First time build- questions on fire clay and test build

Hello from South Africa!

Just in the 'gathering materials' stage of a first time build (8" system) and have a few questions:

1) How much powdered fire clay is necessary for one build? I can get it in 25 kg bags, but from the opposite side of the country, so would like to get it all at once.

2) Can a test build be built with fire clay slip? (ie will it be easy to disassemble, clean off bricks, and reassemble, with the fire clay slip on all the bricks? Never worked with fire clay before so not sure how it behaves)

Thanks!

Hi Jess; Welcome to Permies!

Please give us a few details of your planned build.
I assume as a first build your thinking J tube?  You told us 8". Is it a Piped mass ? Or a brick bell ?
Have you built a rmh before?  Do you have a copy of the builders guide ?
Now, lets try to answer your questions.  

1) How much fire clay? Two bags should be more than enough.  It really depends on your build. Fire clay is only needed near the core unit. It is very possible you would only use one bag, but better to have extra than not enough.
If you are building a piped mass rather than a brick bell then plain clay is just fine to use anywhere away from the high heat of the core unit.
2) Yes! easy to knock apart and all the clay can be crumbled and reconstituted with water. Make sure any sand you mix in is clean of  pebbles. You want your bricks as close together as possible.
When you are using fireclay to assemble, you only need a thin layer, unlike concrete where you use a uniform thicker layer.

Thanks Thomas!

Yes it will be a J tube, piped mass, with steel barrel.

I haven't built a RMH before but do have a copy of Erica and Ernie's guide.

You mentioned mixing sand in- is that necessary for all builds, or only if the bricks are not uniform enough?

I think for the core, even fine sand may be more bulk than you need. You can make a thin slip of fireclay (very liquid), and dip the firebricks in it and immediately place them. You only want enough clay to fill any gaps and provide firm stable bedding - firebricks are so uniform that there will be almost no gaps. If you are using more common bricks, those are likely to have enough irregularity that you need the body of a sand and clay mix that is stiff enough to stay where you put it without running off the sides.

Ah thank you Glenn!

One more question about the ducting inside the mass- I've seen some recommendation that the first meter or so be stovepipe flue-grade (e.g. stainless steel), rather than HVAC ducting-grade... is that necessary?

Jess Reyes wrote:Ah thank you Glenn!

One more question about the ducting inside the mass- I've seen some recommendation that the first meter or so be stovepipe flue-grade (e.g. stainless steel), rather than HVAC ducting-grade... is that necessary?

Caveat: I'm also working up to my first RMH build.  I've been looking into this same question.

The common concern with using HVAC ducting-grade in the early part of the run is reactions of the zinc to high temperatures.  Assuming your HVAC ducting is galvanized, if it gets hot enough then the zinc may oxidize, melt, or vaporize.  It seems like melting or vaporizing are pretty unlikely in most cases but oxidizing might happen.  Another issue is that the HVAC ducting is not quite as robust as stove pipe so it may not last as long.  If you're building up a solid bench around it then this may not be a major issue.

There are a bunch of threads on permies about this very question as well as some discussion over on http://donkey32.proboards.com/.  For my build (also 8"), I've decided to go with about 6' of black stove pipe coming out of the barrel, then HVAC ducting, then black stove pipe for the vertical run up to the high-temperature chimney (which is already in place, required by code, and probably a good idea for all of the ceiling/roof penetrations anyway).

Hope this helps.

Hi Jess;  The first section of pipe is recommended to be standard black stove pipe. Apx. 24" or .6 meter long. After that lite weight hvac pipe may be used throughout the mass. I suggest using one more piece of the black pipe when it comes up out of the mass, not required but this is a spot the pipe could be easily damaged. After that switch back to the lite pipe to go thru the roof.

May I make a design suggestion. If you were planning on your pipes making a 180 degree turn to come back near the core. Rather than using two 90 degree turns with cleanouts to make the 180. Consider building a brick box that your pipe enters and leaves. Add a cleanout door on this box. This will give more room for you to access those pipes and less resistance to flow.
You won't find that idea in the builders guide it is my own innovation I WISH I had built into mine.

Jess,   As Thomas suggested, this is what I had done back in 2016 to reduce drag on the exhaust.
I learned this one from Donkey:  Rocket Bell Retrofit

Thanks Thomas and Gerry! This has got me looking into bell benches now, which are certainly a fascinating option! Do you you of anyone who's made an L-shaped bell bench? I see there's a limitation on internal surface area, but is there an issue with a chamber with internal corners?

Hey Jess;  No problem with an L shaped bench. As long as your ISA is good, your golden.

Have you seen the half barrel option ?  55 gal barrels split in half lengthwise.  Then almost all of the ends removed (just enough left to keep them from going flat).  You can use up to 6 barrels end to end with an 8" J tube.

With this arrangement it may be necessary to use a sliding exhaust pipe that you raise up to the top of the barrels to get started and then drop it down just above the floor after the draw is going.