Woodstove to RMH Conversion - My Planning and Execution Thread

The heating season is behind us, and after one winter in this house burning gobs of wood in a Blaze King, I would like to work towards a RMH to do the heating. The house is large, the Blaze King is the biggest one on the market, and the former owners have some clever natural convection and forced convection features to allow one stove on the ground level to reasonably heat the house. However, I look at all the soot in the chimney (which was cleaned before we moved in last October, right at the start of heating season), and I know we are throwing away a lot of energy up the flue. So, this is the beginning of my RMH journey. I have done masonry work, I have worked a bit around refractories in a foundry, but this will be a learning experience!

The basics - The usable area for the RMH is tucked in a corner, and thus limited in size. It will need to fit in a plan area of 4 x 6 ft, with the chimney aperture about 7ft up the wall, where it transits to the garage and goes vertical for about a 25ft rise to clear the roof.  It's a rather long chimney, as it's a rather tall house. The chimney is double-wall stainless, 8in inner diameter, 10in outer diameter. This will be the chimney for the RMH, as it's the only game in town, and I don't want to change it for laziness and permit-y reasons. Pieter van den Berg of Batchrocket.eu suggests that the riser diameter must be <= chimney, so I can do up to an 8in riser. I expect I may need to lay a fire-rated wallboard layer over the interior drywall here, and I will give some thought and analysis to how close the bell wall should be to the house wall - comments welcome on this if anyone has good data.

My intent is to experiment with one or two bare cores outside, to practice the build, and then build the real one (so it is not my first build - I would like it to last). The Blaze King heated this house for ~17yrs, and is beginning to show its age, as there are a few places where light can find a way through the walls now, though all are in suction areas, thankfully, and the CO2 and CO monitors have never indicated any buildup of either gas. So I have until the next heating season to figure this out, which should, if I hustle, be enough time.

I will keep this thread updated from time to time with progress, and perhaps some of you will be so kind as to critique or comment if I say or do something known by rocket scientists to be a bad idea! I was very much inspired by Glenn's awesome work here, so thank you to all who have contributed RMH wisdom!

Happy home heating,
Mark

How exciting, Mark!
It sounds like you are ready to go.

Are you thinking of a single or a double skin bell?
A 6" batch will fit your build plan much better than the larger 8".
Although your home is large, once you get a bell up to operating temperatures, your home will be amazingly warm.
You can always load a 6" twice to match the 8" in performance.

Take a look at the ISA numbers for a 6" & 8"
The box is bigger on the 8", so it would burn longer, but the corresponding Bell size is larger.
A 6" batch is plenty big, especially as Peter approved up to a 25% increase in the box length.
As I mentioned, you can load a second load of wood if you feel you need more.

It is a radical life change to switch from heating a steel box (and the great outdoors) to heating with bricks.
You will be blown away!

Once you go brick, you will never go back!

Code for masonry heaters (with the code-required double skin) is a minimum 4" gap to combustible walls. For an exterior wall, you would want that gap to allow heat circulation into the space rather than only to the exterior wall.

Howdy, Permies,
I've not been idle, just busy with lots of different things. Thanks to Glenn & Thomas for your input, and to Peter for lots of rocket-y project inspiration in the forums recently, too.

Specific answers to specific questions: I am planning a double brick bell, 2in air gap between the layers (because 4in x 8in brick). I am planning a 6in system, as Thomas astutely suggested, and I get ~58ft^2 of ISA in the layup I have sketched.

I've rough-sketched the space I am working with, dry-stacked a mock core to get a sense for the size and brick count. The core concept is straight from Peter's website, batchrocket.eu, but I do have a few questions.
The chimney as shown is 10 courses high (47in from plinth to top due to base layer), but this will push my roof high, and increase the ISA for the footprint that I am targeting. I do think I may want to extend the body by 1/2 or 1 brick, because some of the log rounds I get are pretty long.

Should I narrow the inner bell to reduce ISA, or can I make the core chimney shorter?

I know the throat, where the draft is pulling into the chimney, is a very sensitive region for setting the fluid flow. Is dead-center best, or is a slightly-off-center throat effective? Dead-center demands a dual-vortex in the chimney, as the flow splits at the back wall. Off-center will set the direction of a single vortex, which will have more energy and less drag. I'm liable to import "conventional" combustion ideas into RMH-land, so I want to be careful. I'm leaning on Peter's spreadsheets for details of the cross-section.

Has an off-center throat been explored?

In order to keep the chimney relatively near the wall, can the outer bell be notched/skipped at the exhaust region? The brick structure would be weaker then, and lots of half-bricks would be needed, assuming it's best to not tie into the inner bell there, but it would buy a much tighter installation.  The wall has a chimney penetration that I plan to use, as it's a busy wall near the electrical panel, and I won't cut new holes in it, so I can't penetrate the wall low and do the rise on the far side.

Is notching the outer bell to embed the chimney a dumb idea?

I'm assuming based on some ash cleanout discussions that a pair of service doors through the bells, near the chimney intake, would be advisable. I'd think this ought to be two separate doors rather than an assembly, to avoid a thermal bridge between the bell walls. The operability of the doors can be poor (like, threaded fasteners), since it sounds like the frequency of use would be every-couple-years.

I'm game to embed thermocouples in the nested structures, and would love some suggestions of where key points might be. K-types should be fine, cable lengths may be restrictive (3, 6, or 9ft)

Thanks again to all the rocketeers here! I'm hoping to get lots of vicarious experience from y'all, to reduce the number of mistakes I have to make along the way, but I'm sure there will be plenty left for me to make!

Happy heating,
Mark

Hi Mark;
Peter allows some variation from the stock numbers.
Specifically, he has said a 25% increase in box length is acceptable, and he also allows for a shorter riser.
From https://batchrocket.eu/en/building#dimension  "The height of the riser is 8 to 10 times the base, measured from the firebox floor."
With a 6" core having a base number of 4.32, multiplying by 8 allows you to have a  35" riser

The port is best located directly in the middle of the box with the six inch secondary tube directly in front of it.
I assure you that Peter checks every possible arrangement, and the one he has posted is the best variation possible.

Hey Mark;
Glenn built a beautiful 6" double bell, and he documented it extensively.
Here is his material list https://permies.com/t/248275/Batch-Rocket-Double-Skin-Bell
Here is the build itself.   https://permies.com/t/238503/Batch-Rocket-Build

Mark Miner wrote:Has an off-center throat been explored?

Since you asked, as a matter of fact, it has been. It turned out that the off-center venturi was only as effective as the centred one when the chimney draw was really strong, wind force at least Bft 7 with gusts. For this reason, I ditched that sideline.

Mark Miner wrote:Is notching the outer bell to embed the chimney a dumb idea?

In my book, it is. Please, try very hard to avoid compromises, otherwise you'll get problems later. Just my two cents.
It would be much more clear to the reader to call every part of the heater by its accepted name. There's a firebox, riser, port, floor channel, bell wall, top gap and whatnot.