What gauge should duct inside rocket heater be?

I have bought the rocket mass heater book, pdf, from the website of the authors.  i've searched their boards, searched here and googled the question of burning point of metal.  We're not talking about buying duct from home depot and putting it inside the thermal mass, especially where it just comes out of the burn area, are we?

chris

no we are talking about ventilation duct that you can pick up at the local recycling center. the heats beyond the manifold are not high enough to cause a problem with the zinc but just to be sure burn it off on your grill or in a bon fire, stay up wind of it. follow the instructions in the book as closely as possible. and if you get stuck either ask here or get ahold of cob cottage company.

Thank you, Ernie! Since I'm in school to learn sheet metal I'd be a little embarrassed to buy duct that I could make. I will definitely burn the galvi off; we have lots of experience in our trade dealing with zinc poisoning.

chris

good deal. since you are in schools for tin work you can make some of the other parts for your stove.  LOL ya galv poisoning is a bit of a bad one, not as bad as it once was but bad enough.

however you will probably be able to get the same gauge as the ducting without the zinc probably cheaper from several places

I have the same question and am still not clear about using the galvanized heater duct.  I've been gathering materials for building a rocket mass heater and was able to get most of the 8" horizontal flue pipe (heater duct) from a recycle center, and did end up buying a few pieces new.  Some of the recycled pieces look different- more mottled and flaky looking than others and the new stuff is much less so, but it's all galvanized...  Am I hearing that older galv. material has more zinc and is thus more toxic?  Just to be clear, most of the stuff from the recycle center looks exactly like what they sell at Home Depot.  How many different kinds of 8"duct are there out there?
I was thinking maybe the the first lengths coming off of the stove might need to be regular stovepipe and then switch to the duct material farther out where it's not so hot, but it sounds like it might be possible to use all duct material if it gets burned off first?  I'm feeling worried about using the wrong material and causing harm... 

Rebecca yes and no
part of what you are hearing is that zinc poisoning can be uncomfortable and that zinc gases at a high temp. what you are not hearing is that it will kill off you and your neighbors. it actually takes lots of exposure to fumes. so here is the relevant data on the temps and off gassing.  there is only one place in the stove that the temps actually get high enough for this to occur: the heat riser. 

But if you are and have been in industries where you have been exposed long term to welding fumes many people develop a sensitivity to zinc. given the culture in the US the number of people with this high sensitivity is very low.

Metallic coating will begin to melt around 427°C (800°F) and the metal will begin to melt around 1510°C (2750°F). This product will proceed to a liquid and will form irritating and toxic gaseous metallic oxides at extremely high
temperatures.

Material will begin softening at approximately 2400 F, will proceed to a liquid and form irritating and toxic gaseous metallic oxides at extremely high temperatures.

When heated to temperatures nearing boiling point, zinc oxide
fumes are produced. Boiling Point: 1663° F (907° C)

At temperatures above the melting point, fumes containing metal oxides and other alloying elements may be liberated: Freezing/Melting Point: Base Metal – 2750 ºF
Metallic Coating – 800-900 ºF

it is irritating to me and most in industry that the general public is mixing two forms of zinc poisoning. the one most hear about is ingested zinc and unless you plan on licking the coating off your vent duct you aren't going to get this form from your stove.  since the heats aren't enough to make zinc a gas you aren't going to get the first type from your stove.

as a note the zinc tablets you take for colds, have in fillings, ingest in several other pseudo medicinal forms is the culprit in the types of zinc poisoning you hear about in the horror stories. do a search for zinc poisoning and another for galvanization poisoning. read up on them from actual industries and journals where this is known and studied.

Ya, I'm in welding classes and work in a metal shop where we weld galvanized steel sometimes so the zinc poisoning i'm referring to is when the coating is burned off by way of welding it.  It's a temporary condition, usually, and it's why alot of metal workers eat alot of yogurt; for some reason milk and yogurt help the zinc leave your system.

I got 8 6" 72 degree elbows from school today, the product of our mid term fitting test. Very happy.

Thank you again Ernie!