Exhaust sizes

We have a chimney liner with a diameter of 12.5cm or 4.9 inches roughly 7 metres long. Are considering building a RMH using this exhaust and would like to know how these sizes would potentially effect the build?  We have the builders guide book and would like to know if there are any specific chapters that deal with sizes etc?

Thanks in advance, Marcus.

Hi Marcus;  Welcome to Permies !

Trying to use a 5" chimney will limit you to a 5" RMH.
You may be able to build a 5" batch box with a brick bell and use your existing chimney. Check out this build , it may be just what you need.  https://permies.com/t/43809/Masonry-stove-diy-build-feasible

EDIT)   I would say that chapter 6 describes the need to keep a constant csa in your exhaust path.

Well, with a tall chimney like this, i would try to go for a 6 inch batch with bell.

Hey Thomas,
Thanks for your reply, when you say we can use our 5" flu with a 5" rmh is that a problem/ normal? The flu gives perfect draw for our wood stove currently ,so we just thought a rmh would give better performance ie less wood more heat plus heat storage in the mass of course.

M.

Hi Marcus;   Normal is same size. But as Max suggested ,with your tall chimney a 6" could be just fine.

Thanks Thomas,
Only reason I ask is that at present we have a wood stove with a 150mm exit attached to a 125mm chimney liner going up a 7m chimney and away. Thing is, yes it all works but to heat the room takes forever and lots of wood that has to be cut just so and for a lot of the time at the beginning the stove is not in the ideal " burn zone" and as such will be creating creosote I'd imagine! So our worry is that if we deviate from 125mm (which we are stuck with and can't change unfortunately) going with 6" for example then our newly constructed rmh might work just as badly! Not knowing a huge amount about all this we just need a few solid pointers in truly simple terms. Anyway thanks for sticking with this much appreciated!

M.

Hi Marcus;    I see your concern and I suggest just going with a 5" batch box design.    I sure that Max is correct and a 6" would work …. BUT … we know that a 5" will work.