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.

Hello again,

Silence was not inaction. The RMH is up and running, commissioned on the last day of fall, so I "got it done before winter" by about 4hrs. It has been a busy fall, both with this and all the other projects (not to mention work), so I am sorry for the lack of in-process posts, but this way all my mistakes can be solely blamed on myself.

We've been burning it over the past few days, and gathering data from the thermocouples, and I am pleased to report that it seems to be working, and even more pleased that my wife agrees that it seems to be working. For lack of a properly formatted SD card (or maybe a small-enough one, my reader is old), I do not have the temperature graphs ready yet, but those will be forthcoming after I fiddle it into cooperation.  We burned a batch around 9pm, and this morning at 6am the outer skin was ~25degC and the inner temperatures were still in the 60-70degC range. I was quite pleased.

I must again thank Peter, Thomas, and Glenn in particular for your excellent advice and guidance, whether or not you knew you were giving it. Glenn's build (linked in Thomas' post above) was the nearest template to what I wanted, and where I have differed, I will take blame for failure; where I have followed, I will give him credit for success.

The final outer bell size is about 3.5ft wide, 6ft high, and 4ft deep, made of clay common brick. Most of the outer bell is mortared with regular Type S masonry mortar, there are a few places near the firebox door where I used Rutland refractory cement. Yes, the masonry is not pretty, that's my doing, we may clean it with an acid and scrub to get it red again (my wife's preference) or plaster it (my preference), but right now it is what it is.

The two K-type thermocouples monitoring it are positioned roughly above the inner bell riser and roughly on top of the insulated firebox (just inside the outer bell skin there, it will become clearer with photos), and we are now getting burn temperatures about 580degC at the top of the riser, and up to about 150degC on the top of the firebox. The outer mass seems to have fully dried after about 3 days of use, and is a pleasant 25-35degC. Here is a short video of it burning: [youtube]https://youtube.com/shorts/xKOSKjSKZBs[/youtube]

The photos are numbered in sequence of the build, and I am supplying a short caption for each.

1) The layout on the floor, loose. I started with a plinth support for the core, but abandoned that in favor of a metal rack for better dimensional control over the core floor height and for cleanout access.

2) Old stovepipe from the wood stove, with a rectangular aperture cut in it to allow the air nearest the floor to come in. Rectangular apertures can be treated equivalent to circular by making the hydraulic diameter (Dh) match, where Dh = 4* A / P where A is the hole area and P the perimeter.

3) The cleanout is a stainless blast gate from Zoro, it didn't have to be that fancy (it's cool down there, relatively speaking), but I like it. This was a major reason to ditch the plinth support and have legs. I can get a shop vac tool all around the bottom now.

4) The basket for the core, which has six legs and also rests on the inner bell walls. Note that I only have a single layer of brick at the front at this point, which is a departure from Glenn's plan. Time will tell whether that was a reasonable call or not, but you will see a double front further up.

5) The door frame, which has its own legs on the floor, and also ties into the core basket with bolts for stability, somewhat hard to see, but they drop into the wide crossbar at the back of the core support, and make an angle brace for the front door support legs, which are nice and stiff.

6) The core takes shape, following Peter's 6in dimensions. Very thin fireclay in the joints, which was a surprisingly fun material to work with, way nicer than the refractory mortar.

7) Assembled core, and I did choose a notched brick arch for the top, it kept the same perimeter of the core, and avoided casting a slab. This did not change the door aperture size, which remains rectangular following Thomas' plans.

8) 1.5in Superwool all around the core, tied on with stainless wire.

9) Here is where I accommodate the arch with a pretty aggressive corbel. I do have a steel plate supporting the top, and this is all done with refractory mortar, since it gets warm, though not, so far, above 160degC, which is not a significant stress on the concrete. This is a very good article about Portland at elevated temperatures (but NOT firebox temperatures!): courtesy of Periodica Polytechnica Civil Engineering.

10) Here, above the body of the core, I add a front face to the inner bell, and transition to firebrick for it. This rides on angle iron, which is supported on the inner bell common brick as well as with a support leg resting on the door support angle iron, so there should be good support all across the span, which is only 25in. The second thermocouple tip is located just below this support beam, more or less on the superwool on top of the core, just inside the single-layer common brick front wall. I wanted to know how hard I was stressing this beam, and so far, so good, it's peaked at about 160degC as noted above.

11) Here you can see the superwool trimmed to the riser top, and the firebrick inner bell growing.

12) Here is where I checked all my interior surface area calculations for the last time, which required one more layer of firebrick to hit the 57sq.ft goal. The final ISA came out as 57.6sqft, though I did not calculate error bars on the value.

13) Many thanks to Thomas Rubino for his excellent and very usable door plan and parts kit. The snotty welding is my own failing, but the design is great.

14) The top is built a little differently, so it may be worth a few words and pictures. Glenn's design has his riser exhausting against the angle iron which supports his uppermost firebrick layer. This did not sit well with me (though I won't knock his design in the least!! accept this as my engineering neurosis). So, instead of layering it steel-brick-wool, I chose to layer it wool-steel-brick at the top, and I formed a basket of ultrahigh temperature stainless rods (303 series, I think, but $7/rod from McMaster, I used 9, not too bad). In this basket I laid my large piece of superwool. Exhaust gases from the riser will hit this refractory assembly first, and the steel angles which support the roof will not get so hot. Thus, the upper superwool is not used for sealing (as I believe Glenn's is), but is used for insulation. More on sealing later.

15) You can see my upper thermocouple wired to this stainless basket, located about over the exit to the riser - it's not exactly at the stagnation point of the flow, so it won't see quite the highest temperatures, but it will be quite close.

16) Another view of the basket and thermocouple.

17) Laying the superwool in the basket

18) I said I'd say more about sealing. The roof of the inner bell is not perfectly sealed, as the firebricks on their beams are set dry on a 1/8in gasket of superwool. The edges are stuffed with the same thin stuff, doubled, and this is not airtight.

19) The topmost common brick layer sits on a shelf I made on the outer bell by turning the topmost bricks on edge, and then sits on the firebrick inner bell top. I used more of the thin superwool to give it relief from shifting, and covered the angle iron joints with it. The top bricks laid in beautifully in integer rows, almost like math actually worked for me. Still not airtight.

20) Perhaps the most gratifying one. The first real wood fire in it, with the stovepipes hooked up and everything. The flames went back and it roared gently. It has gotten a deeper voice as it has grown up, er, dried out, and runs hotter now with a stronger draft. However, a modest but unpleasant amount of smoke oozed out the top.

21) Here was burn #1, which I did rather cautiously. I had actually put tea light candles in the core for a few days, just to start the dryout as I was completing the outer bell, and that may have helped season the surface a bit, but the burns have only heated up from this one.

22) So I fireclayed the top to stop the leakage, and that has been very effective. No more smoke.

23) And here it is with a much more respectable burn temperature. The stovepipe stays hand-touchable.

The following are FLIR camera shots, they are tiny, but fun.
24) Here's the stovepipe. The FLIR puts an averaged temperature number corresponding to the target circle region (+/-).

25) You can see the wet brick at the top, those were the last courses laid, and they are still cool in this frame.

26) The door gets warm. It's finally cured the high-temp paint, which was a bad smell for a while. I just used Krylon BBQ paint from Amazon.

27) Here is the large sidewall, quite comfortable and pretty well dried.

Time will be the test, of course, but so far, I am very thankful that this (long, long) project seems to be at a functional place. I have to tidy up the front with a better faceplate to hide some superwool around the door assembly, and I am sure there will be more to learn, but we're all enjoying the house being warmer now that the days are staying cold.

Happy heating,
Mark

Looking Good, Mark!
Great photos and write-up.
I'm with your wife about the bricks; they are so beautiful with their deep red color,
Cleaning the concrete mortar mess off the bricks can be a real chore, a large portion of my smoke shack is still grey!
After you get them cleaned, try my trick: carefully apply cooking oil; it does wonders!

Good job Mark!

Mark Miner wrote:I chose to layer it wool-steel-brick at the top, and I formed a basket of ultrahigh temperature stainless rods (303 series, I think, but $7/rod from McMaster, I used 9, not too bad).

Mark is it 303 or 309? 303 is just easier to machine compared to 304. 309 - having higher nickel contents, is used for high temperature application.

Mark Miner wrote:26) The door gets warm. It's finally cured the high-temp paint, which was a bad smell for a while. I just used Krylon BBQ paint from Amazon.

I recommend blackening steel with traditional linseed oil, Weed torch, propane bottle and food grade linseed oil. I have blackened hundreds of objects. It took around 30 minutes per oven door.

Nice build!

Yes indeed Mark, I think it looks great too.....what a great Christmas present you gave to yourselves.
I like the way you arched ceiling of the firebox and the interesting way you supported the superwool on the roof of the bell.
May it keep you both warm for years to come.

Thanks all for the kind words.
Cristobal, it is 330 alloy, sorry I anagrammed the number, but it is rated to 2500ishF, so I do hope for good service in that position.

Thomas, you are on the right side of that position, no doubt. Heigh ho for elbow grease!

We are enjoying a thoroughly saturated warm mass for Christmas. Hope you are all happily heated this season!