For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
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thomas rubino wrote:Hi MM;
Well, you get an A+ for thinking outside of the box!
This may work just as you are hoping but only time will tell us.
It may work well at first but then slowly get sluggish as ash collects.
We encourage experimentation.
I would only use a clay-based mortar with no concrete mortar at all.
When you experiment with Rocket Science, you should expect to need to tear down and rebuild if/when design flaws appear.
Good Luck, and keep posting pictures
Metal Monkey wrote:My biggest worry is that the thing won't work after I've bricked it all in for some reason, and also at the moment I'm wondering about the ratio I should go for with the secondary air intake. I'm, thinking about 1/4 of the diameter of the feed tube, and that will go to a grill to break it up a bit and also restrict it a little more just before the stack.
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Timothy Norton wrote:I am not a rocket enthusiast like many here but I wanted to welcome you to Permies none the less!
I'd love to see your project as it continues so consider keeping a running record here if you wouldn't mind letting folks live vicariously through your photos.![]()
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
thomas rubino wrote:Hey Miles;
1000 C = 1832 F
The riser of a J-Tube runs in the 1800F range; do not use your refractory there.
The core bricks will not get that hot; your mortar will hold on to them, but...
As Steven mentioned, it is semi-permanently attached if you use anything other than a clay mortar.
With respirators and chisels, refractories can be removed, but it sucks to do so.
Clay-based mortar pops right off into a bucket. Add water to rehydrate it, and use it again! Try that with a cement-based mortar.
All mortar will crack, even clay.
When clay mortar cracks, a wet finger dipped in a small amount of clay and sand will fix it immediately.
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
thomas rubino wrote:Ok;
Here is what I recently learned about mixing refractory.
You must mix the entire bag at once; any less, and you will not get the proper mix of ingredients.
The amount of water used is critical, close enough will not work. I weighed the water I added to my refractory to be sure I got it right.
As I was warned, it would seem way too dry but absolutely do not add more water.
There was no "pouring" of the mixed refractory. Instead, handfuls were stuffed in our form boxes and then placed on a homemade vibrating table, which was vibrated using a variable-speed saw. (saws-all)
Care must be taken not to over-vibrate as well.
Could you return that mortar?
If not, the pizza oven would work fine, or as the slab.
Clay mortar is made with one part dry clay and three parts dry sand.
Fireclay is suggested, but other bagged clays may work.
Screened medium-graded sand is highly recommended. Concrete sand is not usable without significant work.
Free local sand is usually too fine and always has rocks and debris.
Here is my thread about mixing.
https://permies.com/t/254174/Casting-Large-refractory-Slabs
Miles Teg wrote:Um.... I found something worrying for it being sold in Australia, but wow, this might be a good liner for the fire box and chimney...
NUCLEAR REACTOR FIRE RETARDANT GOOD TO 2700c!!!
WTF is this stuff?? I don't even know if I want it in my yard! haha
https://www.artisansupplies.com.au/product/rtz-washcoat-1kg-nuclear-reactor-cladding/
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
Rico Loma wrote:Well , it is guaranteed that advice coming from this community on Permies will keep you on the side of the angels .....not up there playing along on your own harp!
Seriously though, I know many of us are influenced by videos or advice from at times. But honestly mate, you can't go wrong by consulting folks here. Emphasis on safe experimentation, using mostly or completely natural resources, and suggestions with a smile. You can trust folks here, specifically Thomas, Fox, Gerry, Glenn, and The ultimate friendly genius Peter
In other words: do not look at those shoes behind the curtain, I am the real Oz!
thomas rubino wrote:Hey Miles;
I agree. No, No, No on the nuclear stuff!
Yes, the fireclay and silica sand that have been screened are perfect.
You need 3x of the sand than you do of the clay.
I doubt it is available there, but this is the sand that I use.
https://www.homedepot.com/p/SAKRETE-100-lb-Silica-Sand-65200396/207136556
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
thomas rubino wrote:Miles, the core is the only place where it gets hot enough to spall steel.
Your mass, especially down low, will not be that hot at all.
Rather than hinges mounted in brick (they always work loose), most of us use a slide-in door with some stove gasket to seal the flange.
The plan here, is if you build your RMH correctly and use dry wood to burn, most of the ash is incinerated.
Only an improperly built RMH will create enough ash to quickly plug up the system.
Typically, most of your ash will remain in the feed tube and partially up the burn tunnel, where you will clean it out every few weeks.
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
thomas rubino wrote:So Miles;
A couple of thoughts for you to ponder.
To start, there is no need to wait weeks or even months to build after pouring.
For the slab, pour it one day, and two days later, start mortaring bricks.
It's the same for building the rest; no need to wait before firing.
I build these things, slop on the last cob to seal it all up and build a fire.
Clay is terrific stuff; no need to baby it.
When stacking bricks with clay mortar, there comes a point where you should go work on something else.
Until it dries, it is easy to bump bricks out of place.
Now, the next thing I want to talk about is your J-Tube.
Dimensions are specific; you must stay within the established parameters.
Looking at your drawing, the burn tunnel appears to be too long.
The roof of the burn tunnel should be 10"-12" long; it should not be longer, or you will create excess ash.
The size (area) should remain the same throughout the feed tube and burn tunnel,
Feed tube depth should be no more than 16".
Now, let's talk about your secondary air tube... you do not need it.
J-Tubes do not use a secondary air tube... first-generation Batchboxes use them.
If you deviate from the build specifications, then you are delving into experimental rocket science, and all promises are off.
The J-Tube design has been tweaked by Peter to be as hot and clean burning as possible.
Look on my website for Peter channels to see what they look like: https://dragontechrmh.com
There is also a burn tunnel roof modification called a trip wire you can incorporate into a J-Tube build as well. (search Permies)
This is why Peter moved on from J-Tubes and started refining the batchbox design to be as hot and clean burning as possible.
Any changes to a J-Tube design will not be as efficient as Peter's improvements and could be much worse...
thomas rubino wrote:Miles;
When making clay mortar, nothing but fireclay and clean, graded (sharp) sand is used.
When making cob, you can add all sorts of things, with ash being one of them. (straw, horse poop...)
Also, when making just a cob mix, you can use any clay and sand with debris included; it is just a filler.
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
thomas rubino wrote:Well, Miles, quite the luck you have!
Nobody else could have dead-centered the water line as well as you did!
Brownie points for you! Oh and an apple...
Oh well, drop a gear and keep moving forward, you will get there.
As far as the sand goes, I will guess the 200 is good. We do not grade sand that way.
If it is a sharp silica sand, screened & graded to be uniform it will work.
As far as piping extra hot air out of your mass, maybe if your bell is undersized, you might get this to work.
There is a limit on how much hot a J-tube can push without stalling.
But you will get less heat into the mass, and you will need to insulate (or bury in cob). the pipe.
Metal pipe sheds head rapidly.
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!
Miles Teg wrote:
I haven't heard of anyone running air duct work through one of these things before though, which makes me wonder why.... I have either thought of something new, or something someone else has tried and it didn't work, that is the biggest worry at the moment.
Miles Teg wrote:I'd already be a buildin'
and explodin' and a killin'
if I only had a brain.
De-fund the Mosquito Police!
Become extra-civilized...
Coydon Wallham wrote:
Miles Teg wrote:I'd already be a buildin'
and explodin' and a killin'
if I only had a brain.
Sounds like one helluva project you have going there...
thomas rubino wrote:Hiya Miles;
I was looking at your drawing again... maybe I shouldn't do that, eh?
Take a look at this thread
https://permies.com/t/270559/Stratification-chambers-Bells-explained
You really do not need the European design with serpentine channels.
You and your buddy can whip up a strat chamber in no time!
Is the door you salvaged today for the pizza oven?
Andrew Welser wrote:
Miles Teg wrote:
I haven't heard of anyone running air duct work through one of these things before though, which makes me wonder why.... I have either thought of something new, or something someone else has tried and it didn't work, that is the biggest worry at the moment.
While I haven't looked to see what other people have tried, as someone currently working in the HVAC field my initial thought is that, while it might be less efficient then a rocket mass heater with the mass inside the house, I think it has potential, if the large potential hazard is properly addressed: you need to be absolutely sure there is no way now or in the future you will be pumping exhaust from the rocket into your house!
To give an example of one way this is addressed in the commercial market that I see: older gas furnaces had natural draft chimneys and the fan on the household ducted air is before the heat exchanger and creates positive pressure in the heat exchanger, so if/when it rusts out or leaks in some way the household air leaks out through the chimney, and not the other way around. Newer furnaces have fan driven chimneys, but that fan is placed after the heat exchanger so it gives negative pressure to the exhaust side, which increases the pressure difference between the two sides of the heat exchanger.
In addition to putting your household blower before the rocket heater so if there's a leak you blow air out instead of sucking exhaust in, I would recommend separating your exhaust and ventilation pipes in the mass (rather then running ventilation pipes through a stratification chamber, or doing a tube in tube exchanger), even just by a few centimeters, so you can surround all the ducting with a layer of clay, cement, or similar, to minimize air cross contamination long into the future, even if/when the ducting rusts through; similar to what you would want to do to all the piping if the rocket mass heater were inside your house.
A few other potential downsides I see, and ways to address them:
-I would suggest adding some kind of temperature sensor in the ducting or the mass, so that you can keep the indoor blower from running if the mass is cooler then inside your house, to prevent sucking heat out of the house to warm the mass.
-Since the mass is going to be outside the insulation envelope of your house I would recommend insulating the mass, to minimize heat loss to the outside.
-Your heat will be dependent on electricity to run the indoor blower, but that is a minimal power draw compared to electric heat or a heat pump and could be set up to run off a battery with inverter or generator.
-You'll have to go outside into the cold to start/tend the fire. I don't have any suggestions of how to address this other than good winter clothing, but you can at least take warmth in the mess of the firewood staying outside and heating your house for only pennies! :)
I can't speak to the design of the rocket heater, having not built one myself yet, but the more I think about your design the more potential I see; with the heater outside you also eliminate the issue of a backdraft down the chimney or a cold plug/hard start dumping smoke into your house.
Looking forward to seeing the finished product in action!
Cristobal Cristo wrote:Miles,
The finer aggregate you use, the higher total surface area the particles will have and you will have to use higher clay ratio to make it stick. The general rule is to use particle size to be no more than half the joint height, so for example if your joint is 10 mm then grains up to 5 mm are acceptable. When laying firebricks, small joint of 2 or even 1 mm is desired and in this case the particles should not be larger than 0.5 mm which would be equivalent of US mesh 35. When I prepare the mortar for adobe bricks, or rooftiles I use the ratio of 2.8 (coarse sand):1 (clay), but the same clay when mixed with mesh 18 sand has to go in ratio 2:1 and for mesh 35 the best strength is when I mix 1.5 (sand):1 (clay).
Fine silicas from the ceramic supply store are used to prepare glazes or engobes, coarser ones are used as grog. The finer silica will help to develop lustrous glazes and the coarser will not quickly melt and will help to develop matte effect. For glazes I purchase mesh 325 - it's like wheat flour.
For cob it would be most efficient if you just dug some soil (without top organic matter) on your property and amend it with clay if needed. Some samples would have to be done first to determine the best ratios.
Regarding the idea of blowing air in pipes through the mass I doubt it will work. Air has 4 times lower heat capacity than water, but 1 m of air weighs only 1.2 kg, so the same volume of air has 3400 less heat capacity than water. Forced air heating systems also use air, but it goes through heat exchanger and multiple torches. It could potentially work if the exchanger was closer to the fire, but at the same time it would lower the efficiency of the wood combustion, which is not a problem with gas torches. Nonetheless I believe that it somehow could be developed.
Rico Loma wrote:Miles, with respect, you might consider following a proven, basic design that has worked well for a decade or more. Then a later attempt could be for quirky, esoteric ideas that pop into your head. Folks here are trying to help ensure a quality heater for you, but cannot build it for you. For the sake of your safety and your happiness long term, please consider this idea and ignore various Youtubers and fly-by-night TMH builders. Experts on Permies are legit and generous.
In the name of Prometheus,
Rico
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