I went with 10 inch chimney pipe which works well once the fire is up and running. I found that adding removable tin shrouds helps reduce the intake of ambient air in areas which do not improve draw and helps increase the intake of ambient air in areas where it positively affects draw. Not super good looking but it works!

Unfortunately larger diameter insulated chimney pipe is very expensive. It took me six months of searching Craig’s List and a two hundred mile drive to secure a collection of used 10 inch insulated pipe and the all important roof flashing kit. Saved me a bundle though so it was worth it.

Just because a person/company can make and market an item does not mean they USE them.

If we are all wrong then why isnt your flu drawing as needed? Maybe it was designed to have a exhaust fan, I am sure it will draw fine if you make that addition. It worked for a 6inch flue A friend set up. As long as his squirrel cage fan was running he never any smoke in the shop, and less costly than replacing the existing flue.

P.S. my 12 inch brick side draft needs the burning paper jump start to draw properly also.

I run a 6 inch flue pipe yes it works but also have to pre warm the pipe with a preburn to start a draft

Unfortunately, the time to ask about flue size is before the chimney is built. As others have said, your chimney flue is too small, all other suggestions is the attempt to get around that.

My sentiment too, for what it is worth.

It is simple, your shop is to clean. You need some coal smoke to give it "Smithy" Juju!!

Here is an example of a forge that does not draw correctly. I get tired of explaining to visitor about the smoke/apologizing to them. Some really can't tolerate the smoke or smell.

The flue tile is 8X8 and quite tall. The State of Ohio realized it wasn't working so a fan was installed inside the flu to pull the smoke and gas inside. That still doesns't do it. The best mothod I found is to build a hot wood fire and slowly add the coke with no green coal. Keep the air flow up and get the fire up to forging temp rapidly. By maintining the fire properly, little smoke is produced. Keeping the doors/windows open also helps.

The roof has a very large vent to allow the smoke to drift upward and vent out doors, much like that of a maple sugar camp.

That large hood above the forge is designed to trap and hold the smoke/gasses and fly ash at head level. The hood is not vented in any way.

With the above set-up, there is certainly no problem generating the coal smoke effect of a real/operational blacksmith shop.