Good day,brand new member here and would like to get some feedback on something i have been wondering about.i have been wanting to build a rocket mass heater for a while now,but as of late i have run across a couple questions regardng using cinder blocks as a thermal mass.most of what i have read they say because of the roughness of the blocks it would interfere with air flow.which makes sense to me.but iwas thinking would it possibly work if i were to run 4 inch pipe through the entire length of the blocks rather than just attach pipes to the ends.bearing in mind that yes for the first 6 feet from the stove to the pipe be cob because of the heat coming from the stove initially and the inability of the cinder block to handle that kind of heat coming from the stove side of things.now having said all of this i dont know if a 4 inch stove pipe would even be feasable to use in this scenario.. and the other thing i was wondering was if in fact this may work does anybody have any idea on how long something like this would retain heat as we live in northern manitoba canada the winters arent gentle or short .and yes it would all be covered in cob as a finished product..any feed back is greatly appreciated..i am sure this is only the first of many questions i am going to ask in regards to this lol.thanks in advance
I know that people have used concrete blocks as airflow channels for thermal mass; someone posted a project here some time ago (don't have a link on hand). The cross section for common 8" two-core blocks would probably be comparable to 6" ducting, which is the smallest you would want to use for most builds. A 4" diameter system is said to be very tricky to build just right, and will be less effective and efficient than a larger one. The friction would be greater than metal ducting of the same size, but if you make it larger, either 10-12" block or using both cores of 8" block for the airflow, the friction would be reduced. The thermal performance should be fine as long as you have enough cross section and keep the interior as smooth as practical. You might want to use metal, brick or cob for the first few feet where the temperatures will be hotter, as there is a range somewhere above 500 to 700F where concrete will start to disintegrate.
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