900 sq. ft. home heating needs

[freedomlives]

Hi,

I watched with interest the video on youtube with the cast core by Matt Walker and would like to make a stove based on that.

I have a couple of questions:
What grain size of perlite is better? I can get three different types of Perlite here:
grain size from 0-1mm
grain size from 0-3.15mm, with 75% 1-3.15mm
grain size from 1-4mm, with 66% larger than 1.5mm
The EP100 is practically dust, and I've used it to insulate our attic, it also flows really well. But I feel like it would be bad for making an insulating aggregate. The EP100 is cheapest, the Agro most expensive, but this is a difference of 51€/cu.meter to 78€/cu.meter so for the stove isn't a big deal.

We have subsoil that is pure clay according to the shake test I did when making earth plaster for our walls, so I'm going to try making a core just from that first. Otherwise, because fire cement is 16€($20)/50lb sack, I have the option of just using pure fire cement/perlite mixture.

This is the layout of our house:


Total area that we want heated is 81m^2 / 870 square feet. Right now the stove in the kitchen heats up fine the kitchen and the room adjoining it, except of course during the night it needs wood added several times to keep the room warm enough for my wife. When our renovation work is done, we would be sleeping in the room outlined in purple, with the door to the olive green room with rocket stove open, and the door leading to the rest of the house. During the day, all doors would be open, except the bathroom, pantry, entrance and veranda, though we would also be having the kitchen wood stove going for cooking. So basically it needs to be heating 32m^2 (340 sq. ft) during the night, and 81m^2 during the day.

What size of rocket stove should I try for heating this house? The length along the wall with the chimney where I plan to put it is 2.1meters (7 feet) and the wall is 50cm thick (18 inches), so I wonder if anyone has tried having the pipes not only in the bench but also buried into the wall horizontally and use such a wall as part of the thermal mass to heat. The wall is not structural either, so digging into to put pipes isn't a problem.

Thanks for any advice.

[matt walker]

Welcome FL, glad to have you here. I'll take a crack at your questions here, hopefully others will chime in with opinions as well.

As for the perlite, I tend to prefer the larger grains. It seems to me that they would entrain the most air into the mix, but that's just a guess. I'm sure any would work. I've used pretty fine masonry grade, and really large Horticultural grade, and while they were very different in grain size I didn't notice much difference in working with them. Hard to say if one was more insulative since the builds were very different.

I personally really like the idea of local clay for a casting, but it does come at a price of durability, at least in my experience. It's possible to have a really well suited local clay that would hold up well. My advice would be to try some tests outdoors and see how you feel about it. I believe that if it was pretty good you could cast with that, and then "plaster" the inside of the feed area, where it will see wood abrasion, with something harder, like furnace cement. I believe the whole thing might need some occasional patching and maintenance, but it would probably work fine. My home heater is just fire clay and perlite, no hardener, and it's going strong here going into my third season.

I think you've got a pretty good layout and usage concept for the heater. Of course, if it could share the bedroom wall instead that might be slightly better, but either way it's connected to two rooms, which I think is great. I also think using the wall as some of the mass is a great idea, assuming you have cob walls, or can build the lower part of the wall with cob or something like that.

I'd recommend going with an 8" system if possible, following the dimensions on the cast core video. They perform well at that size, and you will have more flexibility in regards to length of mass run and fuel size, as well as higher potential output. You can run them on less dense fuel, or just burn smaller loads if you don't need the full output.

Hope that helps. I'm sure we'd all love to follow your build if you'd care to share it as you go along.

[mannytheseacow]

I think early in my build I posted that I was using perlite but that was a mistake because I didn't really know what I had. Now that I know a little more I can say that I have no experience with perlite other than gardening.

I used Zonolite, which is vermiculite, and it is very fine grained. I bought it in 2.2 cu.ft. bags for $8 a bag from a local masonry supply place. I'm really happy with it, but again that's all I have ever used so I wouldn't know any different.

I really like the idea you have of putting the pipes in the wall. I think that will be great.

I have radiant heat in my floors (though it rarely gets used) and it is nice to have surfaces radiating heat like that. Especially if you can do it with little input energy such as by using a RMH. I'm heating 750 sq.ft., but 2 stories. I can't talk too much about how mine works since I'm just going into my first season but I'm in a very cold winter climate and I've been fine in the past. Most of this is attributed to having a very well insulated home. I think if you are well insulated you will be well ahead of the game!

P.S. Glad to meet you!

[freedomlives]

Thanks for the replies. I'm going to go with the EP150 based on my experience with plastering, which is that you want for the plaster sand finer grains to fill inbetween the larger ones. After your reply Matt, I realized that I don't know the dimensions of my chimney which would be a limiting factor. Fortunately, it is 8"x8" square, thus larger cross sectional area than an 8" pipe. Now I'm just hoping to turn up some used 8" pipes at my neighbor's junkyard.

The wall is cob/adobe bricks, though as I think about how deep 8" will be going into the wall, I may well just dig them in part way and then add several inches of cob over them. Anyway, it will be about two weeks until I can really start inside doing anything, though I'll be going to the city next week to pick up perlite so I can make a core next week. Perlite is mined somewhere not far from Košice and I can buy it from a store owned by the producer for wholesale price. I did find where vermiculite is mined, but I haven't found it for sale.

By the way, a cool website I found today is . You can search for any sort of mineral and there is a database with all of the producers around the world for that mineral on a navigable google map.

[matt walker]

Whoah, Mindcat is awesome! Thank you for that. Man, I'm jealous of you getting to incorporate your walls, that is going to be a great heater.

[freedomlives]

Yesterday I made the core. Though, I had this idea that I'd make the form, set it directly on a board, mold around it, remove the form, and then set that part of the core on a seperately made base. I mixed equal parts clay-perlite, but didn't have any fibers added for strength (mistake 1), and it was probably a bit wet (mistake 2). Then, realizing drying would be a problem, I loosened the outside box to encourage moisture escape from there (mistake 3). Finally, realizing this morning that it would take a week or more in this weather to dry, I decided just to slide the top part onto the base and burn away the form (pressboard/masonite/pressed sawdust board thing).

I got a great fire going, rocketing flame even out of the top of the 1 meter long "stove pipe" cobbled together from some gutter material riveted together. Going at least 15 minutes. I went inside to get my camera, and right as I turned it on, the "stove pipe" collapsed (into three pieces-- Aluminum rivets == no good), and as I started to take a photo the clay collapsed.

Oh well, at least having the rocket effect going finally in person was encouraging. Just need to go to the store on Monday and buy some fiberglass fibers, maybe also fire cement if they have any in the small town nearby, and a proper stovepipe if I can find one that large. (Otherwise, I'll properly fold seams to make one...)

[pa_friendly_guy]

Sounds like you have had a wonderful Learning experience there Freedom. When Thomas Edison was working on the Light Bulb he had tried many different things and none of then worked. When someone asked if he was discouraged he said No, I now know 963 things that don't work. He just needed to find one that would work. You now know how not to cast a core, the next time you won't make those mistakes again. Keep up the good work.

[matt walker]

Oh, man, sorry to hear that. I have had quite a few experiences like that as I've gone through this process. As a matter of fact, I cast a new core day before yesterday and it didn't go so well, even though I knew better. Sounds like you have the right attitude. Without the furnace cement the core will be REALLY fragile and take forever to dry. That's how my home system is built, but I'm starting to think it was pure luck it held together long enough to get in place. Once they are in place, they are fine, but breaking them from the mold, moving them, all that is very tricky without the furnace cement as hardener. If you can't find the furnace cement, is there any way you can cast it in place? That would be my recommendation. Not to say you can't move it, I somehow did it, but I don't think I realized how lucky I was to get it in there.

The good news is that since you didn't add the hardener you can reuse all the clay/perlite.

[freedomlives]

Well, I did learn something else in fact, which I only noticed a bit later looking at the broken pieces. Around the heat riser end of the core some of the clay turned from beige-brown to red-- brick red. So I am thinking, if one isn't using firecement and wants to feed tube to be harder, then initially one could build a fire using the heat riser side as feed tube and feed tube as heat riser, that way burning the clay-perlite mix there to become brick-perlite mix, which I suppose would be stronger than just dried clay.

I'm going to give pure clay-perlite another go, just a drier mix and with fiberglass fibers in it. I know where I can get firecement for sure, the same place where I get perlite. Its just a 90 minute drive away, so I wouldn't want to make the trip just for a sack of cement. But, since wood heating is quite popular in Slovakia, if the building supply in town doesn't yield it, then the next closer (30 min away) city certainly will.

I'm definitely not trying to remove the mold next time though. Its just particle board... I think moving the top half onto the lower half didn't contribute much to structural integrity, and without waiting for it to air dry (which with fall temperatures could take what... till December?) there's no way I can remove the mold anyway...

Actually, if I really wanted to, I could contact the company which makes the fire cement, and buy from them castable insulating refractory. But I really want to try without any relatively expensive bought materials, though we could afford them if neccesary, but it would be nice if poorer people around here could make these stoves without needing to buy much.

-Andrew

[mannytheseacow]

Hi Andrew,

I replied to this post this morning but for some reason it didn't appear? Anyway, brick red? are you using fireclay?

Otherwise, what I posted earlier is that I did much of what you did when I built mine- and it sounds like Matt did too on his first one. I built very wet, on a sub-freezing day. Don't get too frustrated, you'll nail it this next time around.

Don't be afraid to leave that mold on there. I used plywood too and burned like crazy with it still on. The temps never get high enough on the outside of the cast to hardly feel warm to the touch.

When I built mine, I put down a piece of heavy plywood, then a piece of cement board, and then just set the sides of the mold on top of the cement board. The plywood had the strength to move the core around on a hand truck, and when I finally was ready to move it into place I just slid the cement board off of the plywood base and slid off the surrounding mold. That cement board is still down under my core in the finished build. I felt better about doing that so as not to disturb the fragile core at all.