Thoughts on rocket mass heaters

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justincorhad

New Member
Jul 3, 2021
22
Canada
I know I’m really late to the game here, and I know rocket mass heaters have been discussed before, but after just getting off the rocket mass heater train not so long ago I thought I’d give my opinion on them.

I was fascinated with rocket mass heaters for a while and thought wow why isn’t everybody doing this. It wasn’t till much later that I started to realize there are definitely a few things that make these claims seem more impressive then they are. The wood consumption reports seem to always refer to old smoke dragon stoves, which any modern stove could easily halt your wood consumption or more depending on your situation when making the change.

Secondly it wasn’t for quite some time that I realized nobody was ever mentioning how big their houses were, they always seemed to leave out the part that they were living in a small house with a loft, rarely ever exceeding 1000 square feet combined living area, most I found were closer to the 600 square foot combined living area.

The other thing that got me thinking when people reported their wood usage was this, you damn near have to split your wood to kindling size to feed these things. I know there has been some debate on this subject, but feel free to chime in if you think this may ring true. If you took a cord of wood, either while logs or roughly split for an ordinary wood stove, and then subsequently split it all into kindling size pieces and stacked it again, what would you have? Obviously I have stacked a cord of kindling, but to my thinking you would probably be looking at closer to 1/2 to 2/3 cord? Kindling pieces can be stacked extremely tight, minimizing air gap to an absolute minimum. I actually posed this question to some rocket people before asking them if their “cord per year” that they use was one cord rounds which was then susbsequently split to kindling size, or if they split their wood to kindling size and then stacked it all to form one cord, I never got an answer, just a lot of dancing around the question like it didn’t matter.

But let’s say what they say is true. I had one gentleman tell me he lived in upstate New York in a 600sqft house and he burned 1 cord per year. So let’s do some math, my house is 2400sqft, so logically I would have to burn 4 times as much as he does to keep my home at the same temperature. So already I am up to 4 cords per year. I also live in central/northern Canada with much longer and harsher winters, so let’s add another 1/2 to full cord to adjust for that fact. So I’m the end I would burn 4-1/2 to 5 cords per year to heat my house with a rocket mass heater. 4-1/2 to 5 cords per year is pretty typical for anybody in my area with a similar sized home running a blaze king, so I struggle to see where all these “efficiency gains” they speak so highly of are going?

My final thought is this however, I think it may just in fact be possible that if one was living in a tiny home that you may very well be able to see some reduction in wood consumption with a rocket mass heater vs a conventional wood stove, perhaps even a modern epa stove. My reasoning is this, tiny homes do not take many btus per hour to heat, especially if they are well insulated and especially if you are living in a medium to mild climate. I think someone living in a 600sqft home would struggle to get any wood stove to burn low enough where you weren’t opening windows and sweating like a pig while using it. So the struggle may be that in a tiny home you end up using more wood then needed because you have to more or less heat the home till it’s too hot and crack windows just to keep the temperature comfortable, even with a modern epa stove. So perhaps if you are in a decently insulated tiny home you just may see a reduction in wood consumption? Just my thoughts, feel free to chime in if you think I’m completely out to lunch.
 
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justincorhad

New Member
Jul 3, 2021
22
Canada

stoveliker

Minister of Fire
Nov 17, 2019
2,002
Long Island NY
One remark (otherwise not having experience with RMH s): your 600 to 2400 sqft comparison does not hold because it is the heat leakage that matters (after initially heating up the space), not the volume. Your 2400 sqft will have less than 4 times the leakage of the 400 sqft place because the exposed surface area will not be 4 times larger.
 

SpaceBus

Minister of Fire
Nov 18, 2018
6,301
Downeast Maine
The real downside is that you have to constantly feed a rocket stove rather than batch burn like a wood stove.
 

qwee

Burning Hunk
Jan 17, 2013
134
Idaho
I know one way a rocket mass heater (or masonry heater) would use less wood than any other wood burner - if you had a small place like you suggested and you built your sofa and bed as part of the the RMH's horizontal hot gas run chamber. This way you would have direct contact with the wood burner. Say you spent 1/2 your time sitting and sleeping you would require less wood to stay warm due to direct contact.
 

justincorhad

New Member
Jul 3, 2021
22
Canada
One remark (otherwise not having experience with RMH s): your 600 to 2400 sqft comparison does not hold because it is the heat leakage that matters (after initially heating up the space), not the volume. Your 2400 sqft will have less than 4 times the leakage of the 400 sqft place because the exposed surface area will not be 4 times larger.

Ah I never even considered that, but you’re completely right. I’d be curious to see if you had yo houses built side by side, same insulation same amount of leakage, but one was double the size what the difference in wood consumption would be. Now that you say that it’s entirely possible that they may drop temperature at close to the same rate, but when it’s time to warm them back up obviously the bigger house will require more btus to bring it back up to snuff.
 

justincorhad

New Member
Jul 3, 2021
22
Canada
I know one way a rocket mass heater (or masonry heater) would use less wood than any other wood burner - if you had a small place like you suggested and you built your sofa and bed as part of the the RMH's horizontal hot gas run chamber. This way you would have direct contact with the wood burner. Say you spent 1/2 your time sitting and sleeping you would require less wood to stay warm due to direct contact.

That is definitely another huge factor I believe. I’ve yet to see anyone build an RMH down in their basement out of site and still claim to have such great efficiency. Most of these small homes that use RMHs more than likely have them right in the living room with their bedroom in a loft directly above it.

I feel like it’s the difference between having an electric heater in your bedroom when it’s freezing cold at night or having 1 small electric blanket. The heater will heat your whole room so it’s toasty, but the electric blanket will keep you warm for way less watts, but you will still say that you’re warm. Personally I’d rather not wake up to take a leak and step out into a freezing cold room.
 

justincorhad

New Member
Jul 3, 2021
22
Canada
The real downside is that you have to constantly feed a rocket stove rather than batch burn like a wood stove.

Exactly! I’ve built a few rocket stoves from steel and I can definitely say without a doubt they are impressively clean burning, but reading people try to justify their wood consumption claims by that alone just doesn’t add up. Modern stoves can burn with 80% plus efficiency, even in a perfect world if an RMH burned at 100% efficiency the most wood consumption drop you could see would be maybe 15% or so, so they should be saying they installed an RMH and burn 5/6th the wood they used to, not 1/10th.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,434
Fairbanks, Alaska
I built a few J stoves myself, never encased one in the rest of the hardware to make a RMH. My experience matches your findings.

My local wood is too twisty to maintain a steady burn. Installed, an RMH is going to be very very heavy. The conductive heat of the RMH can be very nice when you are sitting on it, and if you run a good hot burn the masonry should give off "some" heat for an unknown amount of time.

My catalytic stove can handle uglies just fine. I tend to put the real twisty's at the tops of my stacks and burn them off in the fall when I want one hot incomplete load so I have more straighter stuff in the depths of winter. My catalytic is efficient enough that I cannot reasonably expect signifcant drops in wood consumption with an ever so slightly more efficient stove.

What I would like to do someday if we get out of the big house in the 'burbs to a small house on some acreage is put an outdoor RMH on a patio next to the outdoor firepit. Having a warm bench to sit on should dramatically lengthen the time each year my wife and I can sit out doors by a fire in the evenings.

RMHs probalby work fine in very small homes. Even pellet stoves don't really get up onto their efficiency plateau until they are heating about 1500-1800 sqft last time I looked. Begreen's data is I sure much more up to date than mine for that. Under 1000 sqft, as you already noted, a cord wood stove is not a great idea. I guess you could build some little tiny thing and feed it 8" splits...
 

justincorhad

New Member
Jul 3, 2021
22
Canada
Under 1000 sqft, as you already noted, a cord wood stove is not a great idea. I guess you could build some little tiny thing and feed it 8" splits...

Yeah a guy could always go buy one of those $200 tent stoves lol

I really do think this is more than likely one of the biggest reasons these people living in tiny homes may see wood consumption drops, because even in small stoves there’s no real good way to put off a very tiny amount of heat for a long time. These houses require so little btus per hour I don’t think it would even be possible for even a tiny stove to dial back that far and stay lit.

Maybe someone should try building a micro catalytic stove stove?!?
 

SpaceBus

Minister of Fire
Nov 18, 2018
6,301
Downeast Maine
In Europe there are plenty of homes that are 1000 sqft or less, and there are also small wood burning stoves available. They obviously do not burn overnight, but small homes don't need that. Usually they are noncat and burn hot and fast. I think someone posted a thread about a Wittus bench stove not long ago. Most have the option to incorporate thermal mass, but I bet it does more to soften the output rather than extend heating times. BTUs are BTUs.
 
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justincorhad

New Member
Jul 3, 2021
22
Canada
In Europe there are plenty of homes that are 1000 sqft or less, and there are also small wood burning stoves available. They obviously do not burn overnight, but small homes don't need that. Usually they are noncat and burn hot and fast. I think someone posted a thread about a Wittus bench stove not long ago. Most have the option to incorporate thermal mass, but I bet it does more to soften the output rather than extend heating times. BTUs are BTUs.

Btus are btus yes I completely agree, however I think trying to get a cordwood stove to turn down enough to heat a small home over a longer period of time without overheating the house is where the problem lies. Btus are btus yes, but trying to do all that will end up in an inefficient burn the stove was not designed for, and therefore lost btus.

I do however believe it’s possible to heat a small home efficiently with a cordwood stove if your climate is cold enough, which is perhaps why it works better in Europe or Canada. As I mentioned earlier it seems many of the RMH crowd seem to be in mild to medium climates. Yes it may be cold to them, but at the end of the day a cordwood stove burning even at its lowest setting where the burn is still clean is going to overheat a small house in a mild/medium climate setting.
 

qwee

Burning Hunk
Jan 17, 2013
134
Idaho
I've been watching History channel's, Alone: season 8. I pay attention to the survival shelters and the fire burners that are built. Theresa built a 1/2 way underground shelter to use the earth for warming. I thought she might build a RMH or masonry heater but it looks like a fireplace with a lot of thermal mass.

If she only knew she could rearrange those rocks and burn less wood, which = less work to stay warm. If she knew more she could try and find some flat shale-type rocks to build chambers. Plus, a RMH or MH would mean less smoke in her shelter than an open fireplace. Probably, a fireplace would work fine in such a small place, but.....

https://play.history.com/shows/alone/season-8/episode-6
 

EbS-P

Minister of Fire
Jan 19, 2019
1,502
SE North Carolina
Btus are btus yes, but trying to do all that will end up in an inefficient burn the stove was not designed for, and therefore lost btus.
In my experience I can get a good hot top down fire with secondary combustion with small sticks and kindling. Won’t last more than an hour first light to coals. Low btu, fast burning wood. How much efficiency am loosing? I don’t know. I would imagine it’s 10-20% less efficient. Take the assumption that it’s as as much as 50% ( seems silly but makes easy math). Your consumption for a small house would go from 1 to 2 cords. Ehhh. I think I could make new jotul 602 or morso 2b work well down to about 4-500 sq ft of average insulation. If I had a super well insulated small house I wouldn’t heat with wood as it takes up to much space. Heatpump with resistive backup and a portable generator.

RMH are hype. The comparable tech is a small pellet stove. I want to know what happens when you have a chimney fire in a RMH….. I the home brew versions I have seen don’t have an acceptable safety margin for me to install in my house.

my 2 cents
 

justincorhad

New Member
Jul 3, 2021
22
Canada
RMH are hype. The comparable tech is a small pellet stove. I want to know what happens when you have a chimney fire in a RMH….. I the home brew versions I have seen don’t have an acceptable safety margin for me to install in my house.

my 2 cents

Definitely all hype. It’s funny, the way they seem to justify their ridiculous claims is:

1) we burn more clean and completely

2) wood stoves most your heat goes straight out the chimney

First of all even if they burnt at 90-95% combustion efficiency that would result is possibly a 10-15% drop in consumption vs a modern wood stove, not a 90% drop, and that’s still in a perfect world.

Secondly they constantly spread this false rumor that most the heat from a wood stove goes straight up the chimney. I’d love to know who came up with this one and how. Modern stoves do everything possible to retain heat in the stove for as long as possible before exhausting at a safe non creosote forming temperature. If all our heat went out the chimney then we wouldn’t be able to heat any building period.

I do know some of them claim about 150F flue temperatures vs the standard 300-350f, so are they saying 150 degree flue temp change is the difference between 10 and 1 cord per year? Can’t believe they really believe that’s a good argument.
 

Nateums

Member
Dec 11, 2017
52
Southern Tier
The wikipedia page is a great example of what is wrong with that propaganda ridden site.

According to anecdotes a rocket mass heater might reduce fuel consumption by 80–90% compared to "conventional" stoves.[3][4]
"Reliable" Sources: permaculture.co.uk and jizoku.co.uk

In contrast to a conventional wood-burning stoves and fireplaces, in a rocket mass heater, combustion is close to complete. In a rocket mass heater, by-products of combustion, such as smoke, soot, creosote compounds are sucked into the insulated tunnel of the unit where some claim they further combust, releasing even more heat energy to drive the rocket process, unlike a normal fire where they are blown out the chimney.[5]
"reliable" source: permacultureprinciples.com

It's meant to sound scientific, but obviously there is no scientific method involved here, and all statements are completely devoid of empiricism.

I mostly thought of it as a funny sideshow until I saw a youtube video of a father installing a spaghetti style chimney in his daughters tiny mud house.
 

stoveliker

Minister of Fire
Nov 17, 2019
2,002
Long Island NY
I thought the flue gases of my cat stove are not that far from what they claim...
 

Bad LP

Minister of Fire
Nov 28, 2014
1,800
Northern Maine
The best thing I never did was listen to my architect on building one for our house. Hers was a mess with ash and debris. Ok, she’s not a neat freak but it was a huge footprint and frankly I thought her house in MA was cold.
 

justincorhad

New Member
Jul 3, 2021
22
Canada
The best thing I never did was listen to my architect on building one for our house. Hers was a mess with ash and debris. Ok, she’s not a neat freak but it was a huge footprint and frankly I thought her house in MA was cold.

It’s funny I find that the RMH fan club want everybody to build one and to build it the same way they all built there’s.

“Oh you have to use cob and it has to use clay you dug up yourself, don’t try anything new just do the same thing we all do”

But after saying all that they love to talk about how they are pioneers and love trying new things lol

How big was your architects house?
 

Bad LP

Minister of Fire
Nov 28, 2014
1,800
Northern Maine
It’s funny I find that the RMH fan club want everybody to build one and to build it the same way they all built there’s.

“Oh you have to use cob and it has to use clay you dug up yourself, don’t try anything new just do the same thing we all do”

But after saying all that they love to talk about how they are pioneers and love trying new things lol

How big was your architects house?
I’d take a stab at 3000 sqft or about 500 more than our own.
 

kennyp2339

Minister of Fire
Feb 16, 2014
6,120
07462
For larger homes would multiple rocket heaters make more sense?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,187
central pa
For larger homes would multiple rocket heaters make more sense?
Possibly but that would mean sitting there feeding twigs into your heater for an hour or so for more than one.
 
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Poindexter

Minister of Fire
Jun 28, 2014
2,434
Fairbanks, Alaska
Possibly but that would mean sitting there feeding twigs into your heater for an hour or so for more than one.

Exactly. I want to build one on an outdoor patio as above, just to see and to know and to experience.

I ordered to 2 cords of (local of course) birch rounds back in March for my new Parrilla, an Argentine style BBQ cooker. My guy dropped almost three cords of rounds in my driveway.

Once I have them dry enough for the bark to fall off, birch is a pretty neutral cooking wood. Birch bark sucks donkey parts on food.

As I split those rounds I was pulling out quarter split (as opposed to quarter sawn) splits with straight grain and no knots. I hope to start making Windsor chairs in Sept 2021, I have a froe and a glut and a brake and so forth, I was just looking for rounds straight enough to give me the spindles (the wee short pieces) between the chair seat and the chair arms. Out of almost three cords dropped, I have one sled full, about 48 x 20 x 10 inches of wood straight enough to make arm spindles or RMH fuel. And a lot of what looked OK in my driveway at -20dF (about half of it) doesn't look so good for straightness in the summer time.

Given "firewood" costs me ten cents per board foot dropped in my driveway and KD white oak starts at $10.50/ bf I am not complaning about lumber prices. However, if a future reader is thinking about an RMH instead of a decent, reasonably efficient cord wood stove you really really got to look at your local trees.

The two best J stoves I built are very simple.

The first was a "bucket stove" and I was hooked. Five gallon metal bucket with a 4" chimney about four feet tall, and one section of 6" pipe down into the bucket as the fuel load. Do this one outdoors, good luck finding a five gallon metal bucket that is not galvanized. You will spend some time making the holes in the lid for the 4" chimney and the 6" fuel chamber, but it will be worth it. I had six inches of secondary burn coming out the top of the chimney on the first burn, it sounded like I had an F4 phantom in my backyard, and yes my neighbors called both police and fire. Besides doughnuts, cops like brisket just fine. No tickets.

The other best design I came up with was all six inch stove pipe. Two T sections, a 12" piece of straight pipe between the Ts and 4-5 feet of chimney on the exhaust side. Your ash falls down into the bottom of the T on the fuel side, the bottom of the T on the chimney side is just to make it easier to stand the fool thing upright. That one ran OK, but I had to split my fuel down to smaller than 2x2 from the home store. I didn't get any visible secondary out of the chimney, but I had the single wall pipe glowing pretty good and my neighbors didn't call the cops.

Somewhere under 1000 sqft an RMH might make sense. In regular sized homes a modern, up to date EPA certified stove will kick these things to the curb and handle shorts and uglies doing it.

I probably have a grand tied up fooling with ceramic bricks and P channels. The PhD in Holland who has done a lot of work on these burns mostly pallet wood. Dr. P. I am happy for him that he has found an outlet for his creativity. In some cases his designs and plans make sense. Up here, running 200 million BTUs annually, his ideas don't make sense for me as explicated.

I am still thinking about a masonry heater for my retirement home. At this level, this climate, there is a working model at the Cold Climate Housing Research office on the UAF (University of Alaska Fairbanks) campus. It looks like a regular masonry heater, but they have valves on both the intake and exhaust; so when the fire goes out they close off both the air intake and air exhaust to the chimney system and keep all the stored BTUs in the office building. That system works good, and they are plenty over 1000 sqft.

If you are building 1000 sqft or less, live somewhere south of 64 degrees north latitude and have ready access to plentiful straight grained wood, an RMH should be a consideration for your primary heat source. If your wood comes in with bow or crook or knots in in it, feding an RMH is going to be an ongoing problem, but if you are over ~1000 sqft I don't think it wil be worth the trouble.