Building RMH in NH

[rjdudley]

Phew, thanks to Matt I'm finally able to get and chat with everyone about building my first RMH!
So, I've been reading and re-reading the posts here as well as watching/rewatching Matts video about building the form for the burn chamber and I have several questions to start with:
1) Matt, after watching your video about the Cast Core Plans I noticed in the comment section on YouTube that you said it was ok/preferable to use refractory cement. So I bought some Kast-o-lite 26 and this week will use it to make my burn core. Is two inch thickness enough? I'm a little nervous about burning down my house, would you recommend 3"? or stay with the two?
2) Would you happen to know how much water to mix into the Kast-o-lite? I can Google search, but, if you already know that would be great.
3) Do you think it needs to sit and cure for a while or can I just fire that baby up after 24 hours or so?
4) In your video you added insulation fiber. Think I need that or is the Kast just fine alone?
Ray

[matt walker]

Welcome! Sorry about the registration snafu, glad we finally got you on board.

I do think that commercial refractory materials are going to give you a much stronger product than what can be achieved with my DIY mix. That said, it's hard to recommend a thickness for best performance. I'm pretty much an amateur at all this and am learning as I go, so take what I say with a grain of salt.

So, I have been playing with castable refractory for the last year or so and have built quite a few cores in varying thicknesses, and I can't say definitively that thicker is necessarily better. Even heat transfer is the key, and without expansion joints, the core will almost certainly create it's own. Read that last part as "it's gonna crack." Now, my take on it is that cracks are fine as long as the thing doesn't collapse in on itself, so I choose the easy one piece mold approach. The professional way is to create a core made up of pieces with expansion joints, but even doing that I've found that it will still crack. I feel like the heat cycles are too short and too extreme inside these things to expect anything else, but I'm sure somewhere there is a refractory expert shaking his head at me right now.

So, know that going in, plan on using it like a core. As long as it's contained from the outside with cob or something to hold it all together, you should be fine. I think cob is the perfect solution. It can be mixed with Perlite to provide an insulating layer all around the core, and it's fairly elastic so it tends to do a fine job of holding everything together during expansion and contraction cycles.

As for the water, I haven't worked with Kastolite specifically, but I will say follow the manufacturer's directions to a T. The castables are extremely sensitive to hydration variances in my experience. The difference between just right and way too wet is a tiny bit of water.

The professional refractory castings go through a pretty rigorous "dry out" procedure that is basically a slow and steady heating up to being pretty much kiln fired after a few days of consistent, controlled heat. I think they are all different, but the manufacturer should have data sheets up on preferred dry out procedure. That said, since I have no way of doing that, I usually just light em up and watch em crack. LOL. A patient man might find a way to approximate a real dry out, but I haven't found it necessary for my low expectations. Be warned, they also specifically say that flame impingement on the core during dry out will cause spalling, so my mold burn out technique is pretty much a no-no if you want to do it the right way. So yeah, I'm doing it wrong, but it works for me. YMMV.

I have added the fiberglass to most of my experiments, I feel like it just gives a little web across the inevitable cracks my slack process creates and helps keep things from collapsing. I don't know that it's necessary, and I do know that fiberglass vitrifies at way under the temps seen inside these things, so it might be doing absolutely nothing. My theory is that the insulating castable protects the glass fibers buried in it from the highest temps, so some remain to add some structural support across the cracks. I could be totally wrong, but I have the material here from old projects so I use it.

As you can tell, I'm a tinkerer, and I accept a certain amount of sloppiness if the function suits me. If I were a perfectionist and just wanted one heater, I might seriously consider purchasing an off the shelf core from Dragon Heaters. It will be very hard to improve on those since Peter has done such extensive, verified, testing. That said, it's just a heck of a lot of fun to play with this stuff for me, and I think for a lot of folks. So yeah, build it, and be prepared to enjoy it and get a bit wrapped up in the possibilities and experiments.

Finally, as for burning down your house, just be aware that the core is just that, a core only. It will not be insulated enough to set on a combustible material. It will for sure ignite anything under it if it is not protected with a significant amount of high heat insulation and air spaces. There's a lot of ways to do this, and depending on the install location some are more appropriate than others. A raised platform of Durarock or something similar, covered with a significant amount of perlite/cob or vermiculite board or something, is a good starting point. Be sure to test things out somewhere safe until you are comfortable with what's going on underneath the burn tunnel. There is a LOT of heat generated right there.

[rjdudley]

Sweet info!
Any ideas on embedding small wire mesh (something like hardware cloth) encased in the Kastolite for extra strength? Kinda like your fiberglass mesh?
Downside to that might be burning off any coating on the wire could cause fumes.
I was thinking of casting the core and then leaving it to "cure" for maybe a week like concrete, slow and steady. Although I know I'll never make the full week because I'm just itching to fire that puppy up! LOL.
As far as under the core/heating tubes I was figuring on:
1) laying bricks as a base with spacing in between them for airflow around them and also from front to back as well as allowing airflow between the mass and the walls
2) Durarock/hardibacker laid on top of that
3) Foil/reflective barrier on top of that
4) Then the 3-4 inches of cob onto which I'll place the 8" duct work.
Sound safe?
BTW, here's some information about using the Kast-o-lite 26 just in case anyone else is looking for it (cut and pasted):
"Kast-o-lite is mixed with 20% water by weight. Using a whole bag is recommended as the water can be measured more precisely then. ...mixed this in a big 5 gallon bucket with a mixing attachment to my power drill. You are supposed to mix for 3 minutes and then cast within 10 minutes. Afterwards, the pieces need to dry for at least 24 hours and should be covered to slowly dry out the surface.

The curing schedule for Kast-o-lite is quite complicated and basically requires a programable oven. Since the pieces are all 2in thick, the curing schedule is as follows: Heat up to 250F with a rate of 100F per hour. Hold for one hour. Heat up to 500F with a rate of 100F per hour. Hold for one hour. Heat up to use temperature with a rate of 100F per hour." (I'm absolutely sure this will NOT be happening here at my home!)
Ray

[matt walker]

Right on Ray, your plan sounds good with regards to the base make up. Thanks for posting the cure/dryout procedure. You can see why I do it wrong! Lol. As for the wire mesh, I get pretty wary of mixing metals in with the castables, and I'm including cob in this, due to the extreme differences in expansion/contraction. I haven't done what you are suggesting and it just might work fine, but I would expect it to actually tear the thing apart rather than keep it together. Curiously, and perhaps proving me wrong, is that as reinforcing fiber the refractory places sell bags of Stainless Steel needles for exactly this purpose. Good luck on your build, keep us posted!

[rjdudley]

Well, I did it! I built my first burn chamber using Matts incredibly helpful video. Thank you so much Matt! I have some details, some questions and some photos (assuming I can figure out how to upload them).
Details: Built the core chamber using the same measurements as Matt EXCEPT since I'm using Kastolite 26, a refractory cement, I made the walls of the chamber two inches thick. I was going to go thicker, but, after reviewing several sites that showed folks building their metal smelting ovens and using two inch walls I figured since I'm not melting metal two inches should be fine. Don't worry I'll let everyone know if it's too hot and I should've gone thicker.
Detail/Question: Matt have you ever used this stuff? It calls for 9.5 pints of water for every 55lb bag. Dang, it sure looked dry to me, but, all of the other sites warned about sticking to the manufacturer specs.
Detail: I ended up mixing up all three bags of refractory that I had, but, probably only used two and maybe a 1/3 of the last bag. I was a little afraid of trying to figure out the water/cement ratio since it should be done by weight and I didn't have any way to weigh out 5-10 lbs at a time. If anyone else decides to use refractory maybe you could sit down before and make yourself a ratio cheat sheet. Sometime like how much water per 27.5 lbs, then 18.33 lbs, then 13.75 lbs, etc. It wouldn't take long.
I'm really nervous about how dry the mix looked and so I'm slightly concerned about it setting up properly. But, I'll give a couple of days to sit and slowly cure then light a small fire and see what magic happens.
I guess I don't know how to post photos, if someone can let me know I'll put them up
Ray

[Lollykoko]

Ray, I think the easiest way to post photos is to load them to an online server like Photobucket, then put their link in the IMG box (above the reply box as you're typing) to add it to your post.

I'm looking forward to seeing what you've done and hear how it turns out to ramp up my enthusiasm. I am hoping to cast at least one (if not 3) cores before Halloween. Getting started is probably the worst part and the warm, wet weather we've been having isn't encouraging the thoughts of needing a RMH.

[rjdudley]

Thanks for the tip Lollykoko! Now let's see if I can actually do it:









They're not exactly pictures you'd find on the Sistene Chapel, but, they'll give you a good idea of the end product.
Lollykoko, the hardest part for me was the actual "just do it" part. Since this was unchartered territory I was have huge procrastination feelings. Finally I just decided to put one foot into the water mark out the hardibacker according to Matt's video. Once I started that the ball just rolled down hill.
The only thing I think I forgot to do was to put the 8" tube on one side to shape the exit exhaust. Matt, do you think that will be a problem? It should just sit on there and be cobbled on anyway, right?
Next up, the insulated burn tube using 8" and 10" galv. pipe.
Ray

[rjdudley]

One more thing for anyone using kastolite or something similar, looking back and knowing how much I had mixed up (all three bags) I would definitely had made the walls 3" thick. Three reasons: 1) I had plenty of mix left over and since it's not cheap it would have been nice to use it all for the core, 2) filling the small two in walls was kinda difficult and messy. You can see in the photos material was falling in and around the burn chamber. I was using a trowel to put it in, but, it just wasn't cutting it. Reason #3: after mixing and adding each bag I noticed I had to push the kastolite down, tamping it actually, and the two inch wall spacing made it kind of tough to do. I probably should have prepared a small tamping tool for it, but, I didn't think that far in advance.
I hope I got most of the air bubble out and was able to pack it in solidly.
Ray

[mannytheseacow]

Great to "meet" you, RJ. It's good to see another newby on here going through the RMH build process. Like you, I found Matt's videos super helpful and they actually sold me on the idea of a RMH for my house.

I've been heating with only wood and solar for the last 10 years and am taking a gamble switching out my big potbelly for a 6" RMH for a pretty much sole heat source for my house- SCARY! After much researching, though, I'm pretty confident (plus I can easily pipe my potbelly back in, in a pinch). My footprint is about 750 sq.ft. and the house is super-insulated.

Going from burning logs that are up to 12" in diameter down to splitting wood down to the size of my wrist is another new concept for me.

I did the core just like Matt did his except for scaling it down to a 6" system. I would be interested in trying some other materials in the future if I had more time- the clay/perlite mixture is really fragile, although cheap and easy to use. I was talking with a colleague lately and he talked about being able to use 1:1 clay and sand or also 1:1 portland cement and lime for refractory purposes, though I think you would still need perlite for the insulation. We have a ton of lime around my parts so this would be easy for me.

I'm at the stage of starting to fill my bench. I've heard how people do this in a day and I'm amazed! It takes me all day to fill about a 4" depth. I'm being rather meticulous about it so I hope it turns out good. Still, I don't see finishing the bench for another month or so. I'm also adding in 100' of copper tubing into the bench so I can pump water through it.

Good luck on your project!

Also, Lolly: It's great to hear you have some cores in the works, too!!!

[matt walker]

Wow Ray, it looks like you've done a fine job there! Tamping can be tough with that stuff, and I should have mentioned my other trick with working with the castable. I apologize for not thinking to give you this tip, but for future reference, a sawzall with no blade in it makes a great mold vibrator to help settle the mix. You can put the nose of the saw right on the outer mold and vibrate specific locations, moving all over the mold to help settle it into corners and such.

That said, my last cast using an insulating castable had quite a few voids, a couple which penetrated the whole thickness of the wall. Covered in cob and it's all just fine and holding up well, so don't sweat it if it's not perfect when it comes out. It looks great and I'm sure will burn like a true rocket.

Manny, I was terrified to tear out my old wood stove to put in the house RMH, but my neighbor made a great point when he told me I should try it out and if it didn't work he'd help me tear it out. He reminded me that the two of us and a couple hours and a wheelbarrow and I'd be right back where I was before, minus a few hundred bucks. I thought about the 10 cords I was burning a year to stay warm and figured I might as well at least try. I am never going back, I promise you guys that!

As for the size of fuel and workload of processing, I'll tell you what, that's on my mind a LOT! I am spending an amazing amount of time right now processing wood for the upcoming seasons, since I was so busy playing with the stove last spring/summer I didn't get ahead and paid the price last winter. My take on it is that I think I go through as many cycles in processing for the RMH as I did with the wood stove. By that I mean, if, when using the old wood stove, I handled 1000 rounds and split 4000 pieces and stacked 4000 pieces and so on, I do the same with the RMH. It's taking just about as long to build up a season's worth of wood split small enough and straight enough for the RMH. The difference is that everything is about 30% or so of the size it used to be. Not the rounds of course, but I need way less of them to get the amount of splits I need. But yeah, I'm finding the workload is just way easier on my body. The splits are small, a day's worth of wood is small, and in addition they are drying amazingly fast. I'm also processing big softwood rounds as if I still had a stove, and I'm only doing it that way because I did a bunch of clearing to get more sun this winter and I have a whole lot of large firs on the ground that I won't waste. Anyway, my neighbor and I always joke that since we put in our RMH's we now make scale models of real wood piles. It does feel like that, and it feels way more sustainable to me physically than putting up the huge volume of wood I needed before the changeover.