Rick Frey

Not sure what I'm missing but I feel like I saw this basic idea here a bunch of times. My version of the solution was to put a 3' section of 8" pipe over my feed tube, cut a slot at the bottom of it for air and put a lid on it once it's loaded with wood. There's no chimney effect because nothing can get out the top and the air flow direction is through the heater. Yes, the tube gets warm, but it's not hot enough to pyrolize the wood and pre-heating the wood helps combustion. I use bricks to block varying amounts of the opening on the bottom of the feed tube extension. I burn close to 4' lengths of 2x4 and very straight sticks this way all the time. It's been too hot lately where I am to use my heater, but this is the rough setup I use (that is fire insulation under the piece of wood on top of the feed tube extension

I've actually been wondering about not having the air gap on the bottom of the pipe, but actually having it on the top of the tube. Maybe this could only work for me since I use a fan to create the draw, but this would pull the air from the top, letting it warm as it goes down, it would pull any smoke and wood gas down into the j tube and it would avoid the possibility of a small chimney effect smoking out the air gap on the bottom of the feed tube extension.

The version of this that I'm still trying to figure out is how to fill that same 3' section of 8" or 10" pipe with wood chips and let those feed into the burn chamber. I saw one version on youtube where the guy did a ton of welding and built this impressive set up that let it slow feed onto a grate at an angle, but I don't have the space he did and the expertise to build what he did. Every version I can think would have the bottom of the metal pipe in the actual burn area and the cost of the tubing it would take to last in those conditions is more than I want to spend. I can picture some kind of auger feed, but the wood chips I have easy access to aren't that regular. So for now, it's either feed my heater pretty regularly or be happy when I get scrap 2x4's, old pallets or long straight sticks.

Sounds like fun, you're a more rigorous experimentor than I am (even though I do research professionally . I can't remember who said it, but someone was talking about too much air not necessarily being a good thing. Imagine the two extremes. On one end, you have no fan, the heater doesn't get enough draw at all, the air speed is basically 0, and the fire burns inside the feed tube and smokes straight up. On the other end, you put a 10" cannister fan there, 1200 cfm, you blow out your fire (ok, not that bad) but you add so much air to the fire, that instead of supercharging it, you're introducing more room temperature air than your system can heat and your fire burns horribly. Between those extremes is an ideal amount of air that lets any load of wood burn at its best temperature and it sure would be interesting if struggling rmh's could be fixed or boosted by adding a simple fan (kind of like overclocking with a microprocessor or using nitro on a race car).

I know a lot of folks have had significant success by altering the air paths and intake dimensions around the feed tube. It would be interesting to see how similar of effects could be obtained by: a) a draw fan at the end of the system pulling air through and out of the system at variable speeds (that's what I have), or b) an input fan, that blows air into the feed tube. Heck, maybe like a turbocharger with a car, you could preheat the air you blow in by wrapping the air tube around the barrel. It makes a big difference in a turbocharger (or is it supercharger?) to use preheated air for combustion, and I know in wood gassification, they're really good at making sure everything that's going to burn is at the optimal temperature for maximum efficiency. If you could get a temperature sensor inside the burn tunnel that could communicate with a controller that would alter the speed of a fan to adjust the flow of air to an optimal rate, that would be pretty cool.

The strategy of most folks here has been on designing the system in the optimal configuration so that it heats in a way and draws in a way to achieve maximum temperature on its own. My design is non-standard in a couple of respects, but because I knew I was planning on using a fan, I wasn't worried about my deviations from proscribed practice (mainly the length of my ducting - 35' with 4 u-turns, the absence of a chimney and a small amount of space between the outside of the riser and the barrel). I haven't tried to melt copper yet but I've hit 950 on the top of my barrel and I have a 5" gap. So either the need to meet very precise, specific constraints in the design isn't that critical, or, a fan can work some pretty good magic

I have a 6" system in my greenhouse and I use a fan to pull the air through. Mine is a 40 watt, 200 cfm fan on a slider, so I can adjust the speed up and down as I need. Once my system is up and running, I often turn the fan off and if I restrict the air intake on the feed tube, the system still pushes through, even without a chimney (I have no chimney, the final exhaust exit is lower than the heater and the exhaust channel).

I live in the city, so I haven't seen a power outage in 10+ years of living here, and I am doing this in a greenhouse. For an inhouse application, I get everyone's concerns and they are totally valid. Two thoughts about the potential dangers. First, I don't think many people run their heaters while they're not there. Mine is out in a greenhouse, so I leave the room and can't see it, but for most folks, if it's in a living room or family room and you're hanging around there, checking the fire, etc. it wouldn't be that big of a deal if the fan died, the system backed up some and some smoke got in the house. If you had enough wood in it to burn for an hour or so and you have left the house with it running, or you put a load of wood in before going to sleep, that would be a bummer, and probably make the idea of depending on a fan much riskier and not worth it.

But here's the second question I don't know/get. I hear people talking about winds shifting and weather patterns affecting their draft. So what if all that happened and your draft backs up, it starts to burn the wrong direction, the chimney cools, it doesn't restart back the good direction and you have the exact same problem. Now it's a question of how often does that happen for folks with rmh's in different types of settings vs how often fans fail when you're not there to catch it.

My dad used to make parts for the aerospace industry, back in the days of the $20 diode that went up in a satellite until they figured out redundancy and that making one incredibly high quality part that should never fail costs way more than having 2-3 good parts that rarely fail and will almost never fail at the same time. So what about a fan at the exhaust and a small fan at the entrance to the feed tube? Now you've pretty much taken fan failure out of the equation as a possibility for failure and you're left with power outage. In some places, that's a real concern, but in Oakland, it hasn't happened in the last 14 years. Plus, if you were really worried about it, that tiny draw on an uninterruptable power supply would leave you power for hours, more than enough time to finish a burn.

One other piece I'm curious about is the ability of a fan to adjust burn conditions, potentially to more optimal burn conditions depending on the material. I don't know if wood that is too wet or pretty oily would like different conditions or if messing with the air flow could ever improve the burn. I wonder if any of the researchers playing with these have tried this? Some folks mess with restricting air into the feed tube and others work with the path the air takes coming into the feed tube, all of this could be controlled externally with a fan. I wonder if there's a theory/actuality that the fire knows what it needs and it always pulls the right amount of air or, if one could use a fan to increase the air flow in a way that would improve the quality of the burn?

Robert - Great looking pictures and sharp looking design. I have a similar cast core on a 6" system and have experienced some of the same issues. I'm wondering about how much of the issue really can be attributed to the wood. The different heat I get from different types of wood is amazing. I get some wood that burns so hot so fast, it's a bit scary. I've hit 950 on the top of my barrel with a 5" gap. Other nights I burn and I never get over 450 degrees in five hours of burning. I have yet to be super rigorous in testing the exact wood that produces different results, but it's been clear to me that the exact same heater in basically identical conditions produces radically different results based on the wood I use. Since you've had some great results with your heater (hitting 850 on the barrel top is a pretty good sign of serious heat), and if nothing's changed about your design/set up, it seems like the wood is a great place to do some experimentation.

I'd be curious to hear from other folks their experience with different wood causing a major difference in heat/fire characteristics or if there's something about both our designs that only operates efficiently under ideal wood conditions?

As for the metal duct in your riser still being intact, it's not quite true that metal dissolves instantly in fire. I've got a heavy gauge steel pipe as my riser (with 3" of clay/perlite around it for if/when it fails). Part of why you might not be seeing damage yet is by pouring your core, the first 6" or so of the rising happens in the core and that's where some of the hottest temps are. If your duct is going to fail, you'll see it at the bottom of the riser first. After two months of constant burning, my steel tube had some clear splotching on it, signs of having gotten crazy hot then cooling back down again. If you're not seeing signs of that at all, it probably is a sign that your heater isn't getting as hot as it should be.

In terms of covering the core in cob, cob is thermal mass, not insulation, so it would actually pull heat away from the core rather than keep it in. If you have 4" of material around your burn tube and riser, that's even more than I did, so your core should be barely warm to the touch after 5-6 hours of burning. Poured cores are much better at directing the majority of the heat back into the fire, and again, from your experience of a bunch of successful burns, you've seen it do that consistently.

Two quick things you can do to check internal temps. Drop in an aluminum can and see if it melts/vaporizes pretty quickly. Melting temp for aluminum is 1200 degrees Fahrenheit, so that will give you a sense of getting at least that hot. The next one is dropping in a bit of copper pipe. Melting temp for copper is just under 2000 degrees, so that would be a test to see if you're ever hitting those temps. And, if it's not too hard to do, checking the bottom of your riser for any decay/spalling I think it's called would be one other check.

I'm intrigued by the idea, ever since I saw a Cowboy Hot Tub (Cowboy Hot Tub) and built a simple galvanized pipe coil that I would stick in a fire to heat water for camping or use to heat a 150 gallon rubber maid tub full of water.

The problem with all of these ideas is that it takes a lot of energy to heat 50 gallons of water (or 100 gllons of water) along with a tub and any cob or other connected materials hot enough to enjoy as a jacuzzi. To heat 100 gallons of water from 60 to 100 degrees requires around 40k btu's. If you're including heating the tub and a bunch of cob as well, that would be a much larger amount of heat required. Unless you're planning on running your heater all day or you've got some ultra efficient, concentrating design, it's a rough challenge.

I've been playing with and building a couple variants of this idea, a roman bath rmh to heat water in an aquaponics system and using a rmh to heat water for a jacuzzi. The aquaponics rmh works great, heats the greenhouse and gives a bunch of heat to my aquaponics system. I have a plastic liner 1' high sitting on a sheet of permaboard covering a series of brick channels that the exhaust from my rmh runs through. The barrel heat heats the greenhouse directly, and the exhaust heat heats the fish water. It's hard to calculate the heat transfer in my system, I have 600+ gallons of water that is flowing across a table, mixing with other water and coming into contact with other objects. But the exhaust is almost never above 80 degrees exiting the chimney, so it's possible to use a design like this to capture quite a bit of heat. If the barrel were insulated or the exhaust somehow more directly channeled into the ducting system, an even greater percentage of the heat could go into heating the water.

Having played with a couple of ideas for heating the water in a jacuzzi directly, I built and tried a simplified rocket mass heater design, using a radiator sitting on top of an open barrel with the jacuzzi water running through the radiator. As long as the water is flowing through the radiator quickly enough, pretty much every bit of heat generated by the rmh seems to be transferred to the water. Again, the problem I found was that it takes a ton of energy to heat 100 gallons of water from 60 to 100 degrees and even with this direct system, it still took almost 4 hours of burning to get the water hot enough. I've got a thread looking at ways to improve or redesign the rmh -> radiator model, maybe a batch box with 8" riser (mine was standard J tube with a 6" riser) could burn more wood and generate more heat/hr and speed things up a bunch, but while I love the idea, the simple realities of the amount of btu's involved, the energy content of wood, the rate at which wood can be burned in a controlled environment and the degree to which that heat can be transferred to water show why a nice, natural gas 150k btu heater is an impressive invention

My current working on the problem is based on changing a few of the variables. Instead of heating the water from ambient temperature, I'm using a passive solar heater during the day to heat the water to roughly 80 degrees and then covering the hot tub (I use a 150 gallon rubber maid tub) to keep as much heat as possible. That preheating alone cuts the but's required in half. If I can do a bit more work on the insulation, after I heat it up at night, I cover it again, and then when I start solar heating during the day, it'll already be 80+ degrees, so little by little, I'm hoping to be able to keep the water around 90 degrees and then only need a 30-40 minute rmh fire to get it up to a nice, toasty 104 degrees.

Folks say that steel risers can't handle the heat, but that comes with a few technical notes. Something thin, like sheet metal duct, would die after 1-2 burns. I have 1/4" steel for my tube, it's been burned daily for two months, and aside from a little blistering on the bottom 8", it's fine. One of the key factors is that the heat drops off significantly the higher up the riser you go. With a cast core, the first 6" of the heat riser are the cast core material, which protects the steel tube riser. Another trick I'm working with now is to use a metal interior liner for the riser, but mainly as a shape holder. My system is 6", so I have a 6" steel tube with 3" of perlite/fire clay insulation packed around it, and a 12" duct holding that together. If at some point the steel tube in the center of the riser fails, the clay/perlite will have hardened and the riser will be fine. The 12" duct on the outside will be fine forever, temps inside the barrel after the heat riser don't get high enough to hurt metal (except maybe right above the riser, depending on how small the gap is between the top of the riser and the top of the barrel).

Once you have a core liner, some insulation, a riser and something to redirect the exhaust (typically a barrel), you're free to decorate/make it pretty how you please. My poured core is still in the wood box I poured it in, even after 6 hours of burning, I haven't seen a scorch mark on any of the wood. If you had a refractory cement core with much better insulation around it (my entire core mix is Matt Walker's 1:1, you could use 8:1 or something like that) I'm betting it would get warm to the touch at most. And while Glenn brought up the concern about covering the barrel in cob or the like potentially slowing down the heat pumping action, if you covered the barrel with 8:1 perlite/clay that insulated the barrel and kept the heat inside, you could decorate however you want and not worry about passing to much heat to any mass surrounding the barrel.

I've been thinking about similar things and wondering about materials for Core 3.0/4.0/whatever version we're up to. I wrote about the possibility of a hybrid core, making the inner liner out of split fire bricks for strength and durability and packing in 8:1 perlite/clay insulation around them to hold them in place and super insulate the center of the core. If this Kast-o-lite stuff is as impressive as they claim, an inner core liner made of refractory cement wrapped in high insulation perlite clay could be an amazing combo. A buddy of mine tried something like this, he poured a core but "painted" a bunch of refractory cement on the wood inner form then packed something like 15:1 perlite/clay around it. He said his crumbled and broke, which makes sense, it was way too thin.

I used dimensions from Christine Baker's thread back a bit ago on a 6" core to estimate volume of material needed (5.3" wide, 24" long, and I made this shorter, 7.3" tall), for a 1" liner, it would be 1 bag of cement, for a 2" liner, it would be around 3 bags. Since it's super hard and it will be surrounded in insulation, maybe the 1" liner would be enough? Use the standard strategy for pouring a core, make an outer box and an inner form, but I'd make the feed tube and heat riser tubes tall (6" above the top of the burn tunnel). When you pour the core, you could still only put 1" of refractory cement around the inner form, let it dry, then take off the outer box, build a second outer box for the final size of the core + insulation, pack in the insulating mix (8:1 perlite/clay), set the dry core liner into the box and pack insulating perlite/clay around it all the way to to top of the outer box, covering the previously uncovered parts of the feed tube and riser tube. I lined my feed tube with split fire brick so I wouldn't have any problems with crumbling material, something like that could work, or maybe you try to paint on a thicker coat of refractory cement onto the upper parts of the feed tube and riser tube, then pack the insulating mix around it and hope that's enough?

So, the question is, would this be enough of an improvement to make it worth the extra effort? The 3000 degree rating sounds impressive, the hardness would be great, the low heat conductivity combined with surrounding it with a high insulating wrap means it should direct way more heat back into the fire than your typical fire brick build and even more than a 1:1 perlite/clay mix cast core.

You ready to go build one? You've got me curious, I'm just out of places where I need a core. And while I'm loving the experimentation and design thinking, my core works really well and I'm not sure how much it would improve what I've got. If I didn't have a heater and I was getting ready to build one, I think I'd go for the extra effort and do the refractory liner.

Have you worked with the material at all before? I was surprised how heavy and awful to work with the 1:1 perlite/clay mix was, I wonder what the refractory cement is like to work with around forms and packing in and the like ...

Sorry for the side track about CO2 and killing bugs, but it's part of why I'm interested in the exhaust analysis. I get that CO is far more dangerous than CO2, but CO2 is deadly at sufficient concentrations. Around 50,000 ppm it'll knock a human out and around 90,000 ppm for a few minutes will kill a human (see the OSHA standards for CO2 exposure for workers - OSHA report on CO2 safety) Both of these concentrations are right around what you said was the CO2 content of rmh exhaust. The NIH recommends using intense CO2 concentrations to kill bed bugs and food distributors have used high CO2 concentrations as pesticides for years as well (UC Davis report on using CO2 for pest management.

What I'm just now realizing is why the percent CO2 is so much lower than I had understood in the exhaust. The equation for burning wood (CH4 + 2O2 --> CO2 + 2H2O + energy) had me thinking that CO2 was the bulk of the exhaust, but I wasn't realizing how much excess air was a part of the equation. That's part of what your meters were reading that I didn't get. So when I tried pure rmh exhaust with an infested plant and it just put them to sleep, now it all makes sense.

That article on gas analyzing you referenced was great, I get the CO2 calculation now and why the concentrations weren't sufficient for pest killing. Bummer is, rmh exhaust just isn't concentrated enough to do what I want to do. Thanks for the info and the references.

There's a 2.5" gap in the front of the barrel where the connection to the ducting happens. The barrel is only 2.5" bigger than my riser insulation, so there's no room in back and very little on the sides (1.25" each side). There's enough room for the air to exit the barrel and enter the ducting, there's been no problem with drafting and the air is making it through the ducts and out the chimney no problem. So if there's enough room to direct the air into the ducting, is there any other purpose the barrel serves?

This thread has been closed for a while, but if any of you are still around and checking in, I'd love to hear what you finally came up with.

I have an aquaponics greenhouse as well and I built a rocket mass heater to heat the water in a floating raft table. The water from the sump tank gets pumped to the raft table, flows across it and back to the fish tank. The raft liner sits on a layer of permaboard that covers a series of brick channels that direct the exhaust from my heater, transferring the heat to the bricks and permaboard and then into the water in the raft system. The barrel for the heater heats the greenhouse directly, helping keep temps around 60 degrees at night. If you want to check out the thread that covers the project, it's: https://permies.com/t/42229/rocket-stoves/Replacing-cob-bench