Replacing the cob bench

Maybe other folks have an easier time of getting/making cob, but that has been a bottleneck for me on a number of projects. So I've been thinking of ways around it and have seen a number of alternatives that folks have come up with either for minimizing the amount of cob needed for a project, or, avoiding cob all together. Cinder blocks seem to be the most common alternative, but many folks are concerned about the blocks not being able to handle the heat of the exhaust right as it comes out of the barrel, and making turns and connections with cinder blocks is kind of a pain. Cinder blocks are a pretty cool option, but the difficulties got me thinking.

The picture on the bottom shows the layout of my project (a 4' x 8' warmed platform, but the dimensions could be totally flexible) and the idea of using bricks (common, red clay bricks or any other bricks you have access to for cheap) as both the thermal mass for storing heat as well as creating the channels for the hot exhaust to flow through.

The base can be a layer of insulation followed by a layer of some cementaceous board (I actually think drywall could work fine in this application). For the ultimate set and seal, fire mortar could be used, or a small layer of sand and cement to seat the bricks perfectly. The bricks get laid out flat, stacked two high to form the channels. They could go on side, two next to each other offset to help with gaps, there are lots of options. It would make sense to mortar the bricks into place, but I'm curious to see how well the system functions without mortar.

For the top cap, put a layer of mortar, furnace cement, high temp caulk, something like that on top of all the bricks, then lay a cement board on top to seal the ducts. If you want more thermal mass, you could put gravel, bricks, cement pavers on top.

If you have easy access to cob and you know you want this thing to be mostly permanent, then cob is a much better solution. But, if you don't have easy access to cob or you wonder about changing things, moving things, or you have cheap access to bricks, this seems like it could be a pretty nice solution.

A second thing I'm intrigued by about this solution, you could set it all up dry, and test run it to see how the heating is working out. I'm sure there would be some loss of efficiency without tightly sealed joints, but the heat would still distribute. If the issue is that there wouldn't be enough draw, I'm thinking to solve that by having a cannister fan at the end of my system to create the draw for me while I test it (and maybe all the time as well).

If someone else has done this, I'd love to see pictures and hear how it's worked. If this is a new idea, I'm willing to be the guinea pig, but I'd love any feedback or suggestions before I build.

Rick, remove the last block or at least half of a full block in the elbows.

Satamax Antone wrote:Rick, remove the last block or at least half of a full block in the elbows.

The reason for this is to off-set the friction of the walls in the turn. I'm sure there are HVAC info sheets available that discuss these concerns, although I've not tried to locate any. One name for the concept is plenum, if I spelt that correctly. It just identifies a large area of open space into which the air may easily flow and then become redirected in another direction.

The rule of thumb that is in my mind -although I do not know why, so I cannot provide a citation- is that the 180-degree turn ought to be a minimum of twice as long as the passage is wide. I'm assuming that assumes a round or square passage cross-section, but that is exactly that, an assumption.

So if your passage was 8 x 8, make the turn at least 16 x 16.

Other thoughts...

You could use wet clay to "mortar" the joints in the bricks. It is not really mortar, just clay, and would come part much easier, and still provide an air seal as well as transference of heat much better, and thus should perform better as a thermal mass.

At a minimum, it would be wise to do this in the passage, because a seal against escaping gas is wise. So at least trowel on a clay mix onto the walls of the tunnels. If smoothed, this will also help reduce the air friction somewhat.

Concrete board over the top / the "roof" of the passages....

I've heard that is pretty brittle stuff. Will it support the weight above it, or will it be more likely to crack, and collapse into the passage?

Once common option is to use pavers, foot square, 16x16, etc. Just lay them across the top of the passages, and that makes the roof. Seal it of course, so the gases don't leak into the room.

I don't personally expect the heat will be too severe for brick or cinder block in the bench, but one could always line the first five feet or so with fire brick. That would take care of that concern. There are examples I've seen scanning various building codes that talk about how far into air passages that must be lined with fire brick. Five feet is what comes to mind, so that's why I used it as an example. But that too is a guess, just going by what I remember, as opposed to having the citation at hand.

For those concerned with practical matters and not code, just measure the temperature inside the passages. Then look up suggested temperature ranges of the masonry materials you are using, and it should be pretty obvious what is safe (of course, this requires building a test bench, into which you can insert temperature probes).

An easier means, may just be to measure the temp at the clean outs. I'd start with the one at the bottom of the barrel, in the manifold. That ought to be the hottest part of the bench, in terms of air temperature moving through the passages. It is only going to get cooler as it moves farther along, as it gives up its heat to the surrounding masonry.

300 degrees F is the highest normal temp I've heard for the bottom of the barrel. I suspect one could use that safely. My last burn I measured something like 550 degrees F on the outside face of a split fire brick which made up the burn chamber side. So I cannot imagine the manifold would ever get that hot. So if one took the engineering safety factor of times two, use the temp of 600 degrees F at the manifold and only use materials able to take that. I suspect we are now talking fire brick or refractory material.

Obviously, if one can afford to, one could like the entire passage with fire brick. But that is a lot of overkill.

Personally, if I were concerned about it, I'd measure the temps at the various clean outs and the exhaust to the outdoors. Then you'll have several air temperature measures, at least three I'd expect (manifold, u-turn clean out, and exhaust to outdoors). That will help you estimate the temperature drop through your passages. Then you can start with whatever highest temp you are comfortable using, and calculate the same rate of change, and use that to specify the proper lining material along your passages.

I suspect that is a lot more work than safety requires, but if I wanted to know, that's what comes to mind to easily find out.

If you're more technically inclined, thermocouples are cheap, especially at the lower temperature ranges. Like $3 to $5 each (USD). Digital readers can be obtained for as little as $20 USD. Firgure out how much money it is worth to you to really know the temps in the passages and then spread out that many thermocouples, leaving the plug-in ends with free access to the outside of the bench, and then take measurements during your burns. Within the accuracy of your devices, you'll have your answers.

If anyone has already done this, it be nice to see your measurements!

Rich Frey :Yes this has been done in several cultures , The Romans used a system that today we call the hypocaust system and a similar system is was widely used
in Korea, Japan, and northern China!

I expect that you will find that you will need a very tall chimney and/or the permanent help of an exhaust fan ! Which means a 12-24 volt system and two 12 volt
Carbatteries to carry you through The Storm of the century type events ( This seems to be about every 15 years now )!

Also A great deal of attention kinda needs to be paid to Making and keeping the Entire system air tight. Bad things happen in threes, and Carbon Monoxide is a very
bad thing indeed !

Think like Fire! Flow like a Gas! Don't be the marshmallow ! For the Good of the Crafts ! Big AL

Thank you all for the suggestions and input. So for "removing a brick", the bricks are just under eight inches long, so two of them end to end makes a 16" u-turn area, and the passages will be 5.33 inches (this is a 6" system), so the turn area is triple the passage width. So do I still need to remove a brick or could it work as laid out? And if I did remove a brick, which brick would I remove?

Odds are this will be a fan aided system, at least for start up and finish off. I live in Northern California, and we just had our "storm of the decade", they closed the schools, told people to stay home -- it rained mediumly hard for most of the day and was a little windy for a few hours in the morning. We just don't get real storms here and don't get power outages (1 in 14 years so far, that last for an hour or so). Now when the earthquake hits, that's a different story ...

In terms of what goes on top, cement pavers would be easy to work with, but do I have the same problem that I shouldn't use them for the first 5-10' or so? On top of whatever the final surface is, I'll have a 4' x 8' pond basically, an aquaponics system with a rubber liner that I'm hoping will absorb much of the heat coming from below. But what that means is that I won't want to mess around with what's in the bench, so to speak, once i close it up.

I got sidetracked for a while, stuck on a few questions and problems I couldn't quite figure out, but tonight I finished the dry build and gave it a first test run. The core was built using the plans in the 6" core thread, with a perlite, fire clay and furnace cement mix. The riser is 14 gauge steel, wrapped with kaowool. The barrel is a fairly small, 14.5" used oil barrel that I cleaned out.

The table is 4' x 8' with 3" of sand as bottom insulation, then a 1/2" permaboard (cement board) layer, then the rows of bricks making channels for the air. The channels are 6.25" wide and 2 bricks high. The ducts run the length of the table three times, roughly 20+ feet of duct. The whole system of bricks is covered with another layer of permaboard. The final project will include a rubber liner sitting on the permaboard filled with water (part of an aquaponics set up). The liner can handle 185 degrees and it'll have 250 gallons of water in it, flowing through. I am also a blasphemer and instead of having a chimney at the end of the ducting, I have a small, variable speed fan that creates the draw.

I ran the system, dry fit (no mortar on the bricks or permaboard), for about two hours tonight. The fire burned great, things heated up pretty quickly along the channels and more slowly across the whole top board. I didn't have an infrared thermometer, so I couldn't get barrel temp readings. The exhaust air was coming out just over 100 degrees. I put a few pans of water on the permaboard to see how it did heating water, and the heat transferred quickly and helped the permaboard stay a lot cooler. Once the whole thing is covered with a liner and water, I'm betting/hoping it'll do much better at transferring the heat. I'll also go back, take it all apart and mortar it into place to make sure it's air tight.

So a couple of questions. First, I've been wondering about covering the bricks with a thin layer of sheet metal under the permaboard to help distribute the heat better. My thought is the heat in the channels would hit the sheet metal, start heating that up seriously, but the sheet metal would spread out the heat and do a much better job at spreading it out to the permaboard layer. Does that seem like it would work? Any better ideas for distributing the heat better?

Along that line, I picked permaboard mainly because it was the best choice of the easily available cement-type boards I could get at Home Depot. If there's a better board/material at accepting the heat from the hot exhaust and transferring it to the liner, I'd love to know about it.

Last question/idea. I'm thinking seriously about putting a T on the end of the exhaust duct so that I can choose to either send the exhaust outside the greenhouse (what it does now) or let it blow in the greenhouse. When the system is running, the fire is hot, the exhaust is totally clean, and I'd love to have the hot air and the CO2 in the greenhouse. I've also got a radiator I'm thinking to blow the hot air through to get the last bit of heat out of it heating my water.

Here are a few pictures of the current setup and how it's looking. Thanks for all the feedback and suggestions

Looks nice! I hope you are not planning on leaving the 2x4 at the edge under the first channel from the barrel. That will eventually get enough heat to char and burn. Replacing it with brick for a foot each side of the channel should take care of it.

I see you have doubled brick spacers between the channels. There is no particular benefit to the extra brick there aside from filler to keep the channels a constant width, and with the entry and exit positions you have, you could modify the flow pattern to be like traditional underfloor heating systems. A wide-ish header on each end with the lengthwise bricks spread evenly to make five parallel channels would give much less friction to resist airflow, and relatively even heat over the whole surface.
Something like this (dots . represent open space):

.O chimney
|..|
|..|_______
|...............|
|..|..|..|..|..|
|..|..|..|..|..|
|..|..|..|..|..|
|..|..|..|..|..|
|..|..|..|..|..|
|..|..|..|..|..|
|...............|
|_______...|
.............|..|
..............O barrel

You do realize that if you were putting this in your house we would all be yelling "fire hazard!" at you? There is much too much wood in close proximity to high-heat areas. It is possible, depending on how long your burns go, that the table 3" below the first channel could get hot enough to char. If you only burn for an hour or two a day or twice a day, you might be alright for a good long time. The wood around the core casting needs to come off at least on the side nearest the table.
Your steel heat riser will fail before long if you are getting good combustion temps and you will need to replace it with refractory material.

I've got no sense at all for air flow dynamics, so I appreciate any suggestions. A buddy of mine suggested something similar to your revised brick layout with the header and collector at the end. The only thing I thought I understood was that the air would be more likely to flow where it's easier, so wouldn't I get a much more significant amount of air flowing through the middle than around the sides?

The other thing I thought I understood was that time in contact with materials was a key part of heat transfer. So if the air flows through roughly the length of the table and then out, that's a lot less distance (8') than flowing down, back and down again (~21'). Is it flowing so much more slowly that the net amount of time i contact with the thermal mass is the same and then it's just a better flow plan? So if I switched it to your way, would I keep the channel width the same (6.25" x 2 bricks high)? Or since all the channels are open, would I make each channel narrower?

In terms of fire and removing wood, yeah, I'm still in the test phase, so there's more wood and exposed wood than there will be in the final plan. I'm planning on burning 2-3 hrs/day most days, maybe a little longer in the winter, but it does make sense to replace the 2x4 next to the barrel.

In terms of the heat riser, I'll be curious to see how long it lasts. It's pretty easy to remove the barrel on my system, so once I've done a few more tests and finally mortar everything in place, I'll check the riser pretty regularly to see how it's doing.

Thanks for the feedback - Rick

Yes, the air would flow where it is easier, so if you had the inlet and outlet of both headers in the middle or on the same side you would get uneven airflow. But since you feed in one corner and out the opposite corner, the flow will balance pretty well.

You would just separate the doubled rows of bricks to give five channels instead of three. Each one would be narrower, but since they would all be flowing in parallel, each only needs to carry 1/5 of the total flow instead of 100% of the flow as it is now.

The time in contact will be constant given the same overall flow rate and volume of the channels, whether it flows in a winding path down one channel or in a web through the lot. The main benefit will be measurably reduced friction in the system, and easier draft.

A general observation about RMH design prompted by your situation is that they almost universally have a barrel or bell over the heat riser which forces the exhaust to go back down before reaching the chimney. This downward flow is not something inherent to the function of a RMH, but due to the fact that usually the mass to be useful needs to be lower than the top of the heat riser. If you want a raised mass, or your building layout is conducive to having the feed and riser lower than the mass, that would actually give stronger draft than an identical system which forced the flow to descend before entering the mass.

I like the design. Some clean-outs will come in handy once it's all mortared up.

Update on the greenhouse, brick channel, aquaponics raft rocket mass heater project.

So after a few tests seeing how the heat spread out over the permaboard cover, trying a bunch of brick channel options and how it worked with a liner filled with water on it, it's all assembled, mortared, the aquaponics raft is on top with plants growing and I've been heating with it for about two weeks. My water temperature (abut 600 gallons total) has gone from 54 degrees to almost 67 today. I've been burning most nights, for anywhere from two hours to five hours. I often don't get back to the fire in time, before it's died out, so I restart it from embers, put a pile of wood on and come back an hour later or so.

The final brick path turned out to be just one brick wide on it's side, so the channels are 3 5/8" high and 5.5" wide. The run goes to the end, back, to the end again, back again then one last time to the end, so about 35' total of path. I was feeling a bit fancy, so I put some clay/sand mix at each of the corners of the u-turns to round them out. Not sure how much that helps or not, but I figured it couldn't hurt. At the end of the table, I built a wooden box that connects to the end of the table and has a fan on the bottom that blows into a six inch T so I can either send the exhaust out of the greenhouse or use it in the greenhouse for CO2 boosting or for intense CO2 for pest killing. I also put a thin sheet of sheet metal covering the first 3' of the table (across all the runs) to spread out the heat a bit more evenly. Again, I'm not sure how much it helps, but it seemed to have spread out the heat better than when I didn't have it, so I stuck with it.

I want most of the heat going to the exhaust system running through the table, so I cut my riser so there's a 5" gap to the top of the barrel. My highest temperature at the top of the barrel has been 760 degrees, but usually it's around 500 - 550. The final exhaust coming out the end has been around 75 if it's not burning that hot and up to around 85 when it's burning hot.

Obviously I'm pretty early in the whole life cycle of the project, but the table/brick channel system is feeling like a great success. It also leaves me a lot of flexibility about how I want to have the air exit at the end and how I manage the connection from the rmh and barrel. If I need to rebuild or change things up there, right now I don't have any of the rmh covered with anything, it's still the 6" core I poured with a wood frame around it. I haven't seen any charring or felt any kind of serious heat on the wood box I poured the core in, maybe I had enough insulation that it's fine or maybe my core isn't getting that hot, but I've burned for 6 hours straight and didn't have any problems.

A couple of thoughts reflections from the project so far. First, my poured core has some serious cracks. Either I didn't do a good enough job of smashing the material in as I filled the form or it was so wet that as it dried and shrunk it just cracked seriously, I'm not sure, but there are some major cracks. I'm wondering about making a wet clay mix with some fine sand, ash maybe and pouring it into the cracks then letting it dry for a bit. If anyone has some ideas for what to do about cracks in the core, I'm all ears (see picture at the end for what the crack looks like). I like the fire brick around the opening of the hole where I put in wood, but I've been reading a bit about keeping that area cooler, and those fire brick get pretty toasty after a few hours of burning. And while I get most folks cover their cores with some form of insulation or mass, since I'm in a greenhouse and don't mind the heat radiating off, my core is still in the box I poured it in. I've thought about building a bigger box and putting some insualtion around it, but I'm not sure what it would do for me, except maybe help my core not fall apart

I didn't build in anything for ash clean out, and cleaning it out by hand so far hasn't been that bad. I've got a spot for clean out at the base of the barrel, but there hasn't been that much there. Because both ends of the table are fairly easy to access, I'm thinking I'll be able to go in either with a shop vac or a blower and get out the worst of the ash that got left inside. I'm hoping most of it will be on that first run and I can get a vacuum hose down the entire length, so I'll see how it looks in a few months.

Using the fan to create draft has worked great so far. I can start my fires easily with no smoke and at my last fire, when it's down to embers, I turn it off and let it die out. We'll see how it all looks a few months into burning regularly, but so far so good. I had been using a 6", 300 cfm cannister fan, and even on the lowest power setting, it was pulling too much air. I switched down to a metal cased fan, 180 cfm that responded much better to the variable power level, so I can have it spin just about any speed I want.

So that's the update from the greenhouse rmh. In a month or two I'll check the riser and see how it looks (I used a steel riser wrapped in kaowool), check the inside of the first run and hopefully come back saying how easy fixing the cracks in my core was - Rick

I had too many pictures to show for the last post, so here's a picture of one of the cracks in the rmh core I poured. You can see I let my daughter scratch her name in the top, she loves melting aluminum cans in it when its nice and hot

Good update Rick, and a nice system! Are those firebricks embedded in a cast core? If so, that's why your cracks are large. The brick doesn't shrink and as such bricks will blow the core to pieces during the curing of the mix. I apologize I didn't catch that you planned to do that earlier, it's a well known no-no for casting. Still, it should be no big deal, just use cob or something to wrap the core and hold it all together and it should function just fine. Once the cracks are there, the thing is stabilized and will stay as is, so all you need to do is hold it together and maybe keep the worst of them from leaking much. A little clay/sand will go a long way towards the latter goal. Thanks for the update, you've got some cool stuff going on in this build.

As it was drying, I could start to see that the bricks were settling oddly as the clay dried, I just didn't realize how bad. I have a crack or two though that are pretty bad that aren't near the brick, so it wasn't just that.

Do you think I can take off the wood box I poured the core in or will it fall apart? I had been thinking I could take off the old wood box, make a slightly bigger one, put some sand/perlite on the bottom, set the core in the new box, then fill it with sand around it. Bummer is, that core is heavier than heck and it is rough to work with after the fact.

I'll mix up some clay/sand, I'm guessing leave it pretty wet so it can get down into the cracks? And for patching any spots that have crumbled, do you use perlite in that mix or just clay/sand? Any ash ever to help it stick better?

Thanks for the ideas - Rick

You are right, it wasn't just the bricks, the cores will crack either way. Taking off the outer mold is going to be dicey but I think you should do it. Just try to hold it all together and don't let it get away from you. I'd probably go for a cob/perlite surround rather than another box and sand in an effort to encapsulate the core. You could still have a box outside for finish, but the core needs some support. Clay/sand/ash sounds like a great starting point for a sealing mortar. Don't sweat it too much, you'll learn about your materials as you go and figure out the right mix by working with it. There will be lots of opportunities for crack patching as you go through the finishing process here.

I have seen concrete used in a rocket stoves and the results are not good. It burned slowly over time and created a lot of nasty black smoke out the exhaust. Are you not worried about the concrete board failing?

Concrete is only at risk in the combustion core, where temperatures are higher than 5-600 degrees F. In the mass, there will not be dangerous temperatures (unless you have a big heater and a big thick mass).