Mike Schofield : There is a little controversy about the Zinc coating on the Steel ductwork, This is my best understanding of the situation ! During initial firing
the galvanizing layer is heated to a temperature where part of the shiny coating changes in a mechanical way how it is bound to the Steel. There is some out
-gassing of Zinc volatiles at that time, the process is complete when a visual inspection shows that the zinc coating is now a dull or frosted gray that feels less
smooth of slick than the previous surface.
After the Zinc coating has been checked and is physically changed it is safe to use indoors ! If there is any question in your mind then the only 2 further choices
you have are -to only use that pipe out doors, or substitute Black stove pipe for it completely !
Below is a link that will allow you to scroll down through a series of Building and Working Rocket Mass Heaters RMHs the one that shows the way the zinc coated
pipe is physically changed is the one that shows how to prepare a barrel for RMH use Link below:
Mike Schofield wrote:Hi I've got a length of galzanized steel ducting that I was going to use as a flue pipe on my woodburner. Good idea or bad idea?
The other guy says there is "Controversy". No controversy here - don't do it! All welders know that zinc gases are poisonous, and heating a galvanized part is dangerous. The idea of burning off the zinc *inside your house* is not something I would recommend. Galvanized pipe does not belong in wood heat period.
Welding makes extremely high temperatures which will vaporize zinc. You never want to use metal in the parts of a RMH that can get to those temps anyway, and heating the galvanized duct to the point of converting the zinc to a stable form is safe enough.
This is what I have heard from people with professionally educated experience on the subject.
mike, yes you can use zinc covered hvac pipe for your wood stove. however,you will have a huge ammount of creosote buildup inside the pipe if you run it outside.see, the steam off the wood burning will condense in the inside of the pipe because it will reach a piont where the outside temps will cool the pipe below 212 degrees therefore volitiles in the steam will condense,build up layer after layer,and flake off in big sheets inside the pipe.if you use it inside your house,yes,it will burn the zinc off the outside the pipe,but no,it wont hurt anything.also,my current setup at my 2 story rock house is a stick of 6 inch pipe inside a stick of 8 inch pipe and common fiberglass batt insulation packed between the two as an insulating barrier.it keeps the steam from condensing on the inside of the pipe and causing creosote.if you do use it on the inside going to your flue or side exit,put in a tee instead of an elbow,it makes cleanout easier.put a cap on the unused side and pop it off to clean it out.also,if you use it on the outside with no form of insulation and you burn the zinc off the pipe will rust,fast! good luck my friend.
I am a certified welder and welded lot of galvanized metal. I never wore a mask but used a fan or the wind to keep the fumes away. I also was a commercial fisherman.We used galvanized stove pipe because painted steel pipe only lasts a couple years in salt air. Fishing boats are much more confined than the average house or cabin. I'm 70 and have no health issues from a lifetime using zinc coated items.
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