Are RMHs even more than 50 to 90 percent more efficient?

I decided to post this in the rocket mass heaters section since it is my rocket mass heater that brought this to my attention.  I think everyone who heats with wood knows already that a "cord" of wood is a fuzzy term as far as just how many BTUs of heat it contains.  It's really more about the dry weight of the wood.  I do know this.  Still we tend to think and talk about firewood in cords, and our efficiency gains in reduction of cords we are burning.

Anyway, this is my second year heating with my RMH.  Last year I wasn't fully prepared for it.  I kinda made my heater late.  The heating season had already started and my firewood wasn't ideal.  I was splitting up the larger sized split logs I had been using with my old wood stove a bit more to make the fit in the feed tube.  However, I hadn't yet made my kindling cracker to much more easily and safely split the wood up into nice small pieces with lots of surface area, allowing them to really burn hot.  This year I did that putting away something a bit shy of 2 cords of finely split wood in advance.  Last winter I was burning one full wood hod each day to heat my place.  This was down from 2 full hods, plus a couple more logs I'd grab with the wood stove.  Not the most efficient RMH out there but not bad considering I'm limited on space and can't make the heat exchange tube run as long as I'd like.  This year when the cold days finally kicked in I started burning that single full hod of wood each day expecting it would heat the house just as well as it did last year.  However, this didn't seem to be the case.  It wasn't bad, but it just didn't seem to keep the place as warm.  It's like my RMH became less efficient?  I had fully cleaned it out.  It seemed to be burning just as well.  The temps at the top of the barrel were a bit hotter even with the more finely split wood.  What could the issue be?  I scratched my head for a bit and recalled something I seemed to notice this summer as I was splitting all that kindling.  The kindling pile seemed to grow in size quicker than the log pile that I was splitting shrunk.  So I had a theory.  When the logs were larger in size I could fit more of them by weight into a "full" hod than I could when they were split into smaller kindling pieces.

One of of these warm days here recently I went out and filled my hod with the large split pieces from my wood pile.  Then I got out my kindling cracker and split those logs up to the size I was now using in the RMH and tried to shove them all back into the hod.  Sure enough they wouldn't all fit, not by a long shot.  I probably had 1/3 to 1/2 more wood by volume it took up.  I did go and write a blog post about it on my blog site since I haven't read anyone really talking about this issue elsewhere.  I thought I'd bring it up here too as this crowd might be interested to know it.  

Last year when I was using wood that was split only a little smaller than what I used with my wood stove I was burning more wood in a full hod than I have been with that same hod full of much more finely split wood this year.  This is why my house wasn't getting as warm.  In reality this year I have been burning significantly less wood by weight.  My estimate last year of the RMH using 50% to 55% less wood than the wood stove was probably off too since I was splitting that wood smaller, though not as small.  I probably had more like a 55% to 60% reduction in wood.

This makes me wonder if the 50% to 90% improvements others report might even be skewed to the low side if they too have been measuring it by volume instead of weight while also going from larger split logs to more finely split logs and smaller sticks?

When I was splitting wood for the winter I thought I had enough put away to easily cover this year and hopefully most of next year.  Now though I suspect I really just have this year covered with a bit left over for next year, unless that is I can make my RMH more efficient yet.  I'm playing with that and think I have something that's working which I'll post about at another time after a bit more testing.

Let me see if I can add a couple images from my  blog post.


This is the hod full of large size split wood that I then used to split into smaller pieces.  I should note that most of this also had pretty straight grains and split easily into clean pieces that weren't all twisty.


This is that same wood after it was split finer, showing how much of it didn't fit back into a "full" hod.

Nice artwork, the metal work looks great!

Did you build your studio?  It looks awesome (I see you have album available which describes it).

Thanks Orin.  Yes, I did build my studio.  It was a ton of hard labor, but I fully enjoyed it!

It’s funny, I have a tendency to pick up random free things that look useful, not know before hand what I will use them for. Thanks for showing me...

🙏🏼Peter and the people of Hokkaido🙏🏼

Peter, I had that big heavy semi transmission gear for many years, not knowing quite what to do with it.  However, it did look cool and it seemed like it could be useful for something someday.  Then I heard of the kindling cracker and decided to make my own and it was the perfect size and weight to give me a solid base so I don't need to bolt it down to a stump to use it.  Yet, it is still just light enough I can pick it back up and bring it in the wood shed when I'm done splitting wood for the day so it doesn't sit out in the weather.

I can't tell the size of your gear from the photo, but it looks like it might function in a similar way!

David Huang wrote:Peter, I had that big heavy semi transmission gear for many years, not knowing quite what to do with it.  However, it did look cool and it seemed like it could be useful for something someday.  Then I heard of the kindling cracker and decided to make my own and it was the perfect size and weight to give me a solid base so I don't need to bolt it down to a stump to use it.  Yet, it is still just light enough I can pick it back up and bring it in the wood shed when I'm done splitting wood for the day so it doesn't sit out in the weather.

I can't tell the size of your gear from the photo, but it looks like it might function in a similar way!

Pretty sure it will get the job done. Using the rubber tier splitting technique right now as it’s quick and I can bang through a bunch of wood relatively easily, but I think the kindling cracker is a good option so my wife can get in on a bit of fun with not too much of a learning curve. Next build project after we get the rocket a bit more under control.

Thanks again for all the info and detailed explanation with clear and logical photo documentation on your blog. Very easy to follow. Will up date when I get mine crackin...

Cheers, Peter and crew...🏔✌️🏔

Nice that the tire technique can work for you.  My wood seems to have so many knots or twists it fights against splitting cleanly so much of the time.  When you do get around to making a kindling cracker I suspect you, and your wife, will like it.  Though there will probably be more bending over to pick up pieces of wood than you have to do with the tire technique.  Good luck with your RMH too.  I'm still working to tweak mine for better efficiency than I'll do all the decorative tile/stone work to finish it.

I've had the exact opposite experience as you. If I split a cord of wood that has been quartered into fine pieces for my rocket mass heater, I can stack the wood back far tighter then it was before and really end up with only 3/4 of a cord.....which leads me to believe rocket mass heaters are less efficient then most claim.

Don't get me wrong. I love my rocket mass heater and feel like it's the best way to heat a house by far. Just giving my observation.

Eric Hammond wrote:I've had the exact opposite experience as you. If I split a cord of wood that has been quartered into fine pieces for my rocket mass heater, I can stack the wood back far tighter then it was before and really end up with only 3/4 of a cord.....which leads me to believe rocket mass heaters are less efficient then most claim.

Don't get me wrong. I love my rocket mass heater and feel like it's the best way to heat a house by far. Just giving my observation.

Very interesting Eric.  Now I have to wonder why you get that result and I don't?  When I ran my experiment I thought as I was splitting the wood into the finer pieces that I might have set myself up to be able to pack it in smaller because most of the logs I used had nice straight grain.  I figured twisted grains would result in a greater "fluffing" effect.  Are you initial logs fairly straight in grain or are they twisty?  Speculating now I wonder if the larger logs are twisty if that results in much larger air pockets between the logs that equally twisty but smaller pieces can fill up better?  I admit I still expect the opposite would be the case, but I'm trying to figure out why you might be getting a different result.

There's a simple and intuitive way to measure this. Measure a log in circumference, then split it. Now rearrange the pieces in a "rebuilt" log and measure the circumference again. If the second measurement is smaller than the first, I'd be inclined to offer to eat the log except that I wouldn't because it's badly rotten.

Hi Eric;
Reading your post, leads me to think maybe you only had 3/4 of a cord to begin with?
4' x 4' x 8' is a measurement allowing for no air gaps.
Years ago when I was a teen, I cut, 1/4 split and delivered "cords" of dry red fir  firewood (get this, for $45 a cord delivered)
My truck was a 1953 chevy 1 ton with the long "ranch " bed.  9.5' long a step side, so it was 4' wide. Steel side racks made it 4' tall.
So 4 x 4 x well over 8' Almost everybody agreed I brought them an honest cord... Except one old homesteader.
He lived in an old cabin on several hundred acres. Was happy to pay my price but only if I brought him a full cord....
He required me to deliver my load, throw it out and then leave and come back tomorrow.
In his door yard, he had stakes driven in the ground. Yup 4 x 4 x 8
That old man (had to be in his 90's) would split down my cord and stack it.
As you might guess, it never filled his stakes completely. I always had to bring him a little more.

My test this year has been a cord of red oak that I purchased.  Everything is pretty straight. But the wood is borderline OVER seasoned. It's all dry but some pieces are almost rotten and full of ants.  I split everything with a hatchet and a 3 lb. sledge. The area around where I split is just full of woodchips! Some of it just exploded when you hit it.  I figure I lose a tremendous amount of wood in the fragments that fall off.  I've recieved a bunch of free wood this year that is hedge, cherry, hackberry and a bit of walnut. I pre split it green to help it dry and I still end up with a bunch of wasted wood all around my splitting stump.

In a regular wood stove all of this would have just burnt and turned into coals.  It's not like I cant use wood chips around the farm, but it does mean I need to cut more wood.

I'm only in my second year of my rocket mass heater.  I love it!  This year my goal is to heat entirely with wood. I have never done that before, but I really think I'm going to achieve it this year!

thomas rubino wrote:Hi Eric;
Reading your post, leads me to think maybe you only had 3/4 of a cord to begin with?
4' x 4' x 8' is a measurement allowing for no air gaps.
Years ago when I was a teen, I cut, 1/4 split and delivered "cords" of dry red fir  firewood (get this, for $45 a cord delivered)
My truck was a 1953 chevy 1 ton with the long "ranch " bed.  9.5' long a step side, so it was 4' wide. Steel side racks made it 4' tall.
So 4 x 4 x well over 8' Almost everybody agreed I brought them an honest cord... Except one old homesteader.
He lived in an old cabin on several hundred acres. Was happy to pay my price but only if I brought him a full cord....
He required me to deliver my load, throw it out and then leave and come back tomorrow.
In his door yard, he had stakes driven in the ground. Yup 4 x 4 x 8
That old man (had to be in his 90's) would split down my cord and stack it.
As you might guess, it never filled his stakes completely. I always had to bring him a little more.

I guarantee this is the case!  It goes back to the original posters point that a "cord" can vary greatly.

I guess the best measurement may be to weigh it, but I'm far too lazy for that!

If the wood is almost rotten with a lot of bits flying off when split I can see that as a way to reduce the resulting stacked volume.  I remember the first year I bought wood for the wood stove I originally had this is sort of what I got, highly seasoned oak where most of the sap wood was just crumbling away leaving big piles of wood bits.  Though even with the wood I'm using this year I got a thick pile of mostly bark mulch all around the splitting area and still my split wood stacks back bigger than what I started with by a considerable margin.

Has anyone else here ever tested this like I did?  Stack as much as you can into a wood hod or some such thing, then take that wood out and split it up much finer and try restacking back into the hod.  If you have tried this, or do try it, please post your results.  It would be interesting to see the results from a wider pool of samples.

Hi David,   I'm visiting the desert of Arizona right now but when I get back to the woods of Canada in a few more weeks, I'm gonna try this and add to the poll.

Gerry Parent wrote:Hi David,   I'm visiting the desert of Arizona right now but when I get back to the woods of Canada in a few more weeks, I'm gonna try this and add to the poll.

Excellent!  Thanks Gary.  I look forward to hearing what your results are.

David Huang wrote:Nice that the tire technique can work for you.  My wood seems to have so many knots or twists it fights against splitting cleanly so much of the time.  When you do get around to making a kindling cracker I suspect you, and your wife, will like it.  Though there will probably be more bending over to pick up pieces of wood than you have to do with the tire technique.  Good luck with your RMH too.  I'm still working to tweak mine for better efficiency than I'll do all the decorative tile/stone work to finish it.

Yeah. Tire technique is by no means perfect, but it makes smashing and chasing  after big pieces of wood a bit easier. We are lucky as most of the wood we have sourced, other that off cut lumber, is pretty straight grained and has been seasoned indoors for about 8-10 years. Splitting is not so bad.

Will keep you posted when I get around to making a cracker.

Still building and drying mass on our heater, but coming along. Having a substantial supply really helps the drying process, especially this late in the season.

Thanks again for all your great ideas.

Cheers, Peter and crew

I think the days of measuring wood "by the cord" is almost over. As was stated, it is a fuzzy measure at best. In Maine, selling wood by the cord is illegal. Out of habit I often state, "I sold 10 cords of firewood yesterday", but that is not exactly true. I know from experience that a truckload of wood is 10 cords, but I can only legally sell wood by the weight, or by the load. Since I am not about to go find a scale I can roll the truck across, I just sell by the load.

But the interesting thing about wood is, all species of wood have the same amount of BTU's. A ton of White Pine has the exact same BTU's as a ton of Beech. Of course how much SPACE that ton of wood takes up is going to be vastly different, and this is why it is illegal in Maine to sell by the cord. Now a "load" is a defined amount of space, but it is legal because the buyer is agreeing to a specific size, not a unit of measure.

Travis Johnson wrote:
But the interesting thing about wood is, all species of wood have the same amount of BTU's. A ton of White Pine has the exact same BTU's as a ton of Beech. Of course how much SPACE that ton of wood takes up is going to be vastly different, and this is why it is illegal in Maine to sell by the cord. Now a "load" is a defined amount of space, but it is legal because the buyer is agreeing to a specific size, not a unit of measure.

It is disheartening how stupid lawmakers can be.  Did nobody tell them that a specific size is a unit of measure? (shakes head sadly)  I'd think things got even fuzzier there in Maine now.  With a cord as the unit of measure there was at least a convention as to how that wood was to be stacked for the unit of volume measure.  With a "load" I would guess it could be just however one tosses the logs into the unit of volume measure.  Just plain silly.

I do find it interesting that wood, regardless of species has pretty much the same BTU's based on dry weight.  That is obviously a much harder way to sell by though.

David Huang wrote:It is disheartening how stupid lawmakers can be.  Did nobody tell them that a specific size is a unit of measure? (shakes head sadly)  I'd think things got even fuzzier there in Maine now.  With a cord as the unit of measure there was at least a convention as to how that wood was to be stacked for the unit of volume measure.  With a "load" I would guess it could be just however one tosses the logs into the unit of volume measure.  Just plain silly.

I do find it interesting that wood, regardless of species has pretty much the same BTU's based on dry weight.  That is obviously a much harder way to sell by though.

I do not think it is a fuzzier measurement.

With the cord for a measurement, the amount of air space between the wood is going to depend on if it is tree length, cut and split, or 8 foot in length. But when you sell it by the load, it is what two people define it as. So a load of tree length firewood, is a load of tree length firewood. It saves a lot of hassle. But if I sell them 10 cord of firewood, and then they buck, split and stack that load of wood, it will not be 10 cord of firewood and they will be calling the Maine Forest Service saying then got screwed. They did not, they got 10 cord of firewood, TREE LENGTH. Selling by the load sorts that out.

A cord of wood is 128 cubic feet, but that includes air space between the wood. What is an acceptable amount of air space? That is the question, and it changes as the wood gets worked up.

Travis,  I can certainly see how as a firewood seller it is far better for you to sell by the "load" than by the "cord".  It does avoid legal issues and complaints of the seller shortchanging the buyer.  As I see it that way essentially acknowledges the fuzzy nature of measuring wood quantities.  Exactly what a "load" of wood amounts too changes with each load so in terms of pricing the buyer and seller will be negotiating for each individual load coming to an agreed amount for that specific load.  Thus there can be no claims of getting short changed.  It does make it harder for a buyer to comparison shop, but it seems like it should reduce disputes overall.  Perhaps the Maine lawmakers weren't so stupid after all.  ;)  

David Huang wrote:Travis,  I can certainly see how as a firewood seller it is far better for you to sell by the "load" than by the "cord".  It does avoid legal issues and complaints of the seller shortchanging the buyer.  As I see it that way essentially acknowledges the fuzzy nature of measuring wood quantities.  Exactly what a "load" of wood amounts too changes with each load so in terms of pricing the buyer and seller will be negotiating for each individual load coming to an agreed amount for that specific load.  Thus there can be no claims of getting short changed.  It does make it harder for a buyer to comparison shop, but it seems like it should reduce disputes overall.  Perhaps the Maine lawmakers weren't so stupid after all.  ;)  

I think it eliminates involvement with the Maine Forest Service! I am pretty sure it is there way of saying, "We do not want to get involved."

Myself, I have been on both sides of the Maine Forest Service. They have come to my farm with a supena to appear before a Grand Jury, and I have had them take a logger to court who stole 72 tractor trailer loads of wood off me, which is roughly 1100 cords.

I am not sure if you are interested, but here are some rough estimates for wood:


15 cords of wood= 1 tractor trailer truck
7500 board feet of logs=1 trailer truck
10 cord of wood=1 triaxle log truck
5000 board feet of logs-1 triaxle log truck
1/3 cord of thrown in firewood=pickup
1 cord=7 decent sized hardwood trees
10 cords=70 trees
5000 bd ft of logs=100 logs
1 cord of firewood=1 ton of wood pellets
1 cord of firewood=100 gallons #2 furnace oil
2 cords of firewood=1 ton of anthracite coal
Felled, limbed, hauled to yard=7 trees per hour (1 cord) skidder
Felled, limbed hauled to yard=10 cord per day skidder
Felled, limbed hauled to yard=4 trees per hour farm tractor
Felled, limbed hauled to yard=6 cord per day farm tractor
House (conventional build)=15,000 board feet
House (conventional build)=300-350 logs
House=200 decent sized trees

David Huang wrote:I decided to post this in the rocket mass heaters section since it is my rocket mass heater that brought this to my attention.  I think everyone who heats with wood knows already that a "cord" of wood is a fuzzy term as far as just how many BTUs of heat it contains.  It's really more about the dry weight of the wood.  I do know this.  Still we tend to think and talk about firewood in cords, and our efficiency gains in reduction of cords we are burning.

Anyway, this is my second year heating with my RMH.  Last year I wasn't fully prepared for it.  I kinda made my heater late.  The heating season had already started and my firewood wasn't ideal.  I was splitting up the larger sized split logs I had been using with my old wood stove a bit more to make the fit in the feed tube.  However, I hadn't yet made my kindling cracker to much more easily and safely split the wood up into nice small pieces with lots of surface area, allowing them to really burn hot.  This year I did that putting away something a bit shy of 2 cords of finely split wood in advance.  Last winter I was burning one full wood hod each day to heat my place.  This was down from 2 full hods, plus a couple more logs I'd grab with the wood stove.  Not the most efficient RMH out there but not bad considering I'm limited on space and can't make the heat exchange tube run as long as I'd like.  This year when the cold days finally kicked in I started burning that single full hod of wood each day expecting it would heat the house just as well as it did last year.  However, this didn't seem to be the case.  It wasn't bad, but it just didn't seem to keep the place as warm.  It's like my RMH became less efficient?  I had fully cleaned it out.  It seemed to be burning just as well.  The temps at the top of the barrel were a bit hotter even with the more finely split wood.  What could the issue be?  I scratched my head for a bit and recalled something I seemed to notice this summer as I was splitting all that kindling.  The kindling pile seemed to grow in size quicker than the log pile that I was splitting shrunk.  So I had a theory.  When the logs were larger in size I could fit more of them by weight into a "full" hod than I could when they were split into smaller kindling pieces.

One of of these warm days here recently I went out and filled my hod with the large split pieces from my wood pile.  Then I got out my kindling cracker and split those logs up to the size I was now using in the RMH and tried to shove them all back into the hod.  Sure enough they wouldn't all fit, not by a long shot.  I probably had 1/3 to 1/2 more wood by volume it took up.  I did go and write a blog post about it on my blog site since I haven't read anyone really talking about this issue elsewhere.  I thought I'd bring it up here too as this crowd might be interested to know it.  

Last year when I was using wood that was split only a little smaller than what I used with my wood stove I was burning more wood in a full hod than I have been with that same hod full of much more finely split wood this year.  This is why my house wasn't getting as warm.  In reality this year I have been burning significantly less wood by weight.  My estimate last year of the RMH using 50% to 55% less wood than the wood stove was probably off too since I was splitting that wood smaller, though not as small.  I probably had more like a 55% to 60% reduction in wood.

This makes me wonder if the 50% to 90% improvements others report might even be skewed to the low side if they too have been measuring it by volume instead of weight while also going from larger split logs to more finely split logs and smaller sticks?

When I was splitting wood for the winter I thought I had enough put away to easily cover this year and hopefully most of next year.  Now though I suspect I really just have this year covered with a bit left over for next year, unless that is I can make my RMH more efficient yet.  I'm playing with that and think I have something that's working which I'll post about at another time after a bit more testing.

Let me see if I can add a couple images from my  blog post.


This is the hod full of large size split wood that I then used to split into smaller pieces.  I should note that most of this also had pretty straight grains and split easily into clean pieces that weren't all twisty.


This is that same wood after it was split finer, showing how much of it didn't fit back into a "full" hod.

Hi David,
could you do me a favour and tell me what kind of wood stove did you use before the RMH and roughly how old was it? The reason I ask is not to cause trouble just the consumption numbers given by RMH enthusiasts for conventional wood stoves alway seem incredibly high to me based on my own consumption making me wonder what kind of stoves they were. By the way that is an awesome amount of kindling you have stacked there!
Cheers,  David

David Baillie wrote:
Hi David,
could you do me a favour and tell me what kind of wood stove did you use before the RMH and roughly how old was it? The reason I ask is not to cause trouble just the consumption numbers given by RMH enthusiasts for conventional wood stoves alway seem incredibly high to me based on my own consumption making me wonder what kind of stoves they were. By the way that is an awesome amount of kindling you have stacked there!
Cheers,  David

Hello David.  The woodstove I had before was a Hearthstone Tribute model.  This is a small stove rated to heat a space twice as large as what I was heating.  It was probably better than the average woodstove in that it is a model with soapstone that certainly did function as a bit of mass to retain heat longer.  I bought it new.  I don't fully remember how long I was using it.  I'll guess roughly 7 years, maybe a bit more.  During normal winter days I would burn two full hods of wood plus a couple extra logs.  Basically I would shove as much wood into the hod as I could, pick it up with one hand to carry inside and then grab one more decent sized log with my free hand.  In order to stay warm I pretty much had to be burning all the time.  At night I would fill the firebox as much as possible just before going to bed.  If I happened to get up at night for a bathroom run I'd see if it was still burning well enough for me to add more, but otherwise it would go out sometime during the night and generally the house would be cold in the morning.  On really cold days when temps got down to the negative degrees Fahrenheit it was sometimes unable to keep the house warm.  I roughly figured I was using 2 to 3 cords of wood in a winter.  Now I'm certainly down to half that, likely less.  Also with the old wood stove I would be using my propane furnace as back up heat having it set to kick on if the temp dropped below 58.  It would turn on many nights after the fire burned out.  With the RMH I'm using 80% to 90% less propane because it basically never turns on at night.

Gerry Parent wrote:Hi David,   I'm visiting the desert of Arizona right now but when I get back to the woods of Canada in a few more weeks, I'm gonna try this and add to the poll.

David Huang wrote:Excellent!  Thanks Gerry.  I look forward to hearing what your results are.

Back in the woods again and have done the experiment. Loaded a bucket with my regular size wood, then chopped it up and put it back into the bucket. There was one piece of wood that would not fit and the chips that fell off are shown too. I didn't get the more drastic result that you got David but do agree with your point. As Travis pointed out: "A cord of wood is 128 cubic feet, but that includes air space between the wood. What is an acceptable amount of air space? That is the question, and it changes as the wood gets worked up." I would have to agree.
If I ever get old (er) and need to purchase a cord of wood, I will certainly make sure its split as large as possible. But now I wonder if this same theory applies to wood that has not been split at all vs wood that has been split into large chunks? Might have to go do the experiment again to find out?

Excellent Gerry.  Thanks for running the experiment and sharing your results.  Interesting that you got much less expansion than I did.  I should probably run the experiment again myself sometime to see how the results change.

David, what you're proving is one of the basic principles of materials science. It's called bulking factor, and it can be summed up by saying that the more you divide a solid, the more volume it will occupy. Let's say you have a rock that is a perfect cube, 1m on a side. That is one cubic meter of volume. Now, if you break that rock into pieces, no matter how cleverly you arrange them you will not get all the fragments to pack together into a perfect cube 1m on a side. There will be gaps. Same goes for a single log. Split it into halves, quarters, or kindling...you won't be able to recreate the exact volume of the original log (clamps would be cheating, of course). This is also why gluing cracked pieces of china never gives perfect results...there is always an extra gap across the fracture where the pieces fit together, and if you've ever put a cup handle back on (especially one that broke in a few places), you'll know what I mean.

Firewood is a little tricky, because "round" logs can only be stacked so tightly if they have bumps, stubs and bends. So, it is often possible to split logs and then stack the pieces in a smaller volume. But if you split those down even further, you'll start observing bulking factor effects such as your example above.

Phil Stevens wrote:David, what you're proving is one of the basic principles of materials science. It's called bulking factor, and it can be summed up by saying that the more you divide a solid, the more volume it will occupy. Let's say you have a rock that is a perfect cube, 1m on a side. That is one cubic meter of volume. Now, if you break that rock into pieces, no matter how cleverly you arrange them you will not get all the fragments to pack together into a perfect cube 1m on a side. There will be gaps. Same goes for a single log. Split it into halves, quarters, or kindling...you won't be able to recreate the exact volume of the original log (clamps would be cheating, of course). This is also why gluing cracked pieces of china never gives perfect results...there is always an extra gap across the fracture where the pieces fit together, and if you've ever put a cup handle back on (especially one that broke in a few places), you'll know what I mean.

Firewood is a little tricky, because "round" logs can only be stacked so tightly if they have bumps, stubs and bends. So, it is often possible to split logs and then stack the pieces in a smaller volume. But if you split those down even further, you'll start observing bulking factor effects such as your example above.

Old engines used to have connecting rods that were machined flat, and a matching rod cap that was machined also.  We could get a pretty good tolerance on the rod bearings

Now the latest technology for connecting rods is fractured rods where the rod is machined as one piece and then placed in a giant machine that breaks the end of the rod off.  The result is that you have a cap that fits the rod exactly, you cant even see the seam in which the two halves fit together. The tolerance of the bearings is super tight and a better fit then any machining process could ever dream of achieving.

I'm from the show me state, so I'm not buying into the bulking factor lol

Now the latest technology for connecting rods is fractured rods where the rod is machined as one piece and then placed in a giant machine that breaks the end of the rod off.

But if you take a really precise measurement of the length of the rod before fracturing and after the parts are rejoined, they will be different and the refashioned rod will be longer. Yes, you get a better joint because the mating surfaces are perfectly matched, but the roughness of the break is what creates the gap. Even if you cleave a crystal along a plane and then put the faces together in a vacuum, there will be space larger than the molecular bonds that used to be there.

As far as the showing goes, you can look up bulking factors for all sorts of solid materials and laugh all you want, or you can do some measurements and try it out yourself.

Phil, thanks for sharing that bit of material science.  It completely makes sense to me and my experiences.  With Gerry's test using the 5 gallon bucket in my head easily visualized a clear example.  If I had a log that perfectly fit in the bucket before it was split, then I split it up into small kindling pieces I would certainly not be able to stack it all back into the bucket.  At least not without clamps and a ton of careful work piecing everything back together.  When it comes to practical firewood stacking for real life no one is going to do that.  We just split it and pile it.  However, as you note, because of the irregular nature of firewood to begin with it is quite possible, maybe even common that larger logs prior to splitting stack in weird ways leaving larger air gaps that can be reduced when restacking after being split due to the new variety of interlocking shapes.  I suspect this is the point I started at in my experiment.  The large split logs did stack together quite nicely.  Then when I made them much smaller the bulking factor really came into play!

Phil Stevens wrote:

Now the latest technology for connecting rods is fractured rods where the rod is machined as one piece and then placed in a giant machine that breaks the end of the rod off.

But if you take a really precise measurement of the length of the rod before fracturing and after the parts are rejoined, they will be different and the refashioned rod will be longer. Yes, you get a better joint because the mating surfaces are perfectly matched, but the roughness of the break is what creates the gap. Even if you cleave a crystal along a plane and then put the faces together in a vacuum, there will be space larger than the molecular bonds that used to be there.

As far as the showing goes, you can look up bulking factors for all sorts of solid materials and laugh all you want, or you can do some measurements and try it out yourself.

It was a mild attempt at humor. Not to be taken literally.

Phil Stevens wrote:David, what you're proving is one of the basic principles of materials science. It's called bulking factor, and it can be summed up by saying that the more you divide a solid, the more volume it will occupy. Let's say you have a rock that is a perfect cube, 1m on a side. That is one cubic meter of volume. Now, if you break that rock into pieces, no matter how cleverly you arrange them you will not get all the fragments to pack together into a perfect cube 1m on a side. There will be gaps. Same goes for a single log. Split it into halves, quarters, or kindling...you won't be able to recreate the exact volume of the original log (clamps would be cheating, of course). This is also why gluing cracked pieces of china never gives perfect results...there is always an extra gap across the fracture where the pieces fit together, and if you've ever put a cup handle back on (especially one that broke in a few places), you'll know what I mean.

Firewood is a little tricky, because "round" logs can only be stacked so tightly if they have bumps, stubs and bends. So, it is often possible to split logs and then stack the pieces in a smaller volume. But if you split those down even further, you'll start observing bulking factor effects such as your example above.

I will not argue with you at all, it is just in the mining industry we call it by a different name: "bucket swell", and it is a huge part of the equation.

If a contractor has to make a cut through a hillside, they factor in how many cubic yards it would be and then add the bucket swell factor into whatever type of earth it is. The bucket swell factor changes with types of rock, ore and soil types.

Here is an example: I have a gravel pit that measures out to be roughly 1/2 a million cubic yards, but gravel has a bucket swell, a expansion factor, or bulking factor of 25%. That means while as it sits it is 500,000 cubic yards, if I was to dig it out it would be 625,000 cubic yards. At $2 per cubic yard for gravel, that bucket swell factor is incredibly important!

The interesting thing with bucket swell is, the harder the soil or rock, the higher the bucket swell factor. For instance slate...it has a bucket swell factor of 90%. That is because it is so darn dense, break it apart and it really expands. Again this is huge. Let's say I have medium grade ore of 3.5 grams to the ton of host rock. That means I have to move 8.8 tons of ore to get just one troy ounce. But that is by weight. 8.8 tons, at 3000 pounds per cubic yard would mean moving 5.8 cubic yards of material. But with bucket swell, it actually means moving 11 cubic yards. That is an awful lot more work for the same result.

People may be saying, so what>

In permaculture bucket swell should be factored in when making swales, and other earth works like WOFATI construction and the like. It really is a huge factor because it means moving a lot more material then people first realize.

thomas rubino wrote:Hi Eric;
Reading your post, leads me to think maybe you only had 3/4 of a cord to begin with?
4' x 4' x 8' is a measurement allowing for no air gaps.
Years ago when I was a teen, I cut, 1/4 split and delivered "cords" of dry red fir  firewood (get this, for $45 a cord delivered)
My truck was a 1953 chevy 1 ton with the long "ranch " bed.  9.5' long a step side, so it was 4' wide. Steel side racks made it 4' tall.
So 4 x 4 x well over 8' Almost everybody agreed I brought them an honest cord... Except one old homesteader.
He lived in an old cabin on several hundred acres. Was happy to pay my price but only if I brought him a full cord....
He required me to deliver my load, throw it out and then leave and come back tomorrow.
In his door yard, he had stakes driven in the ground. Yup 4 x 4 x 8
That old man (had to be in his 90's) would split down my cord and stack it.
As you might guess, it never filled his stakes completely. I always had to bring him a little more.

I've sold a fair bit of firewood from trees I had to take out on my property.  That old fella is someone I'd not sell to again.  I don't cheat the people I sell to.  I tell them what the price is for the wood they want, and how I'm measuring it, and they pay it, or move on to another seller.  If someone wanted to play that kind of game with me I'd just ignore them in the future and move on to the next buyer.  I'd probably bring him the extra wood ONCE, just so he couldn't lodge any official complaint, even if that complaint lacked any merit.  Then he'd be on my list of buyers to not respond to ever again.  

Right now I've got firewood rounds for sale.  In the ad I say the lengths are random as I was just eyeballing it when bucking the logs.  And that the fraction or multiple of cords will be based on measuring whatever they're using to haul the wood (as they have to pick it up, I don't deliver).  That way there is no chance to argue, or get mad over what they're getting.  Everyone is happy and nobody feels taken advantage of.

David Huang wrote:

David Baillie wrote:
Hi David,
could you do me a favour and tell me what kind of wood stove did you use before the RMH and roughly how old was it? The reason I ask is not to cause trouble just the consumption numbers given by RMH enthusiasts for conventional wood stoves alway seem incredibly high to me based on my own consumption making me wonder what kind of stoves they were. By the way that is an awesome amount of kindling you have stacked there!
Cheers,  David

Hello David.  The woodstove I had before was a Hearthstone Tribute model.  This is a small stove rated to heat a space twice as large as what I was heating.  It was probably better than the average woodstove in that it is a model with soapstone that certainly did function as a bit of mass to retain heat longer.  I bought it new.  I don't fully remember how long I was using it.  I'll guess roughly 7 years, maybe a bit more.  During normal winter days I would burn two full hods of wood plus a couple extra logs.  Basically I would shove as much wood into the hod as I could, pick it up with one hand to carry inside and then grab one more decent sized log with my free hand.  In order to stay warm I pretty much had to be burning all the time.  At night I would fill the firebox as much as possible just before going to bed.  If I happened to get up at night for a bathroom run I'd see if it was still burning well enough for me to add more, but otherwise it would go out sometime during the night and generally the house would be cold in the morning.  On really cold days when temps got down to the negative degrees Fahrenheit it was sometimes unable to keep the house warm.  I roughly figured I was using 2 to 3 cords of wood in a winter.  Now I'm certainly down to half that, likely less.  Also with the old wood stove I would be using my propane furnace as back up heat having it set to kick on if the temp dropped below 58.  It would turn on many nights after the fire burned out.  With the RMH I'm using 80% to 90% less propane because it basically never turns on at night.

I've been skeptical of the fantastical claims made by some RMH advocates, but this is an impressive comparison.  The Hearthstone Tribute is an efficient modern secondary burn stove - assuming you were burning dry wood and operating with a proper installation, reducing your fuel consumption by 50% and propane usage by 80% is remarkable.

The only thing I can attribute it to is the lower exhaust temperature of the RMH.  Have you ever measured the exhaust temperature of your heater?  I've read claims of 150F which would be great.  There are a lot of wasted BTUs in the exhaust plume of even a modern clean-burning woodstove

I heat my home with a Blaze King, which lists 75-80% efficiency.  I asked the vendor about exhaust temperatures and he said roughly 250-600F to prevent creosote formation.  He estimates the stove consumes 60 CFM at an average burn rate.

So 60 CFM * 0.07967 lbs/ft^3 = 4.78 lbs/min of hot air lost to the atmosphere

Specific heat of air is 0.24 BTU/lb F

Let's say we would ideally drop the exhaust gas from 450F to 150F

If we could scavenge that heat, we would have an additional
4.78 lbs/min * 0.24 BTU/lb * (450-150F)  = 344 BTU/min, or 20,650 BTU/hr to heat our house instead of the world.

For reference, the Blaze King brochure lists the burn output from 12,000 to 36,000 BTU/hr.  So the waste heat is basically equal to the usable heat.  But in a woodstove installation with a tall chimeny we can't scavenge this heat safely because it would cause creosote formation.

If the RMH designs can indeed safely scavenge this waste heat from the exhaust heat, it's conceivable that they can achieve the amazing fuel reductions stated.

Not an RMH burner.  This is just what I've gathered from my readings on it.  Someone with better, and first hand, knowledge please correct me if I'm wrong.  

RMH's get that low exhaust temp in large part by intentionally burning the creosote every time it's used.  So you avoid the risk of an accidental chimney fire by having an intentional one when you use the RMH.  This is done safely and in the part of the RMH that still is adding heat value to the home.  So even though exhaust gas temperatures are well below what is risking substantial creosote deposits in a "normal" wood stove this is avoided in an RMH by having already burned the exhaust content that otherwise would condense into creosote.

yes that's what I gather from my reading too

it just now clicked that this is the key to the efficiency confusion

when I ran the calculations on how much usable heat is wasted up the chimney, even on an efficient catalytic woodstove.  

The wood stove sacrifices heat to avoid creosote buildup and the hotter burn of the RMH makes that compromise unnecessary.

Now, if only the RMH builders can come up with a code-compliant design that doesn't include a rusty barrel in my living room, or re-enforcing my floor/foundation!

Davis Tyler wrote:

The only thing I can attribute it to is the lower exhaust temperature of the RMH.  Have you ever measured the exhaust temperature of your heater?  I've read claims of 150F which would be great.  There are a lot of wasted BTUs in the exhaust plume of even a modern clean-burning woodstove

I will note that I had my Hearthstone wood stove professionally installed so I have to assume it was done properly.  I never saw any indication it wasn't.  

I never measured the temps from the wood stove, but I did get one of those temperature guns to measure and get a better sense of how my RMH does operate.  Last year, the first year I had the RMH, I was regularly reading 190 to 240 degrees F heading up the chimney.  From what I read this is high for a RMH, and hence why I say that mine is an inefficient version compared to what they can be.  I just don't have the physical space to get enough heat exchange mass allowing me to pull out all the heat I conceivably could.  As I began burning again this year I was getting the same chimney temperatures.  However, in the past couple weeks I've been trying a tweak with my RMH to slow down the flow of air, weakening the draft.  I plan to write another blog post about this after I've got a bit more data and experience with it.  For the purposes of this discussion though I can tell you I have now rarely measured the exhaust temp at over 200 degrees F.  The vast majority of the time I'm getting readings between 160 and 195, with the higher temperatures only happening at the tail end of my days burn.  I did a longer burn than normal today and saw it hit about 208, the highest I've seen since making my changes.  This would have been on the low end of normal before.   Of course all of these temps are significantly below the normal exhaust temps of a standard wood stove.

As far as code compliance and rusty barrels go, I understand in more and more places these do meet code, or codes are being updated to allow for them.  From my personal experience a RMH is way safer to operate than a wood stove, so I would expect that codes will reflect this eventually.  It's not the RMH design that needs to change in this case.  It's the codes that need to be altered to take in the new technology.  For the barrel part, first I wouldn't use a rusty barrel.  I'd get one without rust.  Beyond that, you don't need to use a barrel.  You could manufacture something else, or have someone else make something different.  However,  in terms of even radiant heat dispersion, from what I understand the round barrel shape is basically what you want.  So you can pay a ton of money for a custom built metal cylinder shape or you can spend $10 for a used barrel.  If you don't want to reinforce your floor, well then just don't make it with all the weight.  In that case I guess it would be a rocket heater instead of a rocket mass heater.  I have to imagine you could still design it give off most of the heat before exhausting out the chimney, likely through more radiant metal sections in place of the mass bench.  However, without the mass you would lose a significant part of their home heating efficiency.  The efficiency in home heating is not just that more of the firewood's potential heat is released into the house, it's also that the mass helps to hold the heat inside the home for a longer period of time.  This is what the soapstone on my Hearthstone woodstove did to some degree as well, but it was only about 300 lbs of mass.

Hey guys;   The design your "hoping" for is a brick bell (stratification chamber)
Much lighter than a traditional mass. Fits in odd shape spaces. No barrel at all, unless you want one for the quicker radiant heat.
Commonly build with a batchbox design rather than a traditional J tube RMH. Although they can be built with either core.
.

Davis and Andrew,   Agree on your findings from my own experience of burning my shop RMH.
Not sure about the code-compliant design, as its different for each area, but chapter 6 of the RMH Builders guide covers "Rules and codes" which may be of help. Basically, you want to use as much of the already established codes for wood burning heaters so that it can then be classified as something that is known to be safe and compliant to what works.
Eliminating the 'ugly' barrel is a topic that has had much discussion and has many options to be replaced with. A masonry bell for example can be made to look quite elegant and can be made much lighter than a traditional cob bench which translates to probably not needing to beef up your floor to accommodate the extra weight.
So there's my RMH sales pitch for you!  👍

When I typically run my rmh, the top of the barrel is between 400 and 600 degrees.  Exhaust pipe leading out of the house is about 125 degrees at chest level and 100 degrees right as it leaves the house 12 ft up.

If I use a lot of small stuff and burn a hot fire, the barrel temp is 800 and the exhaust is about 160 at chest level and about 130 leaving the building 12 ft up