Ducting in pebble mass

I just (mostly) finished watching all of 4dvd set ‘bwh’  It was awesome. But somehow I either missed or didn’t hear what the duct piping is inside of mass. I’m assuming I can’t buy hibachi ducting from Home Depot, but stove pipe seems like over kill. I read someone suggesting aluminum. Where would one buy that other than online?

https://www.google.com/search?q=6+aluminum+pipe+duct+hvac&sa=X&rlz=1CDGOYI_enUS774US774&hl=en-US&biw=375&bih=591&noj=1&tbs=vw:l,ss:44&tbm=shop&srpd=4866792618927279527&prds=epd:6874571121997822976,paur:ClkAsKraX-_yZqE2TfOZF-EGUv80RZOBodB8LYAdnUKjEFDKwL5H_L22CJxUfraNJ2rWwqXSIeZbWYI3rB6fcYIxduCB1YdRQjImQ6AX-BSDYFk57P4slAZU8BIZAFPVH72UQvQX2p5DWT5DxS9QIOsCG0VasQ,cdl:1,cid:15736563221099844753&ved=0ahUKEwiTwKvCn_LYAhXP0FMKHQeaCjUQgTYI1gQ

Could I use this flexible pipe?

If the mass is cob, it can hold itself up pretty well once dry, so I think flexible ducting would work with cob. You'd probably want to stuff something inside the duct to hold it up during the cob process and drying. But if you want to use loose pebbles, then flexible ducting would collapse.

Ted; When you first leave the horizontal transition area , you use one stick of regular black stove pipe,  after that you switch to hvac pipe readily available in 5' sticks at home depot for around $11.00 each.  Use another stick of black pipe when you pop up out of your mass (high abrasion area) the back to hvac up to the roof.  If your chimney is all indoors you will not need insulated pipe until you pass thru your roof.      EDIT )   No flex pipe , the ridges inside resist the flow of hot air.

Hi Ted;  Can I ask why you want to use a pebble mass ? Although an accepted build material,  Its not the best medium for holding your heat.