Oddmar's RMH Basics, a primer

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Oddmar's RMH Basics, a primer

Postby matt walker » Wed Nov 26, 2014 4:35 am

Oddmar made this excellent post in another thread, and I thought it was a great primer for folks who are just starting out.

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


A guy named Peter van den Berg 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.
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Re: Oddmar's RMH Basics, a primer

Postby Nirky » Wed Nov 26, 2014 5:39 am

Big thumbs up, thanks for condensing all the important points of constructing a RMH into one clear picture & post!
I don't think I'll ever get over Macho Grande, those wounds run pretty deep.
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Re: Oddmar's RMH Basics, a primer

Postby ByronC » Thu Nov 27, 2014 11:22 am

Excellent info on a tweaked J-tube RMH design. It deserves to be made "sticky" if not already :mrgreen:

Non electronics nerds (and non ham radio ops;) will probably not have a clue as to what a 555 timer is, at least not without the prefix for a complete part number. I.e. NE555, which is a common integrated circuit that has been around for decades. Searching for "NE555 timer circuits" will bring up many schematic diagrams and application notes.

But being a firm believer in the KISS method <Keep It Super Simple:-> I'd do everything I could in the construction of an RMH with the best chimney available to avoid the need of an electric fan in the exhaust for "forced drafting". But it is not a bad option for a temporary setup etc.
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Re: Oddmar's RMH Basics, a primer

Postby Oddmar » Mon Dec 01, 2014 7:12 pm

A 555 timer circuit could energize the fan for say 2 minutes, creating the necessary startup draft.

It's not meant for full-time use, just for starting the RMH when the heat riser and mass are cold because the furnace hasn't been run in a few days/ months.
Darrell "Jake" Jacob, Oddmar on all the forums, KC9PZN to all you amateurs.
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