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Concrete or cob for best radiant heat?  RSS feed

 
Posts: 126
Location: Springfield, mo
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I'm in a bit of a frenzy and running out of time to get my greenhouse RMH operating at maximum radiant heat output. It will be in the single digits for lows by the end of the week.
I posed this question in my other thread, but I think a new one more to the point would be more helpful to all.

My 6" Rocket heater in a 55gal barrel exhausts into a 6" 30ft effective length in ground duct with a 10ft vertical stack. I'm getting my top of barrel temps up to about 6-700deg, and the cob on the duct @1ft is ~145deg, @12ft ~115deg.

The first 12 ft of the duct is covered with ~800lbs of cob (~2in on top), the rest is covered with 3-4" of small gravel. I need to know which is my best absorption and radiant heat option to add more mass to the cob covered section.
1- Just add more cob (I only have about 200lbs left) to level off the top of the ducts' current cob covering, and maybe a thin layer of the fine limestone gravel spread over/into the fresh cob on top. I might be able to add more sand around the cob, but I am confused as to whether sand is good for absorbing and radiating heat as opposed to being an insulator.
2- Add the last of the cob to the top of the duct and set 2x8x16 cement / concrete cap blocks on the fresh cob. The cap blocks would be placed longways perpendicular to the duct for the 12ft run, and surrounded by the gravel floor.
I'd like to focus on this particular issue in this thread.

This likely won't solve my aquaponics greenhouse heat needs as I can now only gain a max of 30-35deg from outside temps, and most cold nights am unable to maintain more than 10deg above outside overnight. I plan on focusing on heating the water for the fish tanks which in turn will heat the gravel grow beds. The 2800lbs of gravel and 500gals of water will work as the much larger mass to radiate heat.
So on to the rocket heaters in greenhouses thread to address this plan.

 
John Adamz
Posts: 126
Location: Springfield, mo
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3 days and counting to the deep freeze and snow. I had to do something and since I got no replies to this thread before today I went with idea 2, cap blocks on fresh cob, 17 in all. I did get the old cob wet/softened a bit to get good adhesion of the top layer and also dampened the bottom of the blocks. I just hope it will radiate some kind of decent heat when it dries..
Here's the result.
cap_blocks_on_cob.jpg
[Thumbnail for cap_blocks_on_cob.jpg]
cap_blocks_on-_cob_duct.jpg
[Thumbnail for cap_blocks_on-_cob_duct.jpg]
 
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I had hoped you might get some more responses. I am in Springfield and would like to talk to you more on your rmh. S.
 
Posts: 36
Location: Truckee, CA
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John Adamz wrote:3 days and counting to the deep freeze and snow. I had to do something and since I got no replies to this thread before today I went with idea 2, caare

Some nice work you are doing. I am wondering if you have an IR thermometer to measure the differences in the ground. The cob attached to the ducts will always be the better choice. Sand is not a good choice for a heat mass because each particle creates so much more surface area it is hard to put heat into them and have it stay there. You didn't say anything about my little green house within a green house idea of using the cold frames around the media beds so I figured you were going to pursue the floor duct heating to it's end. You do have to understand that any heat being run through the ducts is being cooled by the surrounding soil. Yes there is some fraction be transferred to the cob and tiles but consider the two masses... you have the earth which has basically infinity amount of area to it, that is cold. Then you have your heat ducts/cob and tiles which do have some heat potential in them but compared to what.....? Stick a cup of coffee in a refrigerator and tell me how that can work to overcome the cold of the frig? IF you don't define the area your are trying to heat and then isolate that from the colder area, you are going to be fighting an uphill battle. You really don't need to heat the whole space you need to concentrate on saving the beds themselves.

Not trying to be a negative Nancy, just putting out the facts as I see them. SCIENCE!

 
John Adamz
Posts: 126
Location: Springfield, mo
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I understand what you're saying about the ground sucking up all the heat but the duct is somewhat insulated from the surrounding earth as it has gravel underneath, and on the bottom sides about the bottom 1/3. The cob is going from there up and over the top 2/3, and then the cap blocks on top. The surrounding ground is in the low 50's right now.
Yes I have a good IR thermometer. I am considering the tent over grow beds idea, but I also intend to heat the water in some way.
 
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is setting the tanks over it an option?

The rising heat would worm the tanks.

 
John Adamz
Posts: 126
Location: Springfield, mo
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Can't move the tanks, but 1 of them has a few feet of duct under it.
Does any one have a comment on the concrete or cob question originally posed?
 
Posts: 3366
Location: Kansas Zone 6a
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Get that thing RUNNING to dry out the cob and heat up the mass now to have flywheel momentum going into the cold.

I am not looking forward to this weekend. I have my wood stacked and ready, but that cold without snow cover is just so hard on plants.
 
R Scott
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Location: Kansas Zone 6a
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John Adamz wrote:Can't move the tanks, but 1 of them has a few feet of duct under it.
Does any one have a comment on the concrete or cob question originally posed?



I think you made the right choice. I would have said so earlier but I didn't see the thread until today.
 
Jim LaFrom
Posts: 36
Location: Truckee, CA
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John Adamz wrote:Can't move the tanks, but 1 of them has a few feet of duct under it.
Does any one have a comment on the concrete or cob question originally posed?



John,
I believe, and this is my opinion only, That anything you could do to make the cob one continuous mass is a good thing. If you mechanically connected the concrete tiles to the cob that is a good thing. What wouldn't be as effective is sand or gravel loosely filling the trench. In that example there are too many air spaces in between the individual particles that allows it to cool off too quickly. An good analogy would be to compare one large marble filling up a fish bowl. The second is the similarly sized fish bowl but filled with very small marbles. Which bowl contains more surface area versus which one has the total volume? The smaller marble has less potential to store heat for it's size and yet it has the greater capacity via the total amount of surface area to radiate of any heat.

The resulting conclusion is that you want the cob to be as continuous as possible in a monolithic pour. The straw (if you are using it) would provide encapsulated air pockets. The combination of the two gives the best performing mass for heat storage. Hope this makes your problem and solution more clear.
 
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