Design Question - Should I use a 6" or 8" RMH?

I’m designing a RMH for a 546 ft2 addition in Northern CA, for my wife and I.  We have a woodstove in our existing house, which also provides hot water, the RMH will heat the addition.  The footprint of the RMH is about 8.5 feet by 40 inches wide, about 18” high.  Based on the sizing calculation spreadsheet available on bathrocket.eu, my heating needs needs fall somewhere between a 6” and 8” system.  I am leaning towards a six inch system, largely due to space- I have a fairly small house and for 8-9 months of the year do not need a fire (and then have 3 months of winter with the stove going all the time).  The footprint of an 8” system would be just a little too large for us, though do-able.  I’d love to hear some opinions, especially from folks who own and operate a RMH .What do you recommend, a 6” or 8” system?  Folks who own and operate a 6” system – do you feel confined by the small wood?  Does it require more than 2 firings per day?  What space are you heating with it?

Here are some pros and cons (Thanks to Luke Parkhurst who is helping me with this design and install):

Eight Inch System:
Pros: Can accept larger diameter wood. It will most certainly be large enough to heat the space on even the coldest night. An additional bell/masonry piece could potentially be added in the future to capture more of the heat.
Cons: Will need to change out the existing pipe. Slightly more expensive (core will cost $325 instead of $200, plus the 8" pipe will cost more). Needs a 55 gallon drum/slightly larger core footprint as well. Would likely be sending extra heat up the chimney (i.e. losing some efficiency/burning more wood).

6"System
Pros: Smaller footprint.  Ideal for burning prunings from the orchard and smaller limb wood.  Existing piping is already 6". WE have an existing . Given the size of the bench we discussed the 6" should provide plenty of heat to charge it up.
Cons: Restricted to smaller wood, more time splitting, possibly not warm enough for the coldest nights(?)

Hi Mark;  Big Welcome to Permies!  And a big welcome to the wonderful world of rocket science!

Congrats on choosing to use a rmh in your addition!
Are you thinking of a J tube?
I know that Luke sells the Walker J tube CF board stove.
I heard a rumor that he also  has the batch plans and could possibly produce one. (the cfb portion )

The batch rocket site spreadsheet that you found might be giving you btu numbers for a 6" batch / 8 " batch. Not a J tube.

I have lived with an 8" J tube and traditional piped mass since 2013.
I built a second 8" J tube in my shop last summer.
This summer I have converted the shop 8" J tube into a 7" batchbox and I am hoping to convert my other 8" j tube into a 6" batchbox before this next burning season gets started.

I loved my J tubes, but I love my batchbox even more.  
Here's why...
Way more heat, lasts longer, wood is not split as small and it lays horizontal,fire burns unattended for over an hour... coals last longer,  there is a door so no open flames....
I think they speak for themselves!

So all that to say,  I recommend a 6" batchbox .
A 6" j would need wood every 35 minutes or less, with multiple firings needed to get the mass up to heat.
I think you will be much happier with a batchbox design.

Hello Mark,  Welcome to Rocket Central.

From everything you said, a 6" batch box I think would be the way to go as well.
I have a 6" batch box in my shop that I have converted from a J tube around Christmas time, so half a season of burning with it.
Never have been happier to make the change. A J tube has it place, but not somewhere you don't live in to feed it all the time.

I don't consider a 6" batch to only be able to handle small wood. It may not be able to handle something you could throw into a large box stove and smoulder all night (unless your looking to clog the chimney with cresote), but its much more capable of handling bigger wood I used to feed my J tube and more of it at a time.

I don't live in my shop so 2 firings a day works quite well for me. It really depends though on so many factors how much burning you need to do to maintain the temperatures that make you feel comfortable. How well insulated is your house, type of wood you burn, temperature outside etc....

Thomas and Gerry, thanks so much for the prompt and informed response, it's a huge help!  I communicated with Luke and we're going to look into the batchbox design, sounds ideal for my situation.  I'll be sure to post some photos of the project as it takes shape.  I'm excited to dive in, Ive known Ianto Evans since 1987 and have been wanting to make a RMH for years!

Thanks again for your input, I truly appreciate it!

Mark DuPont

Hey guys- just wanted to show you what Mark and I have been up to. I built my first batch box based on Matt's plans and we fired it up a couple days ago. We had a lot of fun!

FYI- for anyone watching, I received some important feedback from Matt and Thomas and it turns out we are missing a key part in the batch box shown above. We didn't include a primary air intake in the door of the batch box (the plans I was working from hadn't included this- they are now being amended). It wasn't as big of a deal in our trial because there were enough small gaps in the stove for air to come in, but it would become an issue once the stove was properly sealed up.

I'll get that fixed and we'll try and shoot another video in the near future.