J. Tabordiy : Seriously, a good place to go cheap is the horizontal ductwork in your Thermal Mass Bench, all ductwork will rust, from the moment you paint on
the clay slip- through all the wet cob, and then all the products of combustion from your wood fuel, and the 10% moisture content of that fuel when it is considered
kiln dry, for a period of weeks, as long as 3 weeks, and balanced against the Moisture content in the air, the ductwork within your Cob will be constantly wet.
This is one of the reasons we tell people the more dense stone they use in their Thermal Mass Bench the less( high moisture content ) cob they will have to make!
For this reason any ductwork should be considered a sacrificial form for creating the horizontal chimney in your thermal bench !
Here in the States we will have three grades of pipe, a heavy Wood stove black, a Medium Galvanized, and a Lightweight Galvanized Cold air return ductwork.
To that I add one more grade free! Often after house fires due to the amount of water used in fighting the fire- all the houses appliances are trashed and get
replaced, this is usually done by the general contractor long before the H.E.V.A.C. contractor shows up,you will get a mixed bag of 6'' 8'' and bigger, I have had
good success ether removing pipe from the skip or even bribing the contractor foreman with cheap beer ! Hope this helps and is timely ! Big AL
Late note : Click on the link below and scroll down to the 9th video the one labeled rocket flames Rocket mass heater barrel prep ! this is definitely in your future
http://www.richsoil.com/rocket-stove-mass-heater.jsp
Then start at the 1st video and watch them all they are a little dated so Any questions ask here ! A.L.