Rocket Mass Heaters and Masonry Heaters

[Tedinski]

So!
Here's a new thread to pick up from where we left off on the masonry heater thread...
I'm looking forward to hearing more from Q on that thread, as he builds his home & stove.

It's been mentioned that the efficiency numbers for RMH's are notoriously over-estimated.
Has anyone done an actual efficiency test on one? It shouldn't be difficult.

Total mass of wood burned
%H2O
Exhaust temperature
Exhaust volume (CFM)

I should think that would be all that was necessary.

If anyone knows if such numbers have been run, drop a note here!

 

[NW Walker]

Ted, you are asking about overall efficiency, which is different from combustion efficiency. My understanding is that when you are seeing numbers on box stoves in the US you are seeing combustion efficiency numbers, not overall efficiency. I could very well be wrong on that, so take it with a grain of salt. Anyway, the overall numbers on a RMH are going to vary a lot depending on how much mass and how much heat you keep inside the building envelope due to the storage. Not only that, but you will have a short, hot burn with relatively low exhaust temps, but then a long, long heating cycle when there is no combustion. How long that is depends on how much mass and how hot you got it. So, it's really tough to do overall efficiency.

That said, Peter van den Berg, a retired masonry heater designer in the Netherlands who has been designing and tinkering with rockets for a few years, has been very giving and generous with information and designs over the years. Both Dragon Heaters entry and my entry in the design challenge are based around one of his designs. This is a new style of rocket which loads like a wood stove, so it's a bit different, although he has said it performs similar to a standard rocket combustion design, which is called a J tube. Here's one of his Testo combustion analyzer charts from a burn of one of these cores, it should give you an idea of how they burn:

O2 12.9 %, eff. 84.5 %, CO 1103 ppm, Tr 153 C. (307 F.)

He cautions that this test run was done with a recently cast core that still had moisture, and has other test runs with better numbers, but I picked one that was in the middle so as to give a fair idea of real world performance.

 

[Tedinski]

Quite right! Overall efficiency is useful because it includes combustion efficiency.
I can't see the graph here in the forums, but it came through to my email just fine.
I'm surprised at how poor/incomplete the combustion is! I expected much better.
I work for a large furnace manufacturer (steel & aluminum industries). If we had that much CO in our exhaust streams, there would be fines imposed! NOTE: I rechecked those numbers. not so bad! I'm used to seeing %CO. Damp refractory can certainly cause troubles with initial tests though.
You're quite right about it being difficult to determine overall efficiency for the entire class of RMH's. Too many variables! There must be a typical range though...
An averaged exhaust temp of 307 isn't bad. Even using recuperators our exhaust temperatures are typically 600 to 800 F! The sites I've seen that have bothered to measure exhaust temp on the RMH's say they're typically 150 to 170. That's significant! Of course, they don't say how long they've fired the stove or with how much fuel. They don't list CFM of exhaust, either. If you have considerably higher air flow the flue temperature could be low, and STILL be less efficient.

I have half a mind to build one just to thoroughly test it in scientific fashion. I've got the perfect place for one!

The immediate radiant heating that is described sounds right up my alley! I've wondered about that with the masonry stoves. My house is often cold when I get home (I may be gone for a week, or three...) Waiting hours & hours for heat to soak into, and then slowly radiate out of a masonry stove is NOT desirable. They sound perfect for people who have a nice routine. (I envy people with a nice routine!)

 

[Seasoned Oak]

THe 55 Gal drum would be easy enough to camouflage. I saw one picture where the whole stove was incorporated into a kitchen island and the top of the drum(originally the bottom) was just under flush with the counter top and used as a cooktop.

 

[Tedinski]

PS... I am under the same impression, regarding efficiency numbers for stoves. Some of the wild claims for reduction in wood usage are almost believable to me! In order to NOT get toasted right out of the house I've needed to reduce my draft & bank the fire. Lots of smoke.... lots of creosote. I've never measured the flue temperature but I'm quite sure it's pretty high! High efficiency numbers are great if you can run your stove "wide open".
So... even a stove with a very high efficiency rating, run all damped down (like mine) must be very inefficient indeed. Maybe 30%? 40%?

And when run hot, for high efficiency, i'm also sure that a GREAT deal of heat is going right up the pipe!

 

[Tedinski]

THe 55 Gal drum would be easy enough to camouflage. I saw one picture where the whole stove was incorporated into a kitchen island and the top of the drum(originally the bottom) was just under flush with the counter top and used as a cooktop.

I saw a beautiful kitchen "island" design, made with stone, where the drum was mostly hidden. Beautiful design! I should think that burying the drum in stone like that would reduce the amount of immediate heat radiation. I'm intrigued about using it as a cooktop... a REAL cooktop, not just a spot for boiling tea water!

 

[Tedinski]

I have my eye on a 30-gallon stainless drum, with a clamp-on top. I like the idea of an easily removable top to the barrel, for cleaning purposes.
No idea where I would get good clay dirt for cob... the woods all around me are pine with thick peaty/piney dirt. The creek through my property is very rocky with coarse sands. Making a cob stove isn't very efficient if you have to buy a gagillion pounds of clay! Ideas?

 

[Seasoned Oak]

PS...
And when run hot, for high efficiency, i'm also sure that a GREAT deal of heat is going right up the pipe!

Thats what im wondering about the rocket stove. If the draft has to be strong and the fire hole is sucking air to keep the smoke going down and across does that not equal a lot of heated room air going outside needing to be replaced with cold air? PLus does this stove have any way to introduce super heated fresh air into the fire stream as most EPA Stoves do for burning smoke at high temps?

 

[NW Walker]

So... even a stove with a very high efficiency rating, run all damped down (like mine) must be very inefficient indeed. Maybe 30%? 40%?

And when run hot, for high efficiency, i'm also sure that a GREAT deal of heat is going right up the pipe!

You nailed the reasons I think I do much better on efficiency with one of these Ted. It's one of the reasons I'm a bit frustrated with the design challenge in D.C. in the coming weeks as well. They just announced they won't be testing the box stoves at low air settings, one reason was because of concerns about smoke on the mall. While box stoves are capable of clean operation, I believe they are rarely run that way.

Oak, as for the draft and room air, yes, it's true, they pull hard through the combustion area. The thing is, that once the gases are burnt completely the heated exhaust gasses are run through a long run of masonry so a large part of the heat is transferred to the masonry for slow release over time. My typical flue temps are right around 200° when it's running hot, but keep in mind I might burn for 3 hours for every 12 hour heating cycle. After the burn is out the combustion chamber is closed off so there's no more transfer of room air through the mass.

 

[NW Walker]

Oh, as for the heated secondary air, the standard J tube does not benefit from air added downstream. The three T's; time, temperature, and turbulence are all handled well in the J configuration. There are some little tricks to making sure they get air in the right spot in front of the fuel, but mostly they are a wonder of passive, simple engineering.

Ted, like I said, there's charts with much better numbers. Those are just what an average dude can expect with an average build.

 

[NW Walker]

Ted, here's some more Testo graphs. You can follow this link to a build by Dragon Heaters, and see what kind of numbers Sandy is getting from an optimized J tube, again design by Peter van den Berg. If you read the text you will see that she is learning the testing equipment, so there are some weird readings. You can also see the co jumps at loading times roughly every 40 minutes. Here's the link:

(broken link removed to http://blog.dragonheaters.com/category/flue-build-part-1/)

...and here's the results...

(broken image removed)

Much better Co numbers, but still some spikes when fuel is loaded. She notes the rough patch at the end is when the testo insert was clogging, or something like that.

 

[Prof]

I have my eye on a 30-gallon stainless drum, with a clamp-on top. I like the idea of an easily removable top to the barrel, for cleaning purposes.
No idea where I would get good clay dirt for cob... the woods all around me are pine with thick peaty/piney dirt. The creek through my property is very rocky with coarse sands. Making a cob stove isn't very efficient if you have to buy a gagillion pounds of clay! Ideas?

This may be a bit morbid--but it is Halloween: I got some great cob dirt from a grave yard. They all have a pile of dirt in a corner somewhere. It usually doesn't have topsoil mixed in because they use that to reseed the grass.

 

[Seasoned Oak]

So if im reading this concept right the "draft" is produced by the J tube that then pushes the flue gas thru the rest of the structure even if the exhaust tube is horizontal?(Still trying to figure out just how this stove works)

 

[NW Walker]

Pretty much Oak. It's actually immediately after the J. The highly insulated riser is encapsulated in a metal radiator (the barrel). The super heated gasses are rising and expanding in that riser, they hit the barrel and instantly start rapidly cooling. I call it a Differential Engine, although I'm not exactly sure that's a great name, anyway.....those cooling gasses can't go back down the riser, they are falling down inside the barrel outside the insulated riser. It's the combination of the hot expanding gasses rushing up the riser and the same gasses cooling rapidly and falling in the radiator that give the thing it's drive. It will actually burn fine without a vertical stack at all, but the chance of subtle pressure differences in atmosphere at the entry and exhaust make that a real finicky proposition in the real world. They benefit greatly from some added low pressure pull from a warm stack at the end.

 

[Tedinski]

This may be a bit morbid--but it is Halloween: I got some great cob dirt from a grave yard. They all have a pile of dirt in a corner somewhere. It usually doesn't have topsoil mixed in because they use that to reseed the grass.

Hello Prof!
Well, we're both from the same neck of the woods. Perhaps I can locate one willing to get rid of some good clay that's been dug out? I never would have thought of a graveyard... they have to do SOMETHING with all that clay that gets replaced with "other material"...

 

[Seasoned Oak]

Pretty much Oak. It's actually immediately after the J. The highly insulated riser is encapsulated in a metal radiator (the barrel). .

How is the riser insulated ?Would a riser made completely out of firebrick be considered highly insulated?

 

[Seasoned Oak]

Funny, until recently the only kind of barrel stove i knew of was one of these.
(broken image removed)

 

[NW Walker]

How is the riser insulated ?Would a riser made completely out of firebrick be considered highly insulated?

Not if it's heavy brick, which is typically what firebrick is. There are kiln bricks which are highly insulated but less durable that would be.

One way is to build with regular heavy firebrick and surround that with Superwool, kaowool, or a mix of clay/perlite. It's done often, but I think it's a terrible way to build since it's high mass at it's core so you use a lot of heat to heat the mass before it all works correctly. Same goes for the combustion core, heavy brick is the norm since it's easy to find and durable, but I think it's a low performance build. You want insulation everywhere in the combustion zone.

I think the best DIY approach is a mix of fireclay and perlite formed in a mold made of an inner sleeve of flue pipe and an outer canister made with a 17 gallon "grease drum" barrel. It's not worth trying to find anything fancy for the inner sleeve, it will be destroyed within the first few burns no matter what metal you use.

The best risers I know of are formed ceramic fiber that Ted is probably familiar with from his work. You can use ceramic board and build a square section riser, or you can source round section formed ceramic fiber in various I.D., but they are expensive.

 

[Tedinski]

What would the mix of fireclay and pearlite be? (ratio) Is fireclay and pearlite "up to" the challenge of direct flame impingement? I'm betting you'd need to re-pour the riser every couple of years at most, and perhaps more often.

I can get pretty much any quantity of fiberfrax that I want, for free. Large cut off pieces, or even ends of rolls are thrown out every day.

Perhaps IFB (insulating fire brick)? The higher the temperature rating on the IFB, the more they will conduct heat, but they would last longer as well.

Is there a general consensus on the temperatures the J-tube / riser will see?

 

[DevilsBrew]

My novice advice is backing up what Walker said and to stay away from any type of brick for the core or riser in order to create a high performance build. In the future, I am going to be getting away from using brick for my rocket stoves and only use it for the ovens. I have found that brick takes away from the combustion and flow of gas. I'm not even going to waste my time with firebrick. Currently, I am waiting for the right conditions to test out an alternative lining.

 

[Seasoned Oak]

Some type of ceramic as walker said would be the key. Its good enough for the space shuttle belly and for afterburn fire dome in my harman wood stove where temps are in the 2000 DEG range. Im still puzzled as to why there is no commercial parts available for this tech same a s a MH. And as i learn more about it starts to look like the same principle as a wood gassifier boiler on a budget.

 

[NW Walker]

Ted, IFB makes an awesome core, although the feed area will see wood abuse and need patching now and then. Furnace cement or low density castable refractory are good choices for the patching, and the latter makes a great core.

Fireclay/perlite/furnace cement(sodium silicate) at a ratio of around 14/14/1 makes a great core as well, I'm on year three of use as the primary heat source for my home and it's holding up great. It becomes fired and while it is soft, the non abraded areas won't break down. Same as above, I patch the feed with handfuls of the same mix a couple times a season. Mine actually doesn't have the Furnace Cement, just fireclay/perlite. Again, it's fine, but too soft to stand up to impact or movement.

Fiberfrax, by which I assume you mean ceramic fiber board is fantastic material for most of it. Again, wood abuse in the feed is the only caveat there. I use formed ceramic fiber risers and Fiberfrax Duraboard in all of my "high tech" builds, although I'm still a big fan of low cost, found clay/perlite builds for experimenting in the yard and outdoor burners.

I've not seen a whole lot of folks post info who have adequate testing equipment for temps, but I know of two who I trust, the aforementioned Peter and Sandy from Dragon Heaters. Both seem to find sustained temps of around 1800°F with occasional forays up into the 2000°F range.

If you search Youtube for "cast core" you'll find a set of videos I made showing my casting process for an 8" core and riser.

 

[NW Walker]

Oak, Dragon Heaters does sell cores and risers for J tubes. I am considering offering my radiators as an alternative to barrels, but frankly, I'd rather just tell folks to find a fab shop who can weld you up a 16g box. Materials are cheap, if you make the cuts with a jigsaw and take it to a welder who can do air tight welds (think fuel cells) you can get away with the whole thing for a couple hours labor. Shipping one of mine would be a killer.

 

[DevilsBrew]

And as i learn more about it starts to look like the same principle as a wood gassifier boiler on a budget.

I want to learn more about these wood gassifiers. I saw one recently for the first time and the interior seems to be the exact same concept as a rocket stove. Maybe you are right. Thanks for the incentive. I am off to the "Boiler Room".

 

[DevilsBrew]

Let me add to that...a gasifier hooked up to mass.