Cinder blocks for mass ducting?

Saw a video online of a guy using cinder blocks laid on their sides and joined to produce air ducts inside of his mass storage bench. What is the thoughts on this type of air ducting. Will the cinder block hold up. not crack, and are they a good source of storage mass? I would probably add mass over the top and sides of the blocks too, but just wondering what are the characteristics of cinder block and are the adequate when used in this manner?

Hi Hugh,

Welcome to Permies.

Hi Hugh;
Using the cast holes in concrete blocks will not work very well, not nearly smooth enough to encourage the flow of gasses.
Using them with the smooth side to form an 8" tunnel and then burying them in a mass will work.
However, HVAC light gauge ducting will work for the whole system as long as you start with one piece of black stove pipe at the transition area.

Several people have tried this. It can work; you would want to ensure enough aggregate cross section for airflow, allowing for friction of the rough surfaces, and you would need to use larger size blocks to give large enough channels. You would also need to keep the concrete block far enough from the combustion core that it is not exposed to high temperatures. Common brick or cob would be safe in the intervening space.

If you have to buy new material, light ducting will probably be distinctly less expensive than concrete block of equivalent capacity, assuming you can also get thermal mass material free or cheap. If you have recycled material of one type or another, that would tilt the balance.

I would have to buy any ducting material I decide to use. I got two pickup loads of free rock that I would like to use as the covering. Not sure of the type of rock it is, not a very good "rock" person. It was used as fireplace facing. It is quite dense and square and rectangular in shape and would be easy to use as a cover over a cinder block shape, and would be a good heat storage mass. That would eliminate the need for additional "filler" material if I were to use cylindrical ducting, and eliminate problems of attachment of  the rock to any underlying structural element. It would make the construction process quite simple. Use cinder block on its side to form the ducting channels, then simply mortar the rock to the squareness of the outside surface of the cinder block.

Hi Hugh;
You really want a filler between your stones.
Any air gaps are insulators, you want to tightly fill all gaps.
It does not need to be clay or cob it can be free dirt from your backyard.
Sand is not a good choice unless it is super fine, pebbles are not a good option either.
By filling any gaps you create a solid heat-holding mass.

Do you have the cinder block free or cheap?

Another idea is build a horizontal stratification chamber with the blocks seal it (we prefer cob over mortar) and surround it with your rocks with cob infill.

Otherwise you can use 55 barrels for the stratification chamber usually it is cheaper than ducting.

thomas rubino wrote:Hi Hugh;

Any air gaps are insulators, you want to tightly fill all gaps.

Sand is not a good choice unless it is super fine, pebbles are not a good option either.
By filling any gaps you create a solid heat-holding mass.

Backing up Tomas on the sand, I had a 10 foot run that was pretty smooth, and wanted to pull some heat out, and before I knew how things would work, used about 1 foot of SAND by 2' wide, around the 10' long channel, perhaps 1-2" would get really warm, the next 3-4" just warm,  and beyond that hardly a thing. Some cinder blocks were also involved, they did not do much as well. ( by much, I mean-adsorb heat)  Pretty upsetting to carry in  and then carry it all back out. But well learned was the task.

This brings up a question,  I don't intend to do, but can see others that might try.   I have a single large brick bell, 74 sq foot internal surface, so am at the desired goal of MASS when all said and done.  But if I was to exit this bell with say a 8" round duct,  ( it is a 7" system)  and run this for 10 foot, before going into a 7" chimney,  Is this a "fault"  or something that should not be done, or does this not have much bearing if all the stove, and bell is done right. ???