RMH in TN

[mannytheseacow]

I've used sonotubes a couple times, and those poster/mailing tubes, too, for smaller sizes. I'm not crazy about them. They work in a pinch, I guess. I can order 6 and 8 inch sizes but they're not stocked, and they're not really rigid so if you're really ramming your mix in there it will flex a bit. It probably doesn't matter much but I'd use something more rigid if you can.

The first one I used I found in the dump and it was falling apart. My mix was a little wet, too, which added to the problem. I like them (especially mailing tubes) for experimenting with. They seem to be easy enough to find and not an issue if you aren't worried about longevity.

[wileythenord]

After looking around my area and making some calls I think I have a plan. I'll use the 8 inch sonotube for the ID of the heat riser and use flat sheet metal (for roofing) as the OD. I can bend the metal into a circle and rivet it into shape. This allows me to make the OD whatever size I want and the local metal roofing company will sell their scrap pieces to me for little to nothing per foot. He told me $0.50 - $0.75 per foot depending on color, and since color won't matter it shouldn't cost more than a dollar or 2.

I built the inner core this weekend and made all of my cuts for the outer core, and I rounded up about 6 wheelbarrows full of red clay to put near my work area. I will have to sift it a bit to get all the rocks out of it, but I'm moving along. I wanted to get it ready to pour over the weekend, but little league football, fall festivals and big family breakfasts got in the way...

Still haven't secured the duct yet, I've called a few HVAC companies and asked them for their scrap duct but none seem to have 8", so I'll keep on it for another week before I buy any new. Would the metal mentioned above work for ductwork? Most of the duct I've ran across has been 30 ga, this metal for roofing is 26-28 ga?

[matt walker]

Yeah man, I don't see why you couldn't use the same method for ducting. Should work fine.

I'm going to reiterate this here Wiley, since it seems to keep cropping up. When you make your core, especially if you are using found clay, it's going to be really fragile. If you can make it in place, do it. If you can't, burn it dry outside, then replace the outer mold boards to move it inside, and get it wrapped with some good cob asap. Expect it to crack/break, but as long as you can get it in place in it's original form, it will perform fine. Cracks and all.

Man, big family breakfast sounds good! Worth putting stuff on hold for that, for sure.

[wileythenord]

Thanks for the reassurance, I've seen your core and others on this site that have removed their outer mold to reveal lots of cracks, I think i'm ready for it lol. I do hope to circumvent some of the cracking by casting this thing on a cart and easing it into place before removing the mold box. But even then I plan to have my final location prepped and read to start cobbing.

If that metal will work thats probably my best bet. I can pick it up anytime as the metal place is on my way home in the afternoon. Sweet!

Man its exciting! I'm trying to take as many pics as I can along the way so when I get an update ready I can have pics!

[wileythenord]

Well somehow my ridiculus phone lost all of my pictures, but this weekend I cast my core!

It was around 65 degrees saturday and sunday afternoon and today and tomorrow it is supposed to get up to around 70 so I'm hoping that the sun will help dry it out. I won't be home tonight to try a fire in it, so tomorrow afternoon will be my first firing! I believe I am set on everything to build my bench too, I just hope the weather stays above 40 for the next 4-5 days some I'm not mashing frozen clay for cob

[matt walker]

Nice Wiley! The core won't dry much on it's own, so don't be surprised about that. Looking forward to seeing it come together.

[wileythenord]

I think I fixed my phone, but I lost all my pics...and I took in process pics for a redneck tutorial...

Oh well I'll document the next one

Ok Matt, maybe I can fire it in the dark some, give the coyotes something to howl about!

[wileythenord]

Here is some pics






At first it was smokey since it was burning out the inner mold, but once that was gone it cleared up! I only burned about 45 minutes last night but tonight I'm going to burn some slabs for an hour or two! Weekend install if all goes well!

[matt walker]

Looking good Wiley, that flue pipe got plenty hot by the looks of it. Looking forward to the build.

[Oddmar]

Wish i'd seen this thread sooner, but i've been spending time away from the computer, trying to get more work done.

Here is my latest design, with explanation. Hope it helps.



A guy named Peter Vanderburg came up with several innovations. the P-channel, the tripwire, the backsweep, the kicktail, and recently the cold-air box. If i accidentally give anyone credit for something someone else created, i apologize.

A. P-channel. If you look closely, you'll see there's a thin metal plate 3/8" off the inside face of the feed chamber, curling around almost into the burn tunnel. This allows fresh oxygen to be siphoned into the burn tunnel by the inrushing air. A metal box sits atop the feed chamber, with an air inlet on the top left allowing cool air to enter the box, surrounding the wood, keeping the upper part of the thin wood cooler, helping to prevent flame/ smokeback. The air in the box is pulled in by the P-channel.

B. Tripwire. A ridge in the top of the burn tunnel, arrow-shaped, pointing towards the heat riser. Placed 1/3 of the way along the burn tunnel. Hot gases tumble over this ridge, mixing fuel with oxy, for better combustion.

C. Viewing window/ cleanout. Thermo-ceramic glass in a hinged metal frame, allowing you to see the fire (ambiance), and providing access to the burn tunnel for cleaning (like when burning pallet wood, to clean out the nails, and any ash that may collect in the burn tunnel.)

D. Backsweep/ kicktail. Curving the lower back of the burn tunnel reduces friction and keeps the gases flowing fast. An arrow-shaped ridge, much like the tripwire, protrudes into the end of the burn tunnel, just before the heat riser. This tumbles hot gases and mixes fuel/ oxy again.

E. Drum/ riser clearance. Most of my RMH's have had 2.5"-3" clearance here. According to Ernie Wisner, increasing the gap moves the donut-shaped 'torus' of the final burn lower in the drum, decreasing drum-top temps while increasing drum-side temps. I also shape the top of the heat riser to a sharp edge, otherwise ash may build up here and reduce airflow.

F. Exhaust plenum/ cleanout. WARNING! Problem area. This plenum must be quite a bit larger than the cross sectional area of the system to work well. Simply running an 8" pipe into the side of the drum usually chokes the system. YMMV.

After researching for several years online, i found many people complaining of poor system flow, and they all had a small exhaust opening out of the drum. So i built my exhaust plenums big, and have had no problems. A large plenum means lower air pressure which will help ash fall out of the exhaust stream. This area will be the spot where you clean out the most ash...with RMH's, very little.

G. Floor Standoff. While the side of the mass may be coupled to a stone/ brick wall, the bottom should be separated from the floor. A concrete slab floor will suck heat away from your mass. You want to slowly radiate heat into the room, not lose 1/4 to 1/3 of your heat to the earth beneath the slab. The bottom of the burn tunnel/ core MUST be separated from the floor/ wall. I didn't do this in my most recent build (for a friend), and while it heats his home/ shop, the burn tunnel never gets up to the temps required to efficiently burn all the smoke. Smoke out the roof vent and alot of ash production. Creosote buildup in the plenum/ mass ducting due to the combination of smoke with the low exhaust temps of a RMH, which will probably result in a future chimney fire. My friend won't listen to me or let me rebuild it.

H. Duct Sizing. If you have an 8" riser, you MUST have 8" duct through the mass, and 8" exhaust stack. Restricting the pipes here will choke the system. You COULD have an 8" riser with two 6" ducts running through the mass. 8"=50.32 sq in. 6"=28.3 sq in. 28+28=56. You would have better heat transfer with the two 6" ducts as well...more surface area. 8"=28.6" dia, (2) 6" ducts = 37.6" of metal transferring heat to the mass. The ducting through the mass can be larger in cross-sectional area than the riser, such as with Matt's half-drum benches. Feeding an 8" RMH into a 6" stone chimney will Not work well. The burn tunnel can be a bit smaller, this will just result in faster airflow. I like to expose as little of the wood as possible (vertically) to radiant heat from the burn tunnel, to reduce flame/ smokeback.

I. Duct Cleanout. It's a good idea to have a tee wherever possible, to allow for cleaning out the ducting through the mass, should the need arise. Burning damp wood might produce fine black ash that will collect on the inside of the ducts. Right Matt?

J. Priming Tee. If the mass or heat riser is cold (first firing of the season or you've been on vacation?), getting the flame to draw into the burn tunnel instead of back into your face may be frustrating. A tee is placed in the exhaust riser with a bit of expanded metal grill in the pipe (to support burning paper, candle, etc). Prepare kindling/ paper in the feed chamber. Light paper in the priming tee then cap it. This will create an updraft which will draw the flame through the kindling and into the burn tunnel. As soon as the heat riser warms the draw will be self-sustaining.

If you prefer, a three-bladed fan with blades set at a 45* angle (to minimize restriction) could be placed in the exhaust riser, preferably in the attic space. Avoid using large motors (such as in a bathroom exhaust fan) which will reduce airflow when the fan is not running. A 12V cordless drill motor would work well. A 555 timer circuit could energize the fan for say 2 minutes, creating the necessary startup draft.

K. Insulated exhaust riser/ chimney. RMH's typically have low exhaust temps. Hot air rises. If the mass is cold, absorbing most of the exhaust heat, the gases in the exhaust riser may be too cool to rise quickly. This may choke the system. Insulating the exhaust riser will prevent heat loss here and help the exhaust gases retain much-needed heat for proper system operation.

A side note. Many people have made the mistake of venting their RMH out a window or side of their home. This is fine as long as you run the insulated pipe up above the roof. Venting just outside one side of a house works ok until the wind shifts, blowing on that side of the house. This creates air pressure higher than system output, creating reverse flow and flame/ smokeback out the feed chamber. I always run my exhaust riser up 4' above the roof line. The air flowing over the rain cap (no matter the wind direction) creates draft which helps the system breathe.

As always, Your Mileage May Vary.