determining length of thermal mass piping

Is there a formula or other way of determining the maximum lenghth of stovepipe you can use in the thermal mass so as to extract the maximum heat but still leave enough energy for draft in order to vent to the outside? I'm thinking of a bench-style thermal mass in one end of a shop that goes out in both directions behind the vertical riser and circling back to vent out the wall in back of the riser. Trying to determine how long the bench can be if using 8 inch pipe and a stove about the size of a 55 gallon barrel or perhaps a little taller. Thanks in advance for any information.

Cl Riddle : Welcome, You accidentally posted this in Alternate energy, instead of in Rocket stoves.

Building a Rocket Mass Heater with two exhausts off of one barrel rarely works, and is a very difficult 1st build !

An 8'' system can push your hot exhaust gases 50 feet horizontally through your thermal mass! Subtract 5 feet
for every Elbow from the transition zone (below your barrel) outwards to your final vertical chimney! You count
every elbow or "T", but not the final vertical chimney !

Please go to Rocketstoves.com to download your copy of the 3rd ed. of Rocket Mass Heaters

This will save you time, money, and frustration. When you come back here -you will Know that you are using
the same words to describe the sizes, shapes, and specific Orientation of the parts to themselves, and the
whole! For the good of the craft ! Big AL