refers to the amount of air present in the fire relative to the amount theoretically required for complete combustion.  The needed air supply plus 100% would give you about the cleanest burn you can get in a simple device (80-125% EA target range).  Rocket Stoves tend to be in the 600% EA range.  At low power, 2000%.  This high level of EA cools the fire and creates CO + smoke.

The claims one most often sees about any heater are not well-informed.  First they are not expressed in a manner that reflects a good understanding of what to measure and why, and they compare a product that is pretty good with something selected to be sub-standard.  If you were to look at the ISO standards for burning biomass in cooking stoves (an expensive document) they have the metrics correct: CO and PM per megajoule of energy delivered.  For example a pretty clean stove is going to emit 3 or 4 g of CO per MJ delivered to the pot.  For heaters this should be about half because heaters are more efficient at delivering heat than cookers.  When did you ever see a heater rated with CO expressed in mass/MJdelivered?  Never.  Clean burning doesn't mean efficient heating and vise versa.

EPA wood stove performance is mostly examining particulate matter (PM), not CO.  They care about smoke because all their regulatory work is based on stack testing in factories in the 70's.  The applicable standard for wood burning heaters  is CSA B415.1 : 2022, the new version being a substantially reorganized version of the confused document that previously existed.  

Enough about that.  To your specific points:

The RMH is not necessarily efficient because of the combustion efficiency.  The efficiency you refer to in that sentence is the thermal efficiency.  That is the ratio of heat delivered to the home divided by the energy theoretically available from the fuel burned.  You could burn at an MCE of 99% or 97% or 95% and still have a thermal efficiency of 75% or 80% or 85%.  They are nearly independent. The RMH has a high thermal efficiency because it has a low exhaust temperature - same as a heat-storing masonry heater - due to the long and high mass heat-collecting channels. So the "high efficiency" is not because it has a hot central column where gases are burned to completion.  If you had a similarly hot fire in any other shape but the same heat collection and exhaust temperature, the performance would be the same.   The semantic difference is that the combustion chamber and hot central passage are claimed to give good combustion efficiency, while the common term "efficiency" is intended to be the heat transfer efficiency from the hot gases to the air in the room (whether stored in high mass or not).  These two efficiencies are really independent, almost completely.  If you have a really, really bad fire, and have a really, really good heat transfer mechanism, the overall performance will be good, you just have to clean it a lot more often.  

As for the temperatures in the combustor of a masonry heater with high mass storage (5 to 8 tons of brick or rock, typically) again, these are two separate aspects.  They burn just as hot as RMHs - probably hotter due to the burn rate being higher - which delivers good combustion efficiency, and they store and deliver the heat using high mass.  That delivery is the heat transfer efficiency, not the combustion efficiency.  

Your last question hits the nail on the head.  The EU standards are for emissions per MJ of fuel energy or fuel energy delivered to the "load".  The EPA is obsessed with emissions per minute or per kg of fuel.  They are not even directly comparable. Different units.  You can assume that a kg of fuel has a certain amount of energy available, but that is not the same thing as the energy actually available.   The disadvantage American stoves have is there is a certain expectation from the public as to what constitutes a cooking and space heating wood stove.  It is a pretty big design limitation.  The masonry heaters and RMHs evade this preconception by concentrating on space heating, not cooking, though cooking is not ruled out (especially black or white ovens).  The clean burning kachelofens are simply one of the heating systems developed for burning a mass of fuel rapidly in a short time, followed by closing all doors and letting the stored heat into the room slowly.  

RMHs are not "insanely hot" although that is a great turn of phrase.  It is really hard to get a wood fire above 1200 C, and if it did, it would be a huge source of NOx pollution created by splitting N2 from the air and making a copious amount NO (thermal NOx).  There is a reason blacksmiths don't use wood.  It can be done with coal and 50% EA but not in a Rocket Stove of any kind.  Further, it is very difficult to measure the temperature of hot gases.  Personally, I do not believe any claim for a hot gas temperature unless the report includes how it was done and with what instrument.  An ordinary thermocouple grossly under-reports the actual gas temperature because it is radiating strongly.

I conclude with your last question about burning more efficiently that an RMH.  An RMH at high power can in theory have very high combustion efficiency due to the open air entrance, but not at other, lower power levels.  Don't let someone redirect combustion efficiency to thermal efficiency in mid-sentence.  Burning cleanly at all power levels HAS to include primary air control, either by a lever or clever counter-balancing airflow (buoyancy in opposing directions as is done in a Vesto).  Designing the latter requires either a lot of experience or CFD gas flow simulation as shown in the FREPDC link above (it is active in the PowerPoint, not the PDF).  A wood burning appliance can be clean and inefficient at the same time. There is a lot of room for misconceptions and error.

Stay well, burn clean
Crispin

(Sorry for the double post, here. Dunno how my browser fritzed on that post a second ago.

Also looks like a couple other folks were able to respond before I was able to get my reply posted, and theirs looks like good input too.)

Julie Reed wrote:the main premise of rocket heater’s efficiency is the extreme temperatures in the riser burning all of the gasses completely.

This is true, but with a couple caveats. The temperature required to completely burn (oxidize) all of the carbon compounds in wood’s pyrolysis gasses is 800°C (1472°F) and above.
(See: https://www.bios-bioenergy.at/images/bios/downloads/publikationen/Pellets/091-Paper-Brunner-Primary-measures-for-low-emission-wood-combustion-EUBCE2009.pdf)

The riser on a rocketstove can reach temps much, much higher than that… particularly the largest JTubes 8+ inches in diameter, fed thinly split wood (which burns faster than cordwood) and in risers heavily insulated with ceramic fiber (which can accumulate incredibly high internal temperatures) — especially during long burning cycles to heat large thermal storage masses.

Traditional, wood-fired, natural-draft ceramics kilns, for example, can regularly reach temps at least as high 1510°C when continuously fed thinly-split wood in the same manner a JTube is. (The highest officially recorded is 1563°C…and that’s without a ceramic-fiber-insulated combustion chamber. See:
 https://www.guinnessworldrecords.com/world-records/428387-highest-temperature-in-a-wood-fired-kiln )

Achieving vastly higher temps than 800°C wouldn’t be an issue, except for the formation of NOx emissions — from nitrogen (N2 gas) admitted as “ballast gasses” in combustion air — and from the organic nitrogen compounds naturally present in the biomass used as woodfuel.

The higher the combustion temperature, the more NOx is formed. (Production of Nitric Oxides [NOx] from ballast gas Nitrogen [N2] starts at around 1100°C, steadily climbs with increasing temperature, and greatly accelerates above 1300°C) … so getting the temperature high enough to burn all the pyrolysis gasses is important, …but for the cleanest burn, it’s also important to avoid excessively high temperatures which  increasingly encourage NOx.

The ultra-hot-firing formation of NOx also becomes more of a problem when there’s lots of excess oxygen — beyond the amount of O2 which is necessary to oxidize all the carbon to CO2. The excess O2 in the combustion zone(s) is then available to oxidize nitrogen. (At lower combustion temps, nitrogen has less affinity for oxygen than carbon does— which some Kachelgrundöfen designs take advantage of, by providing a “reduction zone” where excess O2 gas is lean, and so residual carbon monoxide scavenges oxygen molecules off of the nitric oxides, reducing them to N2 while oxidizing the CO to CO2.)

NOx production also increases when air is aggressively mixed through the fuel bed — as with a JTube that sucks a lot of the ash through the burn tunnel and riser — or with a bottom-grate design that admits a large portion of the burn chamber’s primary air through bottom of the fuel bed.

(Bottom air greater than about 5% also encourages carbon monoxide formation, and it creates more particulate matter [PM 2.5] in the exhaust, by mixing and vaporizing more of the ash components —particularly the potassium and sodium fraction, which vaporize (boil) at ~759°C and ~883°C, respectively — forming secondary compounds and micronized particles as they cool and solidify in the exhaust stream.)

(From https://www.babcock.com/home/about/resources/learning-center/nitrogen-oxides-nox-primer)
“NOx formation is promoted by rapid fuel-air mixing. This produces high peak flame temperatures and excess available oxygen which, in turn, promotes NOx emissions. Combustion system developments responsible for reducing NOx formation include low NOx burners, staged burning techniques (overfire air), and flue gas recirculation (FGR). The specific NOx reduction mechanisms include controlling the rate of fuel-air mixing, reducing oxygen availability in the initial combustion zone, and reducing peak flame temperatures.”

But the Kachelofen, as I understand it, has a primary and secondary combustion the same as a typical epa stove has. No extremely hot riser.

The riser in a rocketstove is its secondary combustion zone. In the “riserless” batchbox rocketstove designs like the Walker Riserless Core, Double Shoebox Rocket, and Vortex stove, the “riser” is turned sideways, effectively making it a secondary combustion zone very much like Kachelgründöfen, (albeit with a greater reliance on the shape and size of the ports than on the sharp-angled turns and taper of the flue pathway.)

EPA steel or cast iron box stoves are designed to emit heat directly from the primary combustion in the firebox, which neither Kachelgrundöfen nor rocketstoves are designed to do.

Being made of thermally-emissive steel, (which robs heat from the combustion chamber before secondary combustion is complete) typical EPA stoves do not operate at temperatures as high as rocketstoves or Kachelgrundöfen. Because of this, they typically require a catalytic combustor in the secondary or tertiary burn zone to clean up their emissions (the rare mineral elements like platinum in the catalytic combustor are what “catalyze” the burning of soot/creosote-forming hydrocarbons at lower temperatures, usually between 210°C-600°C, up to a max. of 816°C … the lower combustion temperatures also being needed within the steel box to prevent metal fatigue, warpage, cracking, spalling, and/or (at the very highest temps) beginning to melt.

In contrast, both Rocketstoves and Kachelgrundöfen are made of high-duty firebrick composed of kaolinic fireclay and “chamotte” (grog particles made by calcining high-alumina fireclay, instead of the silica sand used in standard building bricks and the low-duty firebrick splits that sometimes line EPA steel box stoves.) Chamotte firebricks and fire tiles retain heat in the primary and secondary combustion zones, allowing for enough heat accumulation to burn off pyrolysis gasses above 800°C, so they don’t need a catalytic combustor.

So… if that’s true, and yet European standards for emissions are even stricter than USA, how are the Kachelofen burning so clean? And by extension, burning possibly even more efficiently than an rmh?

I’m not sure that Kachelgrundöfen burn cleaner than *batchbox* rocketstoves in terms of CO emissions, but they do burn a little cleaner than J-Tubes.

I’m pretty sure that Grundöfen designs (which intentionally lack a bottom grate, and don’t have a narrow “port” or the upright riser which create aggressive suction on the firebox) produce less NOx and particulate matter, because they don’t stir up the fuel bed or disturb the ash in the firebox. The Grundöfen design kinda-sorta(ish) works like a TLUD in the way it pyrolyses the fuel and burns the gasses without aggressively mixing fuel and air in the fuel bed of the primary combustion zone. …At least, that’s my understanding.

Kachelofen heaters can be good, bad or between. I grew up with mediocre ones - they could never heat the rooms sufficiently and they were coal fired.

There are a lot of firebox designs that add unnecessary complication and may be also counterproductive - for example slow down the movement of gases that requires a better chimney. A lot of stove builders build them this way, because their masters did, because their masters did and so on. They may not have too much respect for the principles of gas physics.
On the other hand - rocket type firebox, especially batch-box type is well documented, tested and SCALABLE. The good results can be replicated by anyone if the right ingredients and construction techniques are applied as per recipe.
Kuznetsov fireboxes are also wonderful.
The bells that are built for these fireboxes rely on a simple and beautiful principle of free movement of gases: hot stay at the top and cooler gases go down.
Any of the masonry stove can by covered with kachels, even the worst imaginable one.

A lot of misconception also comes from comparing various efficiency ratings calculated differently.
The efficiency of the entire heater is a multiplication of 3 efficiencies:

EF_total = EF_firebox * EF_massAbilityToAbsorbHeat * EF_bell
where:

EF_firebox = efficiency of combustion process (how much energy is extracted from the energy source)

EF_massAbilityToAbsorbHeat = (T_firebox - T_exhaust) / (T_firebox - T_room)
where:
T_firebox = firebox temperature
T_exhaust = exhaust temperature
T_room = temperature of the room air

EF_bell = E_received / E_emitted
where:
E_received = energy received by the bell
E_emitted = energy emitted by the bell

By multiplying these 3 values a 0.95 efficiency firebox can be reduced to, for example, 0.3 efficiency heater, which would be rather low.

If you want to see gorgeous kachels, please check the galleries of this manufacturer.
The heaters were built by various builders so they may have different firebox designs and efficiencies:

Kafel-Kar kachels

Cristobal Cristo wrote:Kachelofen heaters can be good, bad or between.  

If you want to see gorgeous kachels, please check the galleries of this manufacturer.
The heaters were built by various builders so they may have different firebox designs and efficiencies:

Kafel-Kar kachels

There some really great looking stoves in the galleries. I was curious how the exterior veneer/facade/facing of these glossy tiles are attached?  This leads to the second question of how all this expands and contracts with the various temps found with any mass type of stove. I personally use silicon (there are many types) but  many of  these stoves have been built long before silicone came along.

Just curious