Procrastinating? Don't... Here's why you NEED to build a batch rocket heater!

The #1 reason why you need to build a batch rocket is: It simply delivers long lasting wonderful bone warming heat. Just think about what gives you that warm toasty feeling inside when you look out the window and snow is blowing sideways... flannel PJ's, fuzzy slippers, a hot cup of coffee or tea, a bowl of cinnamon raisin oatmeal, homemade chicken soup, a snicker of bourbon? What's your warm and fuzzy on a frigid day? I guarantee you that a rocket mass heater will soon be at the top of that list.

I finished building my RMH at the end of the heating season last spring so this is our first fall/winter with the system running. It is located in our shop/multipurpose area that is an extension of our small house and is an insulated 2,000 sq ft area with a 15' high ceiling, so a substantial area to heat. As my wife and I are basking in the warmth we came up with our list of 7 more reasons to build a build a batch box rocket mass heater.

1. The warmth the body feels produced by a radiating mass is far superior to any other form of heat.

2. It's simple to build a fire. Just load up a batch of wood in no fancy manner, large wood for 3/4 of the chamber, smaller on top of that leaving a few inches, then 5 or 6 sheets of waste paper (junk mail or newspaper is free), topped off with some dead twigs/small branches from the yard. Light it and voila, sit back and hear the dragon begin to roar.

3. No fussing with the fire once it's going. Some just do a single batch and let it burn out, it kind of depends on your system design, room size and desired temperature. Some folks build a steel barrel system which will give off lots of radiant heat. If a mass is incorporated with the barrel then you'll need some consistent burn time to heat the mass. Or in my case, I built for maximum heat retention with no steel barrel but 7,000 pounds of brick mass to absorb and slowly radiate the heat into the room. So, depending on our temperature needs I may continue to add wood and keep the fire burning for a while longer.

4. The satisfaction we have from designing and building our heater from scratch.

5. The efficiency and simplicity of the combustion core and system design. The core can run in the 1,000 to 1,400 degree range and yet the exit temperature of the gases as it leaves the bell into the chimney pipe is in the 175 to 225 degree range. This means that all of that wonderful heat is being absorbed by the mass for release into the room, not out into the atmosphere. A huge thank you to Peter van den Berg for his decade-plus efforts to develop, fine tune and publish batch rocket combustion core designs. For more information, visit his website at https://batchrocket.eu/en/.

6. There is virtually no smoke, mostly just some steam coming out the chimney since the combustion is so complete as evidenced by the temperatures reached in the combustion core. This also means a clean chimney with no need to get up on the roof to run a brush down it.

7. The batch rocket system is a beautiful addition to the room even when it is not needed for heating.

From a performance standpoint the following is where we came from and what we are experiencing today.

At this time of year the shop was normally around 50 degrees first thing in the morning before I would light our large Jotul wood stove at about 5:00 AM. We would then keep the stove fired up for a good 5 hours, burning lots of wood to raise the temperature into the low to mid 60's. The shop area includes a food processing & prep area that we use daily, an area for hanging laundry to dry, a dining room table (which is adjacent to the dining room tablesaw), a bar, a bunkhouse for guests and a workout area. So it is really an extension of the house and we need to keep it at a reasonable temperature. Low 60's during the day used to be our goal so we had to keep the door between the house and shop closed to keep from cooling the house in the winter.

The addition of the batch rocket has been amazing. The exterior brick walls measure 16' around and 6' tall, equaling 96 sq ft of surface area. With the mass up to temperature the wall temps run about 150 deg first thing in the morning, 10 - 12 hours after the evening burn had coaled out. After firing it up again in the morning and allowing the internal heat to migrate out to the brick skin it is running in the 170 to 185 deg range depending on just a single batch or feeding it a few times with a little bit of wood once the first batch has burned down. The morning temperature in the shop when I wake up is now at an astounding 66 degrees, 16 deg warmer that it used to be. Talk about a game changer! We rarely ever got the shop to 66 degrees buring the Jotul stove all morning and feeding it tons of wood. Within 2 hours of lighting the first batch and a quick burn in the Jotul stove and the shop is now 71 deg. In the past we would have to burn for a good part of the day in the Jotul and have full sun to get it up to 70. Today it has been snowing all morning, no sun and it's blissfully warm in the shop. We don't yet have a clear idea of the comparison of wood usage before and after but I can see it will be significantly less. Perhaps 50% less and at the same time the shop temperature is much more stable and ultimately much warmer.

If you have any thoughts about justifying the effort to build one for yourself or unsure if the hype is really real, don't waste any more time... figure out your needs, come up with a design to meet those needs, start gathering your materials and post all of your questions here... That's all I did and the community has been incredibly supportive to allow me to successfully transform the heating of our homestead into a simple and effective effort.

If you already built and are enjoying your batch rocket please chime in with your experience. If you haven't, what are you waiting for?

Thank you Glenn for your feedback! I'm in the building process myself and your build process inspired me.
As I'm used to think in the metric system, I made a conversion for others:

Intro: 2000 sq. ft. = 186 m2.
15 feet ceiling = 4.57 m
So 850 m3 to be heated.

Reason n°5: 1000 °F = 537 °C
1400 °F = 760 °C

Reason n°6: 175 °F = 79 °C
225 °F = 107 °C

Reason n°7: 50 °F = 10 °C
60 °F = 16 °C
16' circumference = 4.88 m
6' tall = 1.83 m
96 sq ' = 8.92 m2 exterior surface
150 °F = 66 °C
170 °F = 77 °C
185 °F = 85 °C
66 °F = 19 °C
71 °F = 22 °C

I was surprised by your exterior wall temperatures (higher than expected), is this at the top of your bell? My TV will be very close to the left of my bell so I will have to see how I can protect it from the heat.

May I ask how you reload your batch? Do you wait for it to burn down to coals, ad some small wood to reignite it and then some large chunks to prevent it from overfuelling? These, I believe, are Peter's instructions.

Congratulations on your happy heater Glen!

I take it it is this one from your project thread?


Glen's build thread

Julian Adam wrote:Thank you Glenn for your feedback! I'm in the building process myself and your build process inspired me.
As I'm used to think in the metric system, I made a conversion for others:

I was surprised by your exterior wall temperatures (higher than expected), is this at the top of your bell? My TV will be very close to the left of my bell so I will have to see how I can protect it from the heat.

May I ask how you reload your batch? Do you wait for it to burn down to coals, ad some small wood to reignite it and then some large chunks to prevent it from overfuelling? These, I believe, are Peter's instructions.

This is great Julian... the primary reason for my post is share how amazing a batch rocket heater performs and hopefully inspire more people to realize the benefits of efficiency and comfort.

Thank you for your metric conversion to make sense of my numbers to the metric community. Good reminder for me to make my numbers understandable to all.

Regarding your question about exterior wall temperatures. The next post after your by Nancy Reading is the picture in my shop. Amusing to me is the fact that the hottest spot on the wall is centered about 16" (40cm) down from the top right at the nose of the deer bling I mounted on the wall. The nose gives me a perfect target to aim the IR gun. You see our camper parked on the opposite side and it is 42" (107cm) from the heater wall. I measured the temperature of the wall of the camper yesterday as 100F (38C) when the exterior was at 185F (85C). Keep in mind the large area that I am heating. Depending on your situation you will likely not need to raise the temperature of your bell this high. Are you building a single or double skin bell? Note that Tom Rubino just finished his Shorty core masonry heater with single skin bell and in his pictures you can see one of his comfy chairs quite close. Perhaps he can chime in with his comments in this regard.

As for reloads, yes I wait for the batch to fully burn out to just coals and then reload. At this point the air draw through the system is still significant so no need to add any small small wood. I'll just lay some large split wood right on the coals, fill the rest of the box about 3/4 full, close the door and it instantly kicks back to a healthy roar. As for overfueling, I'm unsure how to determine this other than observe the chimney exit for smoke. I probably need to check more frequently after a refuel but so far all I've ever seen is steam, no smoke. If someone can tell me a better way to determine overfueling I'm always curious to learn.

Nancy Reading wrote:Congratulations on your happy heater Glen!

I take it it is this one from your project thread?


Glen's build thread

Yes Nancy, indeed that is our Happy Heater, Fantastic Furnace and Wonderful Warmer!

It is important for me to mention that I do have some issues with fuel/air mixture in the initial phases of a first burn. My issue has to do with the fact that we live at 8,000' above sea level (2,438 meters) and the air density is significantly lower than the majority of the folks who have contributed to the specifications for a successful build. Knowing this in advance I did make my primary air intake oversized from the specs published by Peter van den Berg, but it was strictly a guess and Peter did not have experience at my altitude.

At the present time I keep my door cracked open about 2" (50mm) when I first light. I can then monitor my pyrometer for riser temperature and gradually close the door watching that the temperatures continue to climb. I'm not in a rush to make changes since I can manage the air by cracking the door. I'm sure this would cause Peter's Testo meter to log lousy combustion metrics but it is for a short period of time while the batch get's up to over roughly 1,000f (540C) in the riser. I'll probably just wait until the heating season is over and make some modifications to my existing door rather than building a whole new door. I'm currently thinking multiple air inlets with sliding apertures low in the door for greater air volume when wide open and more precision control as the fire takes off. This would also allow me to add a larger window to enjoy the flame.

Any thoughts from the community are absolutely welcome!

Glenn Littman wrote:I


At the present time I keep my door cracked open about 2" (50mm) when I first light. I can then monitor my pyrometer for riser temperature and gradually close the door watching that the temperatures continue to climb.  I'm currently thinking multiple air inlets with sliding apertures low in the door for greater air volume when wide open and more precision control as the fire takes off. This would also allow me to add a larger window to enjoy the flame.

Any thoughts from the community are absolutely welcome!

I just wanted to add, and this is solely dependent on so many things!  ( does that make sense )

What I mean, is that, I feel the same stove build, will work slightly different depending on where it is built.  This covers flue type and size, altitude as Glenn pointed out,  air inlets and door types.  But... in general they ALL seem to be working well if built  they are within the build and size parameters.

These things are quickly learned, ( what works best in one location, may not be the same at the next location)

Just as a "for instance"  I can light 3 crumpled news paper sheets, a few small wood pieces, then fill it up,  if I light the paper with the door open, it just lingers and will take off in time.   But if everything is the same, and I light and close the door, I have a instant full draw of air, and nearly a blast furnace with full burn in a matter of a few minutes.   Again, results may vary. Great fun in exploring the "best way"   Also, I feel I can if I want, add wood at any time without risk of over fueling.  And this year, I close it down long before I used to, meaning if I am down to coals covering the bed of the stove I use to wait until they were burned out.  Now I close things up quickly and feel I have net gain in stored heat vs letting heat up the riser at the very end.

I don't hesitate to suggest the largest window you can fit or afford to watch the development of the fire and the red coaling end.  And for a few more dollars the temp probes.  Again, to help discover what works best, monitor things and provide relatable information. While I would like another year or two before I can fully endorse, my use of silicon on my second shell layer of tile or brick or granite ( read- not the inner shell)  has worked extremely well so far.

Enjoy and best of success to all!

Glenn, could you elaborate specifically on how you changed your air inlet CSA-wise?

I did some calculations, and allround, I believe your air inlet CSA should be increased by about 30%. Will add my calculations later.
Edit: here they are.

I work for a company that makes biomass boilers (for heat, power or both). Altitude increase is a known element to take into account. Usually you have velocities and pressure drops to take into account, but that is assuming that the air is not supplied naturally but with a fan.

Glenn Littman wrote:

Thank you for your metric conversion to make sense of my numbers to the metric community. Good reminder for me to make my numbers understandable to all.

Glenn, it is a good reminder for me too, that the metric system is not (yet) used by the whole world! Will try to state my numbers in both metric and imperial as well!

Glenn Littman wrote:
Regarding your question about exterior wall temperatures. The next post after your by Nancy Reading is the picture in my shop. Amusing to me is the fact that the hottest spot on the wall is centered about 16" (40cm) down from the top right at the nose of the deer bling I mounted on the wall. The nose gives me a perfect target to aim the IR gun.

You got me thinking with this comment... It seemed unlogical that the highest point of your exterior wall would not be the hottest. But then I realized at the inside, this is probably the location where the hot flue gases exit the riser. These are probably the highest temperatures the inside skin 'sees' due to the high part of radiative heat transfer which is proportional to the 4th (!) power of the temperature. (contrary to convective heat transfer)

Glenn Littman wrote:
You see our camper parked on the opposite side and it is 42" (107cm) from the heater wall. I measured the temperature of the wall of the camper yesterday as 100F (38C) when the exterior was at 185F (85C). Keep in mind the large area that I am heating. Depending on your situation you will likely not need to raise the temperature of your bell this high. Are you building a single or double skin bell? Note that Tom Rubino just finished his Shorty core masonry heater with single skin bell and in his pictures you can see one of his comfy chairs quite close. Perhaps he can chime in with his comments in this regard.

I'm building a double skin bell with outside skin around 8.5 cm (3 1/2 inch) wide red brick and inside skin firebrick on edge (6 cm wide). My TV will be a lot closer to the exterior skin, less than 10 cm / 4 inch. I'll probably end up installing a heat shield on this side of the tv, and maybe add some insulation between inside skin and outside on this location.

I'm also not sure on how to determine overfuelling. Probably Peter could tell with his Testo and maybe it is not easily seen with the naked eye looking at the exiting flue gas. Good to hear however how you do it. I don't really expect / plan on reloading it a lot, but may end up doing it.
Thank you for your input!

Julian Adam wrote:Glenn, could you elaborate specifically on how you changed your air inlet CSA-wise?

I did some calculations, and allround, I believe your air inlet CSA should be increased by about 30%.

Julian, glad you raised this question and thank you for your scientific approach. I'm good at building stuff but don't ask me the theory or science behind it .

During my build I raised this question with Peter and his guess at the time was inline with your calculation. If you refer to his calculations table for a 6" system (150mm) you will see that at sea level he specifies a primary air intake of 5.625" CSA (3,534 sq mm). My door has a circular inlet of 3.17" diameter which equals 7.89" CSA (5,090 sq mm) and that leads to a piece of channel the forces a 90 deg turn downward for several inches. The channel has a CSA of 7.24" (4,671 sq mm). This equates to roughly a 30% increase in CSA however it does not account for the frictional loss of forcing the 90 degree turn in air flow.

In my simple way of thinking my approach would be to increase by perhaps another 30% which is probably overkill but if I engineer the air intakes with sliders, which would make them infinitely variable, I can dial in whatever I need as the burn ratchets up.

As always, I'm never to proud to toss my ideas in the trash in favor of a better approach. If anyone has other ideas feel free to share.

Julian Adam wrote:

Glenn Littman wrote:
Regarding your question about exterior wall temperatures. The next post after your by Nancy Reading is the picture in my shop. Amusing to me is the fact that the hottest spot on the wall is centered about 16" (40cm) down from the top right at the nose of the deer bling I mounted on the wall. The nose gives me a perfect target to aim the IR gun.

You got me thinking with this comment... It seemed unlogical that the highest point of your exterior wall would not be the hottest. But then I realized at the inside, this is probably the location where the hot flue gases exit the riser. These are probably the highest temperatures the inside skin 'sees' due to the high part of radiative heat transfer which is proportional to the 4th (!) power of the temperature. (contrary to convective heat transfer)

One other factor to consider. My inner skin is red clay brick up to just below the level of the top of the riser and then I transitioned to refractory brick. The point of high temp on the external wall is adjacent to this transition. I suspect this may be a bigger contributing factor to the external high temp location than anything else but that is just a guess. This brick transition can be seen in the photos on page 2 of my Build thread.  

Glenn Littman wrote:A huge thank you to Peter van den Berg for his decade-plus efforts to develop, fine tune and publish batch rocket combustion core designs. For more information, visit his website at https://batchrocket.eu/en/.

Thank you for the compliment, it has been a real pleasure to see your build commencing. And of course, ALL you said about the batch box rocket system is entirely true, no doubt about it!

The question about how overfuelling can be spotted is easy to answer: thick black or dark grey smoke from the chimney. Not to be missed, I'd say.

My method of starting the heater: load it as you described and light it on top, in my case with a single barbecue lighter. Lots of paper tend to fill up the ash space on the bell's floor, surprisingly quickly. When lit, I keep the door open a generous crack, until the chimney pipe temperature rise to 110 ºC (230 ºF), after which I'll close the door in two steps in the course of, say, 10 minutes. Much like you are running your heater, I'd say, minus the newspaper.

Some of the temperature conversions are quite a bit off, sorry Julian.

5. 1000 °F = 537 °C might be 1000 ºF = 537 ºC ?

6. 175 °F = 79 °C might be 175 ºF = 79 ºC ?

7. 50 °F = 10 °C might be 50 ºF = 10 ºC ?

All the rest is correct as far as I can see. Maybe 5. ,6.  and 7. are just numbers of a list?

Peter van den Berg wrote:The question about how overfuelling can be spotted is easy to answer: thick black or dark grey smoke from the chimney. Not to be missed, I'd say.

My method of starting the heater: load it as you described and light it on top, in my case with a single barbecue lighter.

Thank you Peter, as always, for your review and comments. It is great to understand the smoke conditions of overfueling. I can confidently say that I have never seen anything other than white color emissions from my chimney that I associate as steam since it disappears within a few feet of the chimney exit.

Today I used for the first time fire starters that my wife made rather than using paper. She collects the cardboard tubes from the end of the toilet paper roll, spreads some petroleum jelly on the inside of the tube then stuffs in some lint from the dryer. It worked great.

Glenn has it right, Procrastinate no longer, you want a Masonry stove in your future!
I have been building RMHs since 2013.
All of my builds have been in outdoor, uninsulated buildings.
In eleven years I have not lived with an RMH in an insulated building.
This year I remedied that!

Peter has developed a new design Batchbox core named "Shorty Core"
I watched his development closely.
Here is a link
https://permies.com/t/234638/Development-compact-batchrocket-core  

Having retired from active work, I decided to build this core and install it indoors in our 100-year-old log cabin.
In June of this year, Permies Staff member Gerry came to visit.
While he was here we built and test-fired the first "Shorty Core" in the USA.
Here are the links.
https://permies.com/t/254283/Shorty-Core-Montana-Version-burn
https://permies.com/t/254292/Airframe-Construction-Shorty-Core
https://permies.com/t/254174/Casting-Large-refractory-Slabs

In early October Gerry returned and we built Shorty a brick bell and moved her indoors...
Ten days later, we lit off Shorty in the house for the first time!
https://permies.com/t/267527/Shorty-Core-bell

Shorty replaced a homemade wood stove  made from a 2' section of Alaska pipeline that was 3/4" thick
I installed this stove in 1986 and it has been the main source of heat in our house for almost 40 years.
It was not easy to convince Liz that this was a good idea.

Now, I'm currently burning only ONE load of wood a day!    
24 hours between loads.
No fire all day.
No fire all night and the external brick temps are still holding 85F each morning.
450 plus clay bricks at 85F makes for a warm home!
Admittedly, we have only been burning her for less than a month, and the outside temperatures are nowhere near mid-winter temps yet.
But OMG! This old house has never had such even temperatures!

Before my comfy chair would get too hot and three feet away on the couch, Liz would be cold.
The floor was always cold and slippers were a must.
The fire smoldered 24 - 7 from September through April-May, consuming apx 3 cords of dry Douglas fir and larch.
Now we are both warm, from our toes to the ceiling.
It is a balmy 72-73 F all day, and all night, even if we open the bedroom window for half the night!

As real winter arrives, I expect to burn Shorty perhaps, two times a day.
What a difference not having to check the fire, and adding wood to smolder all night.
Leaving for 8 hrs on a big city run with NO fire and not being concerned about things freezing while we are gone.
I expect to use one cord or less in the cabin this season, certainly a welcome change!

Built with a single wall bell, so far, nearby things are not getting uncomfortably or unsafely hot.
The wooden wall 5" away has not gone over 90F  even with the bricks running 180F.
My comfy chair is 8" away and is simply warm to the touch unlike before at 18" when it often became uncomfortably hot.

thomas rubino wrote:My comfy chair is 8" away and is simply warm to the touch unlike before at 18" when it often became uncomfortably hot.

Sounds like that chair will be less useful as a torture device if the Spanish inquisition happens to show up...

Glenn Littman wrote:This equates to roughly a 30% increase in CSA however it does not account for the frictional loss of forcing the 90 degree turn in air flow.

I'm not sure this 90° would mean a significant pressure drop towards the secondary air. However, I think it may be possible that the (well made by you and well designed by Thomas) door directs the air flow too much towards the secondary air tube, and it has more difficulties to go up and over the threshold to supply the primary air. In essence you are making the air bend in a sharp 180° way. Not sure on how the larger air intake would aggrevate this effect compared to Thomas' original design.

Peter van den Berg wrote:
The question about how overfuelling can be spotted is easy to answer: thick black or dark grey smoke from the chimney. Not to be missed, I'd say.

Good to know Peter, thank you. So in essence, even though there will inevitably some CO increase for a few minutes, as long as you see only white water vapour/no smoke and NOT black smoke, you are not overfuelling at the moment you reload the batch?

Peter van den Berg wrote:
Some of the temperature conversions are quite a bit off, sorry Julian.

5. 1000 °F = 537 °C might be 1000 ºF = 537 ºC ?

6. 175 °F = 79 °C might be 175 ºF = 79 ºC ?

7. 50 °F = 10 °C might be 50 ºF = 10 ºC ?

All the rest is correct as far as I can see. Maybe 5. ,6.  and 7. are just numbers of a list?

Exactly Peter, the numbers were to indicate the numbers on Glenn's list where I made the conversion. I will edit this to make it more clear!
Could I ask which temperatures you are measuring on the outside skin of your red heater, Peter?

thomas rubino wrote:...Now, I'm currently burning only ONE load of wood a day!    
24 hours between loads.
No fire all day.
No fire all night and the external brick temps are still holding 85F each morning.
450 plus clay bricks at 85F makes for a warm home!
Admittedly, we have only been burning her for less than a month, and the outside temperatures are nowhere near mid-winter temps yet.
But OMG! This old house has never had such even temperatures!

Before my comfy chair would get too hot and three feet away on the couch, Liz would be cold.
The floor was always cold and slippers were a must.
The fire smoldered 24 - 7 from September through April-May, consuming apx 3 cords of dry Douglas fir and larch.
Now we are both warm, from our toes to the ceiling.
It is a balmy 72-73 F all day, and all night, even if we open the bedroom window for half the night!

I've been anxiously awaiting your comments Tom but I know you've been busy the past few days. Your experience so far is further testimony to the significance of radiant heat and the benefits of a thermal mass. It is difficult to not sound like a paid advertisement but the ability to achieve such comfortable long lasting heat, burning less wood and burning that wood in such an efficient and environmentally friendly manner makes one wonder why everyone isn't rushing to build one in their home.

I am familiar with Tom's requirements and building a masonry heater in a 100 year old cabin requires some consideration. First and foremost is the weight and ability for the floor to provide support. For this reason Tom was limited to building a single skin heater. Even so, he is still reaping huge benefits. Obviously, the more mass, the more thermal battery storage, but also more weight. If you can build a double skin heater, so much the better... if you can't, don't worry about it, it will still perform beyond your wildest expectations. If you've ever woken up in the morning or walked in the house from being away all day and the chill is just hanging in the air... you can say goodbye to that feeling with a masonry heater. It just works.

Congrats Tom on your Shorty core build and pioneering the first Shorty core heater in the US. And thank you for all of your guidance and encouragement during my build.

Peter van den Berg wrote:...When lit, I keep the door open a generous crack, until the chimney pipe temperature rise to 110 ºC (230 ºF), after which I'll close the door in two steps in the course of, say, 10 minutes. Much like you are running your heater...

After thinking about your procedure for initial lighting by adjusting the air with the door cracked open I realize that I'm actually not far off from what you are doing. I'll have to time my process but it's probably just a little longer that I am adjusting the door opening before closing it completely and letting the system work as intended. Interestingly, my chimney pipe temperatures run a good 20 ºF (7 ºC) lower than yours. I'm probably over thinking the initial fire-up phase and air requirements and would be best served by not trying to reengineer my door.

Julian Adam wrote:So in essence, even though there will inevitably some CO increase for a few minutes, as long as you see only white water vapour/no smoke and NOT black smoke, you are not overfuelling at the moment you reload the batch?

Yes, I'd think that is exactly what I meant.
But... the finalized Shorty core is quite a bit different in this respect. It simply REFUSED to kick into overfuel mode, wharever I threw at it.

Julian Adam wrote:Could I ask which temperatures you are measuring on the outside skin of your red heater, Peter?

Yes, you can.
I am running our heater once a day in the evening, with soft coniferous fuel, about 4.5 kg (9.9 lbs) each run.
At this very moment it is about time to start up again, the surface temperature is now 30 ºC ( 86 ºF) on average. Running just one load without refilling, the surface temperature will rise to 40 ºC (104 ºF). When we have frost, we switch to twice a day, probably in smaller batches, temperature will rise then to about 50 ºC (122 ºF). The red bell heater is a slow cooler, build like that on purpose. Our dwelling is a full passive house of 180 m² (1940 sq ft), including workshop, living room temperature is 22 ºC (71.6 ºF). A bit more when the heater has been run.

Our fuel consumption in a full heating season is about 1.5 m³ soft wood species, equivalent to 53 ft³ , or 0.41 cord.

Peter van den Berg wrote:
But... the finalized Shorty core is quite a bit different in this respect. It simply REFUSED to kick into overfuel mode, wharever I threw at it.

I think this will be an added bonus for the people who have a larger house to heat/a not so well-insulated house with high heat losses. I personally see an advantage in long-term maintenance. It seems the shorty core, because it is compact, could be replaced by only opening up a small portion of the bell (unlike the regular batch box which has the high riser). Unfortunately it is not an option for me because I have only limited depth to my bell.

Thank you for your (objective!) input on how you operate your heater Peter. Together with Glenn and Thomas their input this gives a good indication on how hot my bell's outer skin will get.
It was something which I did not find alot about reading through the forums, so I believe it will be of value to others as well.

Burning pallet wood will also be my way to go. Environmentally, it just makes no sense here in Western Europe to burn beech or oak coming from Eastern Europe + pallets are often completely free if you're able/willing to do some cutting work. Good to have an indication on your consumption. My house will never be as well-insulated as yours, but I will only heat about 45 m2.

Julian Adam wrote:Thank you for your (objective!) input on how you operate your heater Peter. Together with Glenn and Thomas their input this gives a good indication on how hot my bell's outer skin will get.
It was something which I did not find alot about reading through the forums, so I believe it will be of value to others as well.

I am very happy Julian that this thread has been of value to you and thanks for making this point. This is precisely the reason that I wrote this post in the first place (along with some encouragement by Tom). You are correct, there is not a lot of real user data and experience to draw from as a person makes the decision to build a batch rocket (or a more simple J tube). It is wonderful to see a growing interest and a growing user community of contributors to this topic.

Julian Adam wrote:Burning pallet wood will also be my way to go. Environmentally, it just makes no sense here in Western Europe to burn beech or oak coming from Eastern Europe + pallets are often completely free if you're able/willing to do some cutting work. Good to have an indication on your consumption. My house will never be as well-insulated as yours, but I will only heat about 45 m2.

Free wood is a wonderful thing and great to hear that you are leveraging this option in Europe. If one keeps an eye out as they travel about one often sees businesses with stacks and stacks of old pallets. In many cases, the business is happy to have you make them disappear with the added benefit that they are typically hot burning hard wood.

Another thought for folks is something that I found earlier this year. Our mountain community has a growing Amish population, many of whom are in construction. There's a nearby family with 2 small sawmills on their properties to supply their own construction needs and it was easy to see their growing slab pile. I asked them what they planned to do with it and that I was happy to buy truckloads at a reasonable price. The main guy said he was going to have to find someone to haul off the slabs and he would be thrilled to not deal with it, that I could have all I wanted for free. SCORE! We have lots of land for places to create my own slab piles. Given our high arid environment in southern Colorado, wood does not rot, so I told him that I'm his guy to take care of the problem and as long as he's not in a hurry I'll eventually take it all. My piles are growing, his are shrinking and I've started to identify needy folks in the community that I'm sharing wood with. In rural communities there is often a small sawmill. Even if you have to pay a reasonable price it is a great resource. The wood is typically douglas fir, you eliminate the risks of cutting down standing trees and it's simple to cut to size to your needs. As can be seen in the photos, there is plenty of meaty wood on the slabs and the wood burns great.

Glenn Littman wrote:
You are correct, there is not a lot of real user data and experience to draw from as a person makes the decision to build a batch rocket (or a more simple J tube). It is wonderful to see a growing interest and a growing user community of contributors to this topic.

I hope to add some info to the knowledge base as my build continues. In the far future (still renovating my house), I hope to make a flowchart guiding newcomers like myself through a full design & build and helping them to make decisions - as I believe this may be the hardest part of a build.

Glenn Littman wrote:
The main guy said he was going to have to find someone to haul off the slabs and he would be thrilled to not deal with it, that I could have all I wanted for free. SCORE!

Incredible that you can get these amounts of good wood for free, Glenn. Something I could only dream of! To me, there is something ritual about collecting and processing wood and ultimately producing your own heat.

Glenn, something just popped to mind.
In your build it was unclear to me to what extent you used the octagonal shape at the bottom of your heat riser. Did you just do the 'triangles in the corners' trick at the bottom facing the port, or did you make the full bottom octagonal.
I read something a while back of not mixing octagonal / square riser shape, but it seems logical that an octagonal shape at the bottom would promote good vortex formation where it matters most.

Julian Adam wrote:Did you just do the 'triangles in the corners' trick at the bottom facing the port, or did you make the full bottom octagonal.

It's a full octagon. I didn't include this picture in my build thread but here it is.

Thank you Glenn, I understand you continued the octagon shape to about the height of the top of the firebox? Will have to look for a few extra IFBs, I only took the triangles into account, but this seems superior.

Julian Adam wrote:Thank you Glenn, I understand you continued the octagon shape to about the height of the top of the firebox? Will have to look for a few extra IFBs, I only took the triangles into account, but this seems superior.

Julian, all of the brick used in the lower portion of the riser, including the triangles shown, are regular firebrick not IFB's. I only transitioned to IFB above the level of the firebox.

Here are pictures of the core prior to adding the firebox roof and IFB riser, as well as the full core before wrapping in superwool.

Note that I used Sairset refractory mortar buttered between all of the bricks of the core to ensure no air gaps as well as added stability. I then used ITC 100 HT ceramic coating painted on all firebrick (not the IFB's) for additional thermal retention inside the core. These steps are not required but I wanted to do everything possible to help maximize performance and extend the life of the core.

I have two more questions...

Glenn Littman wrote:
Note that I used Sairset refractory mortar buttered between all of the bricks of the core to ensure no air gaps as well as added stability. I then used ITC 100 HT ceramic coating painted on all firebrick (not the IFB's) for additional thermal retention inside the core.

1. Why did you not coat the IFB? I was actually planning on doing the polar opposite: not coating the dense firebrick, but coating the IFB with a zircon coating. Do you have any idea on how the coating is holding up in your firebox so far?
2. I was going to make the entire riser with IFB. Could you explain your reasoning behind using dense bricks for the first few rows?
Thanks again for taking the time to respond to my questions!

Julian Adam wrote:I have two more questions...
1. Why did you not coat the IFB? I was actually planning on doing the polar opposite: not coating the dense firebrick, but coating the IFB with a zircon coating. Do you have any idea on how the coating is holding up in your firebox so far?

Two reasons (I'm not sure the reasoning is actually sound but this was my thinking)... The IFB already has great thermal insulating properties. Considering that they are very porous in comparison to regular firebrick I felt it may actually be detrimental to the insulating properties.

Julian Adam wrote:2. I was going to make the entire riser with IFB. Could you explain your reasoning behind using dense bricks for the first few rows?
Thanks again for taking the time to respond to my questions!

The highest temperatures are reached higher in the riser and it was easier to build keeping the walls of the core and lower riser consistent with the regular firebrick. For what it's worth my firebrick is Jet brand industrial grade refractory brick rated for 2,600F. Perhaps overkill but consistent with my desire to have the heater last my lifetime.

If you want to know all about the pros and cons of coating refractory then you could read this ……
https://donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

Glenn, thanks for explaining your reasoning and motivation.
James, I have read through that thread a few months ago and refreshed my memory just now.

I will coat all surfaces with a zircon coating. I'm still doubting the use of IFB up to the top of the firebox. As Forscythe mentions in his thread, direct flame impingement will degrade the materials more rapidly, making the bottom of the riser, where the vortex is formed - although not under a lot of mechanical stress - still a critical point as the combustion gases are directly 'hitting' this surface.

It's good to know your rocket starts rocketing quite soon, Glenn. That means the IFB all the way to the bottom is probably less critical than I'm making myself believe. (I also don't want to rebuild it every few years, just the same as you).

James, your wonderful video's on yt have given me an idea about using ceramic glass as a firebox liner. I can get it for free from reclaimed stove tops here. Its very low porosity should be beneficial to resist the slag formation/ash penetration in my opinion. Do you think this would work? I wouldn't mind recutting the liner say every 5 years.

Julian Adam wrote:

James, your wonderful video's on yt have given me an idea about using ceramic glass as a firebox liner. I can get it for free from reclaimed stove tops here. Its very low porosity should be beneficial to resist the slag formation/ash penetration in my opinion. .

Julian, has it been wrote or discussed here that there is a problem with slag formation/ash penetration to fire brick? I have been running fire brick lined stoves for a lot of years, and have never seen such?  Maybe I have been lucky?   Just curious.

Thought with the discussion of risers, some photos of 7" If anyone is pondering that,  utilizing the "cut corners" as a very good  riser strength enhancer on the outside.    Done like Glens for 6" or mine for 7" really boost the strength of the riser in my opinion.  Like Glen, just as soon as I got above my fire box height, I went with IFB of very good quality.  ( there is a big difference in quality)

While any riser is hard to inspect once a total brick bell is built, I do have ways to check mine, and it still looks great.

Fox James wrote:If you want to know all about the pros and cons of coating refractory then you could read this ……
https://donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

Fox, thanks for posting the link to this thread. This was the thread that got me tuned into the idea of using a coating and different coatings available globally. It's a very technical and in-depth discussion for anyone interested in learning about what happens to refractory in a harsh, high temperature environment.

Julian Adam wrote:Glenn, thanks for explaining your reasoning and motivation.
James, I have read through that thread a few months ago and refreshed my memory just now.

James, your wonderful video's on yt have given me an idea about using ceramic glass as a firebox liner. I can get it for free from reclaimed stove tops here. Its very low porosity should be beneficial to resist the slag formation/ash penetration in my opinion. Do you think this would work? I wouldn't mind recutting the liner say every 5 years.

It would depend on the size of the stove you are building, my little 4” vortex stove and my 6” vortex J tube seem to work at around the same temperatures and the glass will survive OK but I dont think it will fair so well in a 6” batch box.
I actually have a piece of ceramic glass in the roof of the J tube. I really expected it to break after a few burns but it is still going and still clear after a dozen or so long fires!

Scott Weinberg wrote:

Julian, has it been wrote or discussed here that there is a problem with slag formation/ash penetration to fire brick? I have been running fire brick lined stoves for a lot of years, and have never seen such?  Maybe I have been lucky?   Just curious.

Scott, I don't think many actual problems have been described here, only the post from Forscythe.
I work for a company that makes biomass boilers, and I can tell you that refractory degradation is real. HOWEVER, it is least prominent in wood-fired biomass boilers as the minerals responsible for slag formation / refractory degradation are present in far less quantities than e.g. agro fuels.
This being said, in my opinion it does not hurt to protect the core while you are building the heater. Repairs in an industrial context are easier to do, as you can physically enter the combustion / post combustion room.

Scott Weinberg wrote:Thought with the discussion of risers, some photos of 7" If anyone is pondering that,  utilizing the "cut corners" as a very good  riser strength enhancer on the outside.    Done like Glens for 6" or mine for 7" really boost the strength of the riser in my opinion.  Like Glen, just as soon as I got above my fire box height, I went with IFB of very good quality.  ( there is a big difference in quality)

While any riser is hard to inspect once a total brick bell is built, I do have ways to check mine, and it still looks great.

Thank you for adding insights on your riser construction scott. Was your reasoning for using dense firebrick up to the firebox height the same as Glenn's - mainly a practical one?
For some reason I had not seen your build yet but thank you for documenting well! I'll go through it.

Julian Adam wrote:

Thank you for adding insights on your riser construction scott. Was your reasoning for using dense firebrick up to the firebox height the same as Glenn's - mainly a practical one?
For some reason I had not seen your build yet but thank you for documenting well! I'll go through it.

Yes, on Practical reasons ( and if you believe in saving some dollars) even more practical reason to me,  as my IFB of high quality cost more than my good standard firebrick.  On the other hand,  I have tremendous draft,  but can not contribute this to the rest of the riser in IFB, or the fact my flue is very tall  (35 feet)

I have not had to tweak hardly anything on the stove so am pretty happy.  Different than most,  I could get inside my bell for any repairs if needed.

 One change I did make, was after a year of use, I decided that my roaring fire, was actually putting a higher volume of air ( cubic feet)  through the system.  This can only be done by physical size of the system, to be bigger than planned, or in my calculations actual speed of airflow through the system.  With my temps all reading just about perfect,  I decided that while X temp of air, was right where it was suppose to be, the actual flow of air was to much.

Sure don't want to dampen the exit side as that is like trying to stop a bull after it has started running,  so this year I added a inlet door that I could adjust easily, and this has worked really well,  I can tone down the fire, with zero over fueling issues.  And when coaling, I can shut the system down easily.   I am happy.  

Some have told me they don't like all the variations of second skin testing on my bell,  well this was my experiment, very few see it, and it has proved that silicon works fantastic for bonding the second skin, and can handle all levels of expansion and contraction.   Again, I wanted to know, and now I do. it worked just as well on areas where I installed a facade of large 12" x 24" tile.

Best of success.

Julian Adam wrote:Was your reasoning for using dense firebrick up to the firebox height the same as Glenn's - mainly a practical one?

Using the bottom part where the vortex is created makes sense:
-to resist flame erosion
-to resist slag fluxing
-to prevent accidental hitting with wood/poke

However, I don't know what is the durability of the IFB in the wood flame path and I'm not sure who knows it. I'm considering such setup (bottom hard, top IFB) for my masonry heater, but I still don't know if it will last at least 20/30 years, because nobody have used it for this long as the riser material. On the other hand, the longevity of hard bricks is well documented.

As far as ITC-100 is concerned it makes sense to paint entire interior of the firebox and the riser - it reflects heat back into the fire and actually helps IFBs to work better, because they lose their insulative properties in higher temperatures when the infrared radiation is main carrier of energy. It's a known solution used in gas kilns built of IFBs to increase their heat reflectivity and lower firing cost. I'm going to apply it this week on the upper inner walls of my gas kiln to equalize firing temperatures.