Question about bell for batch box RMH

I have a chance to get all the brick I need for making the bell for my RMH this weekend, but they are the red bricks that have the holes in them.  Can I just fill the holes with sand as I build the bell, or do they need to be filled with something more dense like cob?  Sand would of course be a lot quicker and easier, but I don't want to build the bell only to find out I have to redo it because sand doesn't work.  If sand won't work, any other ideas rather than using cob?

Forgot to ask, anyone know roughly how many bricks it will take for the bell for a 6" batch box?  I was going to buy 400.

Hi Trace;
Shorty Core is a single skin and she used around 500 solid clay bricks.

If you fill the holes with sand, you are insulating rather than heat-holding.
You do not need cob but, you do  want plain old clay, dig it up from a creek or roadside small rocks , some dirt woody debris no matter.
You want a solid heat holding filling.

Glenn in Colorado used 900 clay bricks on his 6" double skin build.

thomas rubino wrote:Hi Trace;
Shorty Core is a single skin and she used around 500 solid clay bricks.

If you fill the holes with sand, you are insulating rather than heat-holding.
You do not need cob but, you do  want plain old clay, dig it up from a creek or roadside small rocks , some dirt woody debris no matter.
You want a solid heat holding filling.

Glenn in Colorado used 900 clay bricks on his 6" double skin build.

Clay is in no short supply on my land, so that's good news.  Thank you.

I used granite granulate 0-4 mm (under 1/4th inch). It was cheap here and I did not need that much, only about a bag. I also used leftover mortar that was starting to go off (it was a type N mortar). Your local clay will probably work just as well. I chose granite because it has a high specific heat, because it is self-flowing it was easy to fill the holes and also because the expansion coëfficient is similar to that of red brick, because I wanted to avoid pushing the brick outward and causing cracks. We'll see how it holds up.

The problem with any loose filling is it will also hold a lot of air between the grains and air is very insulating.
That is not to say the bell wont hold heat, just that it is not the best way to go if you want max performance.
So it is not the actual sand or granite but the air trapped between the individual grains that prevents a dense mass.

Hi Trace,

As Thomas mentioned, I used 900 clay bricks for my double skin bell. My BB-RMH is a 6" system and my bell ISA is per Peter van den Berg's specifications on his website. If you are interested, I listed all of the specs and a detailed materials list in the first link that appears in my signature below. The 2nd link is pretty much a running Q&A while I was building. While it's a lengthy thread perhaps it will help answer some questions along the way and keep you on track.

Pretty much the only error I made was placing the bypass too high in the bell. It should have gone through the clay brick just below the transition to refractory brick. So far no issues with the high heat and the blast gate that closes off the bypass but I'm keeping an eye on it. Honestly, I've only needed the bypass at the very beginning of the heating season while the mass was ambient temperature. Since then their is no need to use it.

Have fun with your build and keep posting questions as they arise.

I would have figured that you'd just use the mortar you were using anyway, but I guess that's too thick to be easily pressed into the holes?

Normally very wet pourable cement or concrete  is bad, because it is weak.
In this situation it could be great,if it saved time over stuffing holes with solids.

You could mix Portland cement with soil to make soil cement.
The only advantage over just using soil and water to make a pourable slip is Portland guarantees the mixture will set up.

Overall, I think I'd use rebar and cement slurry if I were trying to make it bomb proof.
Otherwise I'd use nothing, since I prefer a structure that's easily undone and redone.

I would suggest to use semi dry mix and compact it well in the brick holes with a stick. When I was building my compressed block home I had 30% bricks with holes for vertical reinforcements, but in most cases half of the holes would not get any rebar, and I wanted to plug them solid. At the end of the day I scraped all semi dry mortar in the wheelbarrow that was used for joints, fill the holes and tamp them with a round stick. The drier the mix - the less possible cracking.
I have also used soil with some cement for filling voids in structural bricks when building the gate. The loamy soil (with 10-20 % clay) works the best. Dig it from a deeper depth to make sure it has no organic contents, add 5% Portland cement by volume and mix it well. Then add only such an amount of water to make it slightly moist, so when you squeeze it with your hands it will keep the shape. Cement addition will accelerate drying.
If your soil has high clay contents, you can mix it with some sand first.
You can fill the holes as you go - during the construction of the wall.