RMH in TN

[wileythenord]

Back from a week long beach vacation, now I'm gathering materials.

Picked up 2 bags of 4 cu ft Horticultural grade perlite for $12.98 a piece at my local Farmers Co-op. And I've called a couple of hvac contractors to locate some 8" duct. I've found tons of flex but no metal yet.

I do have another question though, would the insulated flex duct be ok for a temporary chimney? we are building our house as we go and don't have the permanent chimney installed yet, and if the rocket stove works out for us I'll probably put one in the basement and one on the main floor using the same chimney, but until then could I use the flex duct to run it out the top of the roof and be fine? I know the exhaust temps won't be high enough to create an issue, but will fiberglass insulation be enough to keep the gases moving?

[matt walker]

It might work Wiley. You don't mean the really thin aluminum stuff, like for dryer venting, do you? I assume you mean the heavier hvac flex ducting. The biggest downside to that stuff is that it can add a lot of drag, essentially acting like a much smaller duct. If you use it after all the mass and your exhaust is fairly low temp (200°F or so) it will probably work, but it's not ideal. Fiberglass insulation should be fine around that, as long as temps stay low.

[wileythenord]

Yeah I'm talking about the metal flex duct used in HVAC applications.

Well I'm hoping that my exhaust going through 20 feet of twists and turns in my bench will be enough to extract that much heat to get it below 200 degrees. I can get that flex duct in 16" diameter for the same price on craigslist, would that be better for my application than the 8" flex duct?

Thanks for all the advice. I'm going to be working on my mold boxes this week and hopefully making my core in the next 2 weeks. I need to get this project rolling by mid november otherwise I'll have to break out the kerosene heaters again, and I don't want to do that!

[wileythenord]

Pardon the double post.

Here are my updated sketchups:

This one shows the layout in relation to my entry door and double carriage doors leading into the basement, this rocket will be right between them. That back wall is cinder blocks poured full of concrete. I have the "bench" at 36" high in this drawing as I plan to use it kinda like a bar/table. The exhaust duct would go out of the ceiling, just realized I didn't update that.


This shows the layout for my duct, Top View, I'll be doubling back on myself here so really wanted to make sure it was understood what I'm looking at. The bottom duct will come out of the heater with a tee for a cleanout and then run approx 5' to another tee. That tee will act as an elbow and a tee giving me a second cleanout and potential for future bench addition. I hope these diagrams make sense.

Thanks!

[matt walker]

Yeah man, that looks real good. 8" hvac should be fine for the verticals, even if it is flex. Standard advice is to steer you away from it, but if it's not a huge investment I think it's worth a try. I can't imagine it would be a show stopper unless the system is right on the edge of working anyway, at which point you have other issues to deal with. You are using smooth wall for the horizontal runs though, right? I think that design should work well.

[wileythenord]

Yes, smooth galvanized HVAC duct buried in cob and the stones seen a few posts back.

[wileythenord]

[matt walker]

Here you go Wiley, it's in the sticky thread as well, but kinda buried on page 3. Might as well just put it here...

[wileythenord]

Awesome thanks! I must have missed it, I have read that thread about 30 times...

You use a pipe for your inner mold, but that pipe will burn out eventually right? According to everywhere else I've read about these monsters the heat is too much for the riser to be metal. Would it be ok to "cast" it with a sonotube? I was thinking of using 16 inch sonotube for OD and 8 inch sonotube for ID?

If i went with those dimensions, that only leaves me about 3.5 inches between my 55 gallon barrel and the outside insulation of my heat riser. That sounds like plenty to me, but I have no idea how these things work...

Sorry for all the questions!

[matt walker]

No problem on the questions, that's what this is all about man. Yep, the inner metal liner burns away. I've never used sonotube but I think Manny did. I suspect it will work fine. If you can, find the drum for the outer. It kinda keeps the whole thing together, which is important as the mix really doesn't have much structural strength of it's own. I always refer to that little drum as the "canister", since it does more than just act as a temporary form. Also, if you ever need to get in there and make changes you'll be glad you used the drum, you can just lift the whole riser off and set it aside, then re install it with no worries. It's worth tracking one down, local jiffy lube places almost always have them.

Yep, that's about a perfect side gap in my opinion.