And wishing and chilly imagined my Clay Chimnea (aka Mexican Chimney) radiating heat and how much better it worked than the olde fire-pit.
Step 1: Look at the wasted heat
Three flavors of heat Conduction, Convection and Radiant.
Conduction (touch) is The most efficient.
Convection is the heating of the air (no good here since it leaves too quickly.
Radiation... sticks around longer than convective current going thru the vent...