100-yr-old 2-story house, 2000 sq ft, with two chimneys - adding RMHs

I'm renovating a 103-yr-old house in the mountains in Sparta, North Carolina that we bought last year.  I want to install a couple rocket mass heaters in place of the existing fireplaces.  It's a 2-story house, 2000 sq ft, with two chimneys and no other functioning heat source.  Has anyone every done anything like that?  I survived on electric space heaters and $500 energy bills through the first winter.  I'd like to NOT do that again - lol.  I have 2-3 young workers who can help, and me (not so young), but none of us has ever built anything like this, so could use guidance.

(I am reposting this from the "Builders" forum - sorry, not too familiar with forums and stuff.)

More details:  The house is wood everything - wooden walls, floors, ceilings, and also is built with a crawl space underneath - crawl is something of a misnomer because it's not really tall enough to get up on your hands and knees.  There is rock foundation around the chimneys, but I'm guessing an RMH would probably extend beyond the chimney foundation, so we would need to add more support under the house - not an easy task.

I'm attaching some pictures.  I would be interested in seeing some quotes from people who know how to build these and also have some awareness of structure needs for supporting the weight, and would be willing to come to Sparta to help us out.

Hi Rebecca;
Thank you for moving your post, it will get more interest as a stand-alone post than it would have where you originally posted it.
I am glad you figured out how to add pictures, as they help immensely.

Before I overload you with information and terms you may not understand, let me ask how much you know about RMHs.
Have you been reading all about them for a while or did you recently come across them?
They seem overwhelming and difficult to build, but they are not.
You and your helpers can do this with some guidance along the way.

Do you know if you want a J-Tube RMH or a Batchbox RMH?  
I recommend a Batchbox design RMH, rather than a J-Tube, although more technical to build, a Batchbox burns hotter and longer with much less feeding wood than a J-Tube design.
On my website https://dragontechrmh.com/ you can find helpful books on Batchbox core construction and Batchbox door construction, as well as some needed metal parts.
You would be very lucky, if there is an experienced builder close enough to your location, most RMH builds are homeowner-built.
Be glad you have younger helpers to assist you with construction.

Let's start with floor support on the first floor.
A 6" Batchbox RMH with a brick bell will weigh between #2500- #4500  with a footprint of apx  4' x 6'
After looking at your photo of the (belly) crawl space, depending on the exact location of the existing chimney and the number of floor joists the weight spans, you may want to add a few more supports just to be sure.

The photo you showed with a wood burner, appears to sit in front of a bricked-over traditional fireplace with the stove pipe entering through the brick.
This arrangement can be modified and a brick bell can be built in place of that box stove and continue to use the old hearth as a part of your bell.
Have your brick chimneys been inspected at all?

The other photos are a bit confusing so I'll stop for now.
I would concentrate on completing the first-floor build before moving to the upstairs build.

Hi Thomas,

I subscribed to permies under my old e-mail address something like 10 years (?) ago, but was just getting the newsletter and never posted or anything.  Then last year, we bought this property, and I'm guessing maybe a newsletter mentioned RMHs and it caught my attention.  I knew the heating system in this old farmhouse was using an old heating oil tank with ducts and vents and it didn't work, so I was looking for other solutions.  I looked at RMHs, and I figure if they work for people in Montana, they'll be good here too.

In March, I went online to permies and bought the RMH movie for $15 to get a little more info on them.  I finally watched the whole thing this week and bought some more of the instructions so I could get a clear idea on how it works and whether one would fit in the existing chimney/fireplace areas.  That is the extent of my knowledge of RMHs.  I believe I know what a J-tube is.  Is a batch box just one with a bigger box so you can load more wood at a time?  I'm not really clear on what would fit here.

I've uploaded a few more pictures - hopefully they help to clarify our layout.  The house is a little longer N/S than it is E/W.  There are two separate chimneys.  What I'm referring to as "Chimney 2" in my pictures is in the middle of the north half of the house that comes down in the wall dividing a bedroom and an office, with fireplaces facing into each room, sharing that chimney.  I have not actually tried to use those fireplaces yet.  (The picture I took of the fireplace facing into the office in my previous post has a lot of old insulation and construction trash in it at the moment.)  The other chimney, "Chimney 1" comes down in the midst of the southern half of the house in the wall between the kitchen and the living room.  We did use the old wood stove a couple times during the winter and there were no immediate problems, though I agree that an inspection would probably be good before using any of the fireplaces or wood stove regularly.

For floor support, yes, I think we'll need to add more support.  When we were redoing the foundation a couple months ago, we could see that the chimney foundations were about 5 1/2' x 6', but for RMH we probably need a little more.  I wish I had known more about RMHs before we did the foundation, since we could have built in the extra support at the time.  But we can adjust.

I hadn't been planning to put in anything upstairs, though I figured we could maybe run something up to help with heating the bedrooms on the upper floor.  I was mostly hoping that since heat rises, if we heated downstairs, the upstairs would benefit enough from it through circulation.  On the second floor, the north chimney (#2) is completely inside a wall.  The south chimney (#1) coming up from the kitchen/LR is exposed in a bedroom which has access to the attic.

Building an RMH seems not too difficult, but really what concerns me most is the structure, making sure there is enough support underneath.  I'm also not sure how to convert an existing chimney - I'd probably cause it to tumble down while I'm trying to rearrange the bricks.  [facepalm]

Hi Rebecca;
Wow, for a 100-year-old house it sure looks good!
I like what I see of the chimney, it appears to be professionally built and probably is in fine shape.
One nice thing about an RMH, is the chimney air temperature normally runs from 150F - 250F nowhere near the high temps of a regular box stove.

A J-Tube RMH is the original design.  
Wood is split small and stands vertically in the open "feed tube"
Wood is added every 30-60 minutes until your mass and home are heated.
The fire burns horizontally at the bottom for apx. 12" and then goes up a short 4' chimney called a "Riser".
At this point traditionally a barrel is placed over the riser, forcing the heat to go down around the outside of the "riser" and into a horizontal pipe system that is encased with a rock and mud "cob" mixture creating a solid mass that heats up and slowly releases that heat overnight.  
This mass was commonly used as a heated bench or day bed.

Batchbox RMHs differ from J-Tubes in that they burn longer and hotter.
The wood is split but not as small as the J-Tube requires.
The wood goes in horizontally just like a traditional wood stove and a traditional door is used.
Fires last apx.2 hrs and coals are there another hour after that.

These days bells are becoming the predominant form for RMHs to put their heat in.
You can utilize a Bell with a J-Tube or a Batchbox design.
Similar to the Russian Masonry heaters that have been in use for hundreds of years.
A "Bell" also known as a stratification chamber, is nothing more than a large box for your RMH to put its heat into, the chimney outlet from the bell is located at floor level.
The super hot air from the riser goes to the top of the Bell and the slightly cooler air starts sinking towards the bottom where it finds the outlet and rises up your chimney.
As that air sinks it heats the entire Bell and that bell shares its heat for 12-24 hours.
Depending on conditions, a Batchbox RMH is only lit twice a day with no fire at all overnight.
There is zero chance of a chimney fire as the crazy high temps (2100F) incinerate almost all particulates including creosote.
The most common material used for bells are clay bricks, but large steel tanks lined with rock or brick can be used as well.

The preferred mortar to use is clay and sand, and not a concrete mortar.
This is nice for several reasons.  
First, it is easy on your hands, unlike concrete mortar which is hard on your hands.
Second, there is no time limit on mixing mortar before using it, unlike concrete where it hardens if you do not use it up in time.
Best of all clay mortar can pop off, be rehydrated, and used again! Try that with concrete!

Yeah, we redid the siding.  Here are the "before" pictures - old siding with lead-based paint...that had to go.

Thank you for the info.  I'll take a look at your batch box books as well, and we'll see if we can figure this out.  I'll post as we go, though it may take us a while.

Beautiful house Rebecca, thank you for sharing.
I wonder if you've seen the Matt Walker tiny house cookstove? I notice one of your chimneys is in the kitchen, so I'm wondering if you wanted to cook on a rocket heater too? You could have the cookstove in the kitchen, with a bell radiating in the adjacent room. Maybe another small batch/bell in the other chimney for colder nights.
There are some useful links on this therad here although it is a little old and there maybe better examples now. As Thomas said a batchbox needs less tending in use and they have firebox doors. I think they tend to give out more heat for the exhaust size too - this one is a tiny one at Paul's place, Wheaton labs, that may be of interest.

Dunno, but offhand an old heritage house with a century of quirky bits may be an advantage -- for insurance!

They will already assume it will burn down from ancient wiring etc. If your insurance guy walks through and notices a couple of "old school solid fuel brick heating units" they won't be shocked.

PS, make sure they look old, antique, neglected, dusty. Collect spiderwebs and apply haha!

Hi Nancy - thank you for those links.  Yes, definitely want to do some kind of cooking extension into the kitchen.  There's Italian heritage on my husband's side, and a love of baking bread and pizza, so naturally a stove/oven is now a required part of the project.

@Douglas - haha yeah.  In that last picture you can see one of the old wires that was run on the outside of the main wall timbers underneath the siding between the first & second floors.  I was like yeah, we, uh, need to reroute that...  And yes, we have an ample supply of cobwebs and spiders we can dust things up with.  Just gotta figure out how to build it first, heh.

Hey Rebecca;
After hearing about your husbands Italian heritage, I thought you might enjoy seeing my outdoor kitchen with a Walker BBQ black and white oven including a Pizza Port built in the door.

Rebecca!  What an incredible project!  I love what you're doing to renovate the home.  I hope you can install RMH technology into the home. I wish I could have done it with mine, but I was dealing with a  home that was only 50 years old.  Sigh.

I built a smaller RMH in our very old farmhouse and tied it into an existing brick chimney. I posted about it here on permies and the link is below. It might give you some ideas on bracing and construction options. It works incredibly well.

https://permies.com/t/95578/Farm-House-RMH-full-house

Wonderful Caleb - thank you!  I read through it quickly and there's a lot of good information in there.  I will go through it again more slowly.  Do you have any more recent pictures now of the finished bench?

Really appreciate the information!

Rebecca,
I will be down your way in October-November on another "tour" building rocket heaters and earthen ovens across Tennessee and North Carolina. Drop me a note if you want info on events or some design consultation.
--Uncle Mud (aka Chris McClellan)
info@unclemud.com