So if i make the heat exchanger of say, 1 inch tubes spaced 1 inch apart, and packed about 100 tubes in there and welded a baffel sides with intake and exhaust.. That box would be about as hot as the exit flue pipe after a while? Or well until i switch the fan on.. The exit flue is about 200C when it runs properly. So basicly some kind of thermostat in the flue on the exit side of the exchanger and it could switch the fan on and off as it see fit.
In the end i just wanna dump as much heat into the air of my garage/workshop (old shed with some stuffing in the walls). And being able to put a bunch of logs in there and strangle it before i go indoors.