Is there a minimum thickness of duct for the heat exchanger?

http://www.tangra.bg/products_EN_15_118_1.html

Hi All, I watched the recent videos and read Ernie and Erica's book but I must have missed this specific detail along the way.
I am planning for a 8 inch / 20cm batch box. Is a thickness of .5mm sufficient for the duct in the heat exchanger?

Thanks!

Nik

Hi Nik;  If I read your  post correctly you are wondering about the thickness of the pipe thru your mass ?  Common practice is to use a piece of heavy wall stove pipe as the first into your mass , (high heat area) ... after that you can switch to thin wall hvac pipe to travel thru your mass, then one more piece of heavy wall to pop up out of your mass,( potential damage spot)  then back to thin wall to exit your home. The pipe buried in your mass is " sacrificial" it is expected to rot away over time, by carefully surrounding your thin wall with cob you are essentially building a clay tunnel and the thin wall pipe is just a form.   EDIT)  I just looked at your link, You are in Europe ? The spiral style pipe is not recommended as its shape slows the exiting gasses down . If this is your only available pipe you might consider using soft brick to create your tunnel thru the mass instead of pipe.

thomas rubino wrote:Hi Nik;  If I read your  post correctly you are wondering about the thickness of the pipe thru your mass ?  Common practice is to use a piece of heavy wall stove pipe as the first into your mass , (high heat area) ... after that you can switch to thin wall hvac pipe to travel thru your mass, then one more piece of heavy wall to pop up out of your mass,( potential damage spot)  then back to thin wall to exit your home. The pipe buried in your mass is " sacrificial" it is expected to rot away over time, by carefully surrounding your thin wall with cob you are essentially building a clay tunnel and the thin wall pipe is just a form.   EDIT)  I just looked at your link, You are in Europe ? The spiral style pipe is not recommended as its shape slows the exiting gasses down . If this is your only available pipe you might consider using soft brick to create your tunnel thru the mass instead of pipe.

Hi Thomas, Thanks. Yes, this is the best duct I could find to date.
What exactly is soft brick? A quick google search was not helpful ")
Is it worth the effort to as a metal worker to custom form the duct from sheet metal?

Hi Nik;  Soft brick is simply regular red or yellow clay brick, not expensive fire brick.  If you had them flat stones carefully fitted and sealed with cob would work as well.  All you need is a smooth tunnel with a proper size for the heat to travel thru.  Have you thought about using matt walkers 1/2 barrel system instead of a tunnel ?  

Nik; Here is a link to donkey pro boards, it explains matts 1/2 barrel system "donkey32.proboards.com/thread/560"   This may be your solution.