Exiting duct directly from metal radiant bell? Shorter burn chamber? 21' of duct in mass? Heat Stop?

Hello I'm building my first RMH. (8", fire brick(2-1/8x4x9") combustion chamber/riser, cob mass).

I am using Ianto's book, Erica & Ernie's RMH Builder's Guide, and two individuals who have built one RMH for themselves (one who has gone to a Ianto seminar) as resources.

We are somewhat stuck on 2 questions(*) and curious about an additional 2:

*1. My current design has the duct system exiting the combustion unit barrel/bell, do you see any problems with this?





*2. My burn chamber measurement is currently approx. 14- 7/8" (7 bricks on edge) (not including fead and riser). Riser is 36", 42" if you measure to floor of burn chamber. It has been suggested to make the burn chamber as short as possible- potentially lessening ash build-up in J of combustion unit. Do you have suggestions/thoughts?




3.Due the limits of my space it seems I will only be able to fit approx 21 feet of duct in my mass. Thoughts?
(Image does not show final leg to exhaust - which would be left on the image...)


4. I plan to use Heat Stop 50 (http://www.heatstoprefractorymortar.com/productDetail.php?HEAT-STOP-II-HEAT-STOP-50-1) use a thin/liquidey mix and dipping bricks (as exampled by Ernie Wisner - "kiln building method" in one of their videos). Any thoughts/suggestions?

Many thanks to all ye permies folks! I am excited to take the next step in my RMH journey!!!

Christopher of NC-USA

I am about to try a side barrel exit as you did, on my first build of a RSTMH. I think it will work, but testing can only tell. Four things though. #1. You should know that you bought galvanized duct. Steel is "galvanized with zinc which burns off at a relatively low temperature. What that temperature actually is, I do not know. Especially for the first section or two of pipe attached the the barrel, OUT OF DOORS you need to soak the galvanized metal in a mixture of one cup muriatic acid to 1 gal of water for at least two hours when its warm out, and 3 hours if it's cold to etch the zinc off. #2 especially for the first couple of sections of pipe off of the barrel, because of the heat, I am using 24 gauge duct, not the thinner 26 gauge. #3. I do not know if this is going to be better, but in coming off of the barrel, I am first using a 12" to 10" reducer - then a short section of 10" pipe and then reducing again to 8", and because my heat riser is only 5.5" in diameter, I am further reducing it down to 6". Flowing air likes to be reduced only in stages. #4. I would test fire the system BEFORE burying horizontal exhaust vent to see just what the heat throughout the system is going to be to determine whether to lengthen the run or shorten the run.

Check your ratios, they sound off to me.
Top of feed tube to floor = B.
Horizontal length from farthest points of feed to riser = D.
Fire riser = E. And personally I measure from the roof of the burn chamber.
Then:
E >= 3B, and E >= 2D.
I'm testing just three on-edge " bridge " bricks, not seven; I suspect that will bite you.

Manifold: Peter van Der Berg says more like a 10" pipe off the barrel if it is at least 2" above the barrel rim. If. Lower, go to a 12" pipe.

I don't remember the thread name, but it's over at Donkey's forum.

Thanks to you both for the feedback.

David are you certain about the ducting needing that treatment. I did not feel it was galvanized just steel like the barrels we use. I feel I'm using the same thing that others use in their designs like in the Ernie & Erica video below:

[youtube]<iframe width="560" height="315" src="//www.youtube.com/embed/U9Pg-Ee8-M0?list=UUH1WWFSPxXjPD7PSKHpYs6Q" frameborder="0" allowfullscreen></iframe>[/youtube]

Erik: thanks for the formulas - very helpful. I'm not familiar with Donkey's forum, would you share a link?

Since I posted this, I've tried the design from E&E's Builder's Guide: 16"feed, 24"burn chamber (3 bricks, and 48"+ riser (I actually did 50") it burned really well from the start - nice rocket sound! Seems like I'll have to use two barrels if I go with this design, or build up the masonry. Only problem was at one point the fire began to climb pretty close to the rim of the feed side? I'm thinking about reducing it's height to 14". I'd love to hear thoughts?

Thank you all!

video link again:
http://youtu.be/U9Pg-Ee8-M0?list=UUH1WWFSPxXjPD7PSKHpYs6Q

Christopher D Brown wrote:Only problem was at one point the fire began to climb pretty close to the rim of the feed side? I'm thinking about reducing it's height to 14".

Lets assume that there are no household appliance fans exhausting to the outdoors and pulling a vacuum on the house, which will cause an RMH to reverse draft.

When the feed is not filled completely with wood, the excess space will slow combustion air velocity and can result in fire creeping up the feed. Using a brick or two to partially block the feed opening will correct the situation.

Christopher; Try adding a "peter channel to the feed tube. Check out my post (experimental peter channel) for pictures of mine in action. Made mine roar louder and get hotter instantly! As far as your transition , bigger is always better, go 12-10 -8

This is one of the interesting links over at Donkey's forum that is discussing CSA and the manifold between the bottom of the barrel the entry to the thermal mass:

http://donkey32.proboards.com/thread/337/helpful-hint

There are a number of other interesting threads on that forum, and I've enjoyed browsing it's contents. I'd certainly recommend reading the threads which discuss the development of the P-channel, or Peter-Channel.

As to the galvanized question. I am not absolutely sure, but as far as I can tell, there is black stove pipe and galvanized, unless you go to stainless. one way to test is to scratch it with a knife and then put a soaking wet wash cloth soaked in salt water. Within a day if you see rust start to form only at the part where you scratched it, it is definitely galvanized. But after my first week of burning, in my particular unit - I would say that only the first piece of duct off of the bell needs to be treated. But it is really easy to do in a 5 gallon bucket, even if you soak one end for two or three house and then the other. I had originally said that I started off of the barrel with 12".. I was wrtong. I started with a 10 to 8 reducer, a short piece of 8, then a 8 inch tee with a clean out, then a 8 to 7 reducer, and then a 7 to 6 reducer.

I have seen lot of designs that use a 4-brick bridge. 3-brick is sometimes too tight against the barrel. But both are a lot less than 7-brick bridge. Just wanted to add that small point that 4 might be easier than 3. It depends on your specific barrel placement of course.

i believe galvy vaporizes at around 700deg, which is why many people including myself have used galvy in the horizontal ducting, by the time it enters the duct, its more like 300 deg, and much less by the end of the road