Permsteading.com

New to this forum. I like it.

Was originally going to get a conventional wood stove to heat my 480 sq ft home on 4 acres of pine forest in N. Idaho. But after learning of rocket stoves & mass heaters on YouTube these past two months, have decided to build my own instead. No pics yet, but they will come soon.

I saw Matt's YT vid of his core cast, and it got me thinking if more Perlite could be used for better insulating. 1 part fire clay to 2 parts Perlite to same amount of cement? Or maybe 1 to 1 1/2?

For the mass part, my plan is to run 8 inch vent piping in a box constructed of cement board, and then fill with concrete. Then tile over with ceramic tiles. Make a tile box to completely hide the 55 gallon drum. Or is there good-looking substitute for the drum?

I know cob is what was first taught in these systems, and so pretty much everyone uses it, but has anyone used concrete as their mass? Or has cob proven to be a better heat sink?

Thanks!

Oh and is there anything cheaper that can be used in place of refractory cement? At my local building supply it's $80 for 3 gallons...

Welcome, Nirky. Glad you found your way here.

I'm sure one of the guys who have built (or are currently building) a rocket stove or two, will come along soon with answers for your questions. In the meantime, I suggest that "it depends". I don't have any hard data on hand and those "materials" classes were 30 years ago, but I think the thermal mass equations for cob and concrete are very similar.

Welcome Nirky! Good to see you here. I'll take a shot at answering your questions.....

More Perlite might be awesome, for sure I think the more insulated the better. This mix is already pretty soft, so I'd do a few small test bricks first and make sure you are comfortable with it.

I think your tile box idea is a good one for the mass. I have seen a build with a tile surround around the drum like you describe. It seemed to work for him, I'll see if I can find a link.

I've also seen a few concrete mass builds, and the builders seem happy with them. Personally I'm a big fan of the cob for the seating area. It has a wonderful soft way it radiates the heat, and it's actually very soft and forgiving to sit on. It can be changed/added to/removed fairly easily, it's free, and so on. Concrete does have a lot more mass for the volume though, so it's got that going for it. And, for all I say how much I love the cob seating, I've never had the pleasure of sitting on a concrete one. Might be awesome.

As for the furnace cement, it is expensive. I believe you could use waterglass (sodium silicate) in place of it, but I haven't done it and don't know the ratios. I believe that is mostly what I'm getting out of the furnace cement in my mix.

You might think about casting the core out of a true castable refractory if you are near somewhere you can source such a thing.

Greetings Nirky.

I am no expert but will throw in a couple cents worth.

I have been toying with covering my barrel too. I am more interested in the heat getting into my (yet to be constructed) bench with water heating element. I was thinking about a variety of covering options, including insulation, thinking that would help "move the heat" along.

Come to find out it, insulating the primary bell would do exactly the opposite. The cooling of the gasses as it drops down the outside of the heat riser is one of the key functions of the RMH unit, it's part of the pumping process! That said, there are theories out there that if, like in your case it's more mass and less insulation, then it will still take heat from the gasses fresh out of the heat riser, cooling it (albeit slower than a steel drum exposed to ambient temps) and playing it's role pumping the goodies along.

A note about perlite. I too am casting a core and while shopping for ingredients was informed that using perlite is akin to adding marbles to a mix, it offers no structural integration, integrity. I was told that adding vermiculite was akin to adding oatmeal to the mix, absorbing some of the water (read slower dry) but it becomes part of the core/cast, not just an added aggregate.

That said many have used it, Matt of course showing us on his great core creation video (my inspiration to cast my own ) uses to great affect perlite. I was unable to get the stuff, and my masonry supply place had honking bags of vermiculite for 15 bucks so that's my deal!

I struggled with the fire/found clay issue for months (kudos to many on the board here for being patient with me) and recently found a source of fire clay (called a pottery place three 1/2 hours away) and perhaps even found clay, but that hasn't panned out and I will cob with that, not cast.

My thinking was to take Matt's recipe and tweak the ratios, ratcheting the fire cement up to 50% of the mix. HeatStop in a 50# bag is 45$, to me that is an investment in making a bombproof core that I wouldn't have to fiddle with. I will be casting in the next week or so and will document the progress here. Whether or not it works will be for everyone to see, and for our general edification....

My hope of course is for it to work swimmingly and to create a highly efficient moderately priced core, somewhere between going with straight castable refractory {spendy but awesome} and homebrew cob combo {less expensive but needs some tlc along the way and a cob surround.

Would love to see your build, its pretty hard to think you'll regret it, but like so much in life, we play the balance of pros and cons and learn along the way. Hopefully we can do this dance with respect for our hostess and eke out a sustainable future...