Inanc Akcan : Just a little difference in the overall smoothness of the interior wall of your existing chimney will make or break this build ! Lets talk about chimneys-
I am going to talk about Two (2) chimneys, the 1st is the Internal Chimney/ Heat Riser, It will gain a lot of "Draft" from just a little increase in its height as compared
to the depth of the Vertical
Feed Tube, there is a point where a really tall Heat Riser Will 'Draft' so well, even discharging through a 5'' diameter final exhaust Vertical
Chimney as to push the hot exhaust gases past the Thermal Mass so fast that very little heat energy gets transferred into the Thermal Mass!
A high enough External Final Chimney with no internal restrictions, and that rises above all other building parts, a Couple of meters above the Highest point of the house
and other close objects, should 'draft' well. Increasing the height of this chimney will also increase this effect ,but not as dramatically as can be achieved with an increase
in the Height of the internal Chimney- The Heat Riser !
So- Allowing for the other points where a slight variation in the Constant Cross Sectional Area can cause the
Rocket Mass Heater R.M.H. to fail, it is possible to build a
6'' system that will 'draft' through a very smooth walled 5'' Final External Chimney, I expect that you will need to continue your 6'' system through your Thermal Mass
and create a very
generous final clean out that will allow you to build a small fire to prime your systems 'draft', so that your final 5'' chimney will accept the hybrid
system you are trying to create.
As I visualize what you are trying to do- Yes, I think it is possible to build a R.M.H. with a 6'' System, and exhaust it through a pre-warmed 5'' final vertical chimney,
IF
you are willing to
create a Custom Heat Riser for your system to allow it to draft ! How much more additional Heat Riser height you will need to create the 'Draft' that
your system has to have, and
STILL Allow for the Hot exhaust gases to flow slowly enough through your Thermal Mass is something that can only be discovered by
you ,And will probably require your to build a complete system, and give it a good long trial through multiple heating seasons, possibly re-building it several times, to see
what will work for you in that location with that final chimney !
I would not try to create a hybrid system that Uses 5'' pipe for both the Horizontal flow through your Thermal Mass and your final vertical chimney ! I hope this is clear
and that it will help you make your final Decisions ! For the Good of the Craft !
As always, your comments and questions are Solicited and are Welcome ! Think like Fire ! Flow like a Gas, Don't be the Marshmallow ! A.L.