Outside wood stove shop heater

This will be used inside a shop? Be careful with tossing fine sawdust into it. It can flash like gasoline. A few years ago, I tossed fine sawdust on a burn pile in the back yard. It flashed and scared the bejabbers out of me. Learned not to do that again. Growing up on a farm, I heard of the same thing causing explosions in grain mills.

That is a terribly inefficient design Theo. Particularly the heat exchanger. 90% of the heat from the fire will be lost without heating the heat exchanger. Heat exchangers work best when there are many small pipes. You want to increase the heating surface area so about 1" pipes would be somewhere around optimal in my opinion because you need to push air through them. If you were heating water then smaller yet would be better.

What would be a much better design would be to surround the heater with a plenum chamber. Cold air in from the shop at the bottom and hot air to the shop from the top. And insulate the chamber to minimize heat lost to the outside. You could also build an insulated shed around the heater. Like John said, you'd have to watch about adding sawdust.

The flexible pipes going into the shop and back are also very inefficient and most of the heat will be lost. I live in Quebec Canada and we have a pretty cold climate so that system would not do well. I have seen some have a wood burning furnace outside that provides heat indoors but they are way more elaborate than just using a metal drum and some flex pipe.