I used clay roof tiles to form channels for the exhaust inside the bench. As long as the cross-sectional-area of the concrete blocks is enough, I don't see what the problem would be. Just make sure to seal the cracks well, or else you will end up with smoke coming in to your room until your chimney is warmed up and providing a draft.
And get a carbon monoxide detector for safety-- this does for any stove in my opinion, even a commercial one. Mine let me know that my exhaust was getting clogged with ash before it became completely obvious from observing the fire-- when the fire had died down to coals there was not enough draft to prevent some of the carbon monoxide coming out the feed hole.
I have a Kidde KN-COPP-B-LPM . It measures down to 10ppm and stores the highest reading. It is useful therefore for checking that all cracks are sealed, etc.
There is a video on youtube "Indoor rocket mass heater" He uses both tubes in a cinder block for the discharge from his small rocket heater. It looks like maybe a 5 or 6 inch riser. Nice little rocket.
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
O.K., I am sure that we will have several members jump in here with comments . Please not only watch the U-Tube Video but watch it on the U-Tube site so that you
can review the constructors Comments regarding his use of this system !
The Video poster created this system as a emergency heater for short term use! And his location is Coastal Connecticut - I am sure that this system will allow him to
retreat into his basement and keep the very core of his house from freezing - minimal protection up stairs - With a 4 X 4 inch feed tube this stove will need lots of
attention and frequent feeding!
This system was build in Dec 2013 ,and only used a few times, so longevity is totally unknown- I am sure we will get updates as the system gets used, as shown in
the video his system runs to hot to touch and for that reason we Must assume that much of the Rockets efficiency is being lost up the chimney.
Also note that this was part of a controlled small diameter Rocket project and much help was received by the builder from this site and Donkey's site, and that he has a
previous video of an outdoor Rocket -Chimnea combination build. Again it was not specifically built with the most efficient use of the wood burned as a primary function!
Finally He recommends that anyone trying to build their 1st rocket mass heaterRMH, Go to Rocketstoves.com to get the brand new edition of Evans' and Jackson's
''rocket mass heaters''.
Until some innovator creates a good sub 6'' system, lives with it for at least one heating season, and then shares a set of building plans ( to be followed closely ) I do
not feel that I can suggest that any one who has never built and lived with a RMH attempt a sub 6'' system for a 1st build ! Y.M.M.V. For the crafts ! Big AL
Success has a Thousand Fathers , Failure is an Orphan