Mass Options question

Seems that everyone talks about using cob for mass in their heaters. Is cob the best material for mass or is it just available and inexpensive. What if any materials would be a s good or better? i.e. stone.

Cob is inexpensive and works really well for DIY. If you're seriously considering building a RMH, I can highly recommend the following. It will save you time, labor, money and answer a 1000 questions on everything from foundations to chimney installations, including step by step example RMH build, and lots of finishing/cosmetic material options:

https://permies.com/t/57365/Rocket-Mass-Heater-Builder-Guide

Hi Jeffrey; Welcome to permies!  Like Byron I recommend buying and reading the rmh builders guide. After you have encased your horizontal pipes in cob ,the rest of your mass should be other things than just cob. Rock is the most common, flat  slate like I used or chunks of granite, as large a stone as you can handle and not crush your horizontal pipes . Large pieces of cast iron , flat steel plate, even an old cast iron sink ... the cob is the filler, below ,around and above each piece of "mass filler" you use, leaving no air pockets. Small rocks and gravel have been used in place of cob but they do not hold heat like a big chunk of stone buried in cob "mud" will.  If your unhappy with how a cob bench looks then build around your cob mass with whatever you like ... stone, metal, wood, brick, sheet rock, concrete board  it does not have to be just cob. Remember cob is clay ... they built many houses out of clay bricks....

In my thinking, cob serves three purposes.  

1.) It acts to seal the pipes so that you have no surprise exhaust leaks.
2.) It is shapeless/malleable and thus forms easily around any object or in any space/cavity.
3.) It acts as mortar to hold and make other objects (rocks, cast iron sinks, bricks, random steel) laid in and on it a part of a single uniform mass.

Thanks for your replies. Cob seems labor intensive so the less I use the better lol. Could the multiple types of mass in the same heater cause "hot pockets" that might create issues? I'm looking to build a RMH as a platform for 3 IBC fish tanks in my greenhouse aquaponics system. I'm currently using 1 IBC tank with a wood burner but can't heat with wood 24/7 for obvious reasons and have to heat the water with electricity. in my new plan the 900 gallons of water sitting on the heater mass should give me additional mass as well. Just have to control water temp somehow.

You would have to create a pretty strong platform above your pipes to support all that water in those IBCs.  You would be regulating the water temperature by burn time and spans between burn times/heat storage times/radiation/conduction times.  Maybe I'm stating the obvious and missing the question on that one?  Hot spots would be nullified with enough uniform mass (like cob) above the assorted densities.  I know you don't want to do the cob... so you could have a platform of slate/stone on top?  Bricks?  Whatever... just make it uniform on the top, and to the point that you don't feel any hot spots.  I think you will want it to be comfortable, not too hot to touch.  The more mass you put in your bench, the more it absorbs and then the less extreme temperatures will be put into your IBC water.

Good point on the strength of the mass. Maybe a brick outer frame for IBC support. That would be a good enclosure for the rest of the mass.

Jeffrey,  i dont think your water would ever heat.


I have a 220mm batch, with a métal tank as first radiator. And a Bell behind, of about four tons. Barely ever goes over 45c°.

I don't think that it would  heat up enough for the IBC's to be hot, but I think it might heat up enough for fish ponds, don't you Satamax?

Since I raise Tilapia the water can get up to 90 degrees, about 28C,or higher without a problem. Even if the mass only assists with the water heating that would be good. If for some reason it's getting too hot I can make an air space between the mass bed and the tanks to lower the water temp. I ill probably start with the original IBC base in place and go from there.

Since you don't have to keep a narrow temperature range, you can probably heat and buffer your IBCs enough with the minimal amount of mass to keep the hottest gases away from the plastic, and support the tanks. You may want to add insulating panels on the sides of the tanks if you experience too much heat loss (assuming the heat is not required for keeping the space warm as well). With 900 gallons of water, it would take a big jolt of heat to unbalance it. You can probably find a good rhythm of firing once or twice a day to keep the water in a good range. The length of each firing would depend on the weather.